A closer look at magic methods in Python

<h2>Preface</h2> <p>In Python, all methods enclosed by "__" double underscores are collectively called "magic methods". For example, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>.<br>Some of the magic methods that we come into contact with the most, we may never encounter again in our lives, so here is just a brief introduction;<br>And some magic methods, use them skillfully You can construct very beautiful code, such as encapsulating complex logic into a simple API. <br>The editing ideas of this article are borrowed from Rafe Kettler’s blog: A Guide to Python Magic Methods, and some code examples are added. <br>The order of introduction is roughly: the common ones are introduced first, and the rarer ones are introduced later. <br>The code examples used in this article can be downloaded from my github. </p> <h2>Construction and initialization</h2> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>We are very familiar with it. It is called when the object is initialized. We generally understand it as a "constructor".<br>In fact, when we call# When ##</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">x = SomeClass()</pre><div class="contentsignin">Copy after login</div></div> is called, <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is not the first one to be executed, <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is. So to be precise, <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> and <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> together constitute the "constructor". <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is used to create a class and return an instance of this class, and <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> just initializes the instance with the parameters passed in. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> will definitely be called during the process of creating an instance, but <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is not necessarily, for example, through <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pickle.load</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> will not be called when deserializing an instance in the <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> way. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>The method always needs to return an instance of the class, and <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> cannot return any value except None. For example, the following example: <p>class Foo(object):<br> def __init__(self):<br> print 'foo __init__'<br> return None # Must return None, otherwise a TypeError<br> def __del__(self ):<br> Print 'foo __del__'<br>In practice, you will rarely use <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> unless you want to be able to control the creation of the class. <p>If you want to explain <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>, it often involves the introduction of <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">metaclass</pre><div class="contentsignin">Copy after login</div></div> (metaclass). <p>If you are interested in going deeper, you can refer to my other blog: Understanding Python's metaclass<br>The overloading of <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is also introduced in detail in the Python documentation. <p>At the end of the object's life cycle, <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__del__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> will be called. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__del__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> can be understood as a "destructor". <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__del__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is defined when The behavior of an object when it is garbage collected. <p> It is easy to be misunderstood. In fact, <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">x.__del__()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is not the implementation of <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">del x</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>, but <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">del x</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is often called when executing <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">x.__del__()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p> </p>.<p>How to understand this sentence? Continue to use the code of the Foo class above as an example: <br>foo = Foo()<br>foo.__del__()<br>print foo<br> del foo<br>print foo # NameError, foo is not defined<br>If <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">foo.__del__()</pre><div class="contentsignin">Copy after login</div></div> is called, the object itself still exists. But if <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">del foo</pre><div class="contentsignin">Copy after login</div></div> is called, there will no longer be foo. Object.<p>Please note that if the object still exists when the interpreter exits, there is no guarantee that <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__del__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> will be executed accurately. So <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__del__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is not a substitute for good programming habits. <p>For example, when processing a socket, close the completed connection in time. <br></p>Attribute access control<h2></h2>There are always people who want to complain about Python's lack of class encapsulation. For example, they hope that Python can define private properties and then provide publicly accessible getters and setters. Python can actually achieve encapsulation through magic methods. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattr__(self, name)</pre><div class="contentsignin">Copy after login</div></div> <h3></h3>This method defines the behavior when you try to access a property that does not exist. Therefore, overloading this method can catch misspellings and redirect them, or warn about some deprecated attributes. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setattr__(self, name, value)</pre><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div></h3> is a solution to implement encapsulation, which defines the behavior when you assign and modify properties. <p> Regardless of whether a certain attribute of the object exists, it allows you to assign a value to the attribute, so you can perform customized operations on the value of the attribute. One thing to note is to avoid the "infinite recursion" error when implementing <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>, which will be mentioned in the code example below. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delattr__(self, name)</pre><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div></h3> is very similar to <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>, except that it defines the behavior when you delete the attribute. Implementing <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> is to avoid the error of "infinite recursion" at the same time. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattribute__(self, name)</pre><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattribute__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div></h3> defines the behavior when your property is accessed. In comparison, <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> will only work when the property does not exist. <p> Therefore, in the Python version that supports <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattribute__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>, <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> must be used before calling <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattribute__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>. <p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattribute__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> Also avoid the "infinite recursion" error. <p>It should be reminded that it is best not to try to implement <br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getattribute__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>, because this approach is rarely seen and it is easy to cause bugs. <p>Example description<br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>'s infinite recursion error:<p>def __setattr__(self, name, value):<br> self.name = value<br> #Every time an attribute is assigned, __setattr__ will be called, so calling itself continuously leads to infinite recursion. <br>So the correct way to write it should be:<br>def __setattr__(self, name, value):<br> self.__dict__[name] = value<br></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delattr__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div>If it appears in its implementation<p></p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">del self.name</pre><div class="contentsignin">Copy after login</div></div> <p> 这样的代码也会出现”无限递归”错误，这是一样的原因。<br>下面的例子很好的说明了上面介绍的4个魔术方法的调用情况:<br>class Access(object):<br>    def __getattr__(self, name):<br>        print '__getattr__'<br>        return super(Access, self).__getattr__(name)<br>    def __setattr__(self, name, value):<br>        print '__setattr__'<br>        return super(Access, self).__setattr__(name, value)<br>    def __delattr__(self, name):<br>        print '__delattr__'<br>        return super(Access, self).__delattr__(name)<br>    def __getattribute__(self, name):<br>        print '__getattribute__'<br>        return super(Access, self).__getattribute__(name)<br>access = Access()<br>access.attr1 = True  # __setattr__调用<br>access.attr1  # 属性存在,只有__getattribute__调用<br>try:<br>    access.attr2  # 属性不存在, 先调用__getattribute__, 后调用__getattr__<br>except AttributeError:<br>    pass<br>del access.attr1  # __delattr__调用</p> <h2>描述器对象</h2> <p>我们从一个例子来入手,介绍什么是描述符,并介绍</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__get__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__set__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delete__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p> 的使用。(放在这里介绍是为了跟上一小节介绍的魔术方法作对比)<br>我们知道，距离既可以用单位”米”表示,也可以用单位”英尺”表示。现在我们定义一个类来表示距离,它有两个属性: 米和英尺。<br>class Meter(object):<br>    '''Descriptor for a meter.'''<br>    def __init__(self, value=0.0):<br>        self.value = float(value)<br>    def __get__(self, instance, owner):<br>        return self.value<br>    def __set__(self, instance, value):<br>        self.value = float(value)<br>class Foot(object):<br>    '''Descriptor for a foot.'''<br>    def __get__(self, instance, owner):<br>        return instance.meter * 3.2808<br>    def __set__(self, instance, value):<br>        instance.meter = float(value) / 3.2808<br>class Distance(object):<br>    meter = Meter()<br>    foot = Foot()<br>d = Distance()<br>print d.meter, d.foot  # 0.0, 0.0<br>d.meter = 1<br>print d.meter, d.foot  # 1.0 3.2808<br>d.meter = 2<br>print d.meter, d.foot  # 2.0 6.5616<br>在上面例子中,在还没有对Distance的实例赋值前, 我们认为meter和foot应该是各自类的实例对象, 但是输出却是数值。这是因为</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__get__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>发挥了作用.<br>我们只是修改了meter,并且将其赋值成为int，但foot也修改了。这是</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__set__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>发挥了作用.<br>描述器对象(Meter、Foot)不能独立存在, 它需要被另一个所有者类(Distance)所持有。<br>描述器对象可以访问到其拥有者实例的属性，比如例子中Foot的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">instance.meter</pre><div class="contentsignin">Copy after login</div></div> <p>。<br>在面向对象编程时，如果一个类的属性有相互依赖的关系时，使用描述器来编写代码可以很巧妙的组织逻辑。在Django的ORM中, models.Model中的InterField等字段, 就是通过描述器来实现功能的。<br>一个类要成为描述器，必须实现</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__get__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__set__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delete__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p> 中的至少一个方法。下面简单介绍下:</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__get__(self, instance, owner)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>参数instance是拥有者类的实例。参数owner是拥有者类本身。</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__get__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>在其拥有者对其读值的时候调用。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__set__(self, instance, value)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__set__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>在其拥有者对其进行修改值的时候调用。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delete__(self, instance)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delete__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>在其拥有者对其进行删除的时候调用。</p> <h2>构造自定义容器(Container)</h2> <p>在Python中，常见的容器类型有: dict, tuple, list, string。<br>其中tuple, string是不可变容器，dict, list是可变容器。<br>可变容器和不可变容器的区别在于，不可变容器一旦赋值后，不可对其中的某个元素进行修改。<br>比如定义了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">l = [1, 2, 3]</pre><div class="contentsignin">Copy after login</div></div> <p>和</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">t = (1, 2, 3)</pre><div class="contentsignin">Copy after login</div></div> <p>后, 执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">l[0] = 0</pre><div class="contentsignin">Copy after login</div></div> <p>是可以的，但执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">t[0] = 0</pre><div class="contentsignin">Copy after login</div></div> <p>则会报错。<br>如果我们要自定义一些数据结构，使之能够跟以上的容器类型表现一样，那就需要去实现某些协议。<br>这里的协议跟其他语言中所谓的”接口”概念很像，一样的需要你去实现才行，只不过没那么正式而已。<br>如果要自定义不可变容器类型，只需要定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__len__</pre><div class="contentsignin">Copy after login</div></div> <p> 和 </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getitem__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法;<br>如果要自定义可变容器类型，还需要在不可变容器类型的基础上增加定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setitem__</pre><div class="contentsignin">Copy after login</div></div> <p> 和 </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delitem__</pre><div class="contentsignin">Copy after login</div></div> <p>。<br>如果你希望你的自定义数据结构还支持”可迭代”, 那就还需要定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__iter__</pre><div class="contentsignin">Copy after login</div></div> <p>。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__len__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>需要返回数值类型，以表示容器的长度。该方法在可变容器和不可变容器中必须实现。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getitem__(self, key)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>当你执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">self[key]</pre><div class="contentsignin">Copy after login</div></div> <p>的时候，调用的就是该方法。该方法在可变容器和不可变容器中也都必须实现。<br>调用的时候,如果key的类型错误，该方法应该抛出TypeError；<br>如果没法返回key对应的数值时,该方法应该抛出ValueError。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setitem__(self, key, value)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>当你执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">self[key] = value</pre><div class="contentsignin">Copy after login</div></div> <p>时，调用的是该方法。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__delitem__(self, key)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>当你执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">del self[key]</pre><div class="contentsignin">Copy after login</div></div> <p>的时候，调用的是该方法。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__iter__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>该方法需要返回一个迭代器(iterator)。当你执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">for x in container:</pre><div class="contentsignin">Copy after login</div></div> <p> 或者使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">iter(container)</pre><div class="contentsignin">Copy after login</div></div> <p>时，该方法被调用。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reversed__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>如果想要该数据结构被內建函数</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">reversed()</pre><div class="contentsignin">Copy after login</div></div> <p>支持,就还需要实现该方法。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__contains__(self, item)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>如果定义了该方法，那么在执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">item in container</pre><div class="contentsignin">Copy after login</div></div> <p> 或者 </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">item not in container</pre><div class="contentsignin">Copy after login</div></div> <p>时该方法就会被调用。<br>如果没有定义，那么Python会迭代容器中的元素来一个一个比较，从而决定返回True或者False。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__missing__(self, key)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">dict</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>字典类型会有该方法，它定义了key如果在容器中找不到时触发的行为。<br>比如</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">d = {'a': 1}</pre><div class="contentsignin">Copy after login</div></div> <p>, 当你执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">d[notexist]</pre><div class="contentsignin">Copy after login</div></div> <p>时，</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">d.__missing__['notexist']</pre><div class="contentsignin">Copy after login</div></div> <p>就会被调用。<br>下面举例，使用上面讲的魔术方法来实现Haskell语言中的一个数据结构。<br># -*- coding: utf-8 -*-<br>class FunctionalList:<br>    ''' 实现了内置类型list的功能,并丰富了一些其他方法: head, tail, init, last, drop, take'''<br>    def __init__(self, values=None):<br>        if values is None:<br>            self.values = []<br>        else:<br>            self.values = values<br>    def __len__(self):<br>        return len(self.values)<br>    def __getitem__(self, key):<br>        return self.values[key]<br>    def __setitem__(self, key, value):<br>        self.values[key] = value<br>    def __delitem__(self, key):<br>        del self.values[key]<br>    def __iter__(self):<br>        return iter(self.values)<br>    def __reversed__(self):<br>        return FunctionalList(reversed(self.values))<br>    def append(self, value):<br>        self.values.append(value)<br>    def head(self):<br>        # 获取第一个元素<br>        return self.values[0]<br>    def tail(self):<br>        # 获取第一个元素之后的所有元素<br>        return self.values[1:]<br>    def init(self):<br>        # 获取最后一个元素之前的所有元素<br>        return self.values[:-1]<br>    def last(self):<br>        # 获取最后一个元素<br>        return self.values[-1]<br>    def drop(self, n):<br>        # 获取所有元素，除了前N个<br>        return self.values[n:]<br>    def take(self, n):<br>        # 获取前N个元素<br>        return self.values[:n]<br>我们再举个例子，实现Perl语言的autovivification,它会在你每次引用一个值未定义的属性时为你自动创建数组或者字典。<br>class AutoVivification(dict):<br>    """Implementation of perl's autovivification feature."""<br>    def __missing__(self, key):<br>        value = self[key] = type(self)()<br>        return value<br>weather = AutoVivification()<br>weather['china']['guangdong']['shenzhen'] = 'sunny'<br>weather['china']['hubei']['wuhan'] = 'windy'<br>weather['USA']['California']['Los Angeles'] = 'sunny'<br>print weather<br>结果输出:{'china': {'hubei': {'wuhan': 'windy'}, 'guangdong': {'shenzhen': 'sunny'}}, 'USA':    {'California': {'Los Angeles': 'sunny'}}}<br>在Python中，关于自定义容器的实现还有更多实用的例子，但只有很少一部分能够集成在Python标准库中，比如Counter, OrderedDict等</p> <h2>上下文管理</h2> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">with</pre><div class="contentsignin">Copy after login</div></div> <p>声明是从Python2.5开始引进的关键词。你应该遇过这样子的代码:<br>with open('foo.txt') as bar:<br>    # do something with bar<br>在with声明的代码段中，我们可以做一些对象的开始操作和清除操作,还能对异常进行处理。<br>这需要实现两个魔术方法: </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__enter__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p> 和 </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__exit__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__enter__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__enter__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>会返回一个值，并赋值给</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">as</pre><div class="contentsignin">Copy after login</div></div> <p>关键词之后的变量。在这里，你可以定义代码段开始的一些操作。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__exit__(self, exception_type, exception_value, traceback)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__exit__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>定义了代码段结束后的一些操作，可以这里执行一些清除操作，或者做一些代码段结束后需要立即执行的命令，比如文件的关闭，socket断开等。如果代码段成功结束，那么exception_type, exception_value, traceback 三个参数传进来时都将为None。如果代码段抛出异常，那么传进来的三个参数将分别为: 异常的类型，异常的值，异常的追踪栈。<br>如果</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__exit__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>返回True, 那么with声明下的代码段的一切异常将会被屏蔽。<br>如果</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__exit__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>返回None, 那么如果有异常，异常将正常抛出，这时候with的作用将不会显现出来。<br>举例说明：<br>这该示例中，IndexError始终会被隐藏，而TypeError始终会抛出。<br>class DemoManager(object):<br>    def __enter__(self):<br>        pass<br>    def __exit__(self, ex_type, ex_value, ex_tb):<br>        if ex_type is IndexError:<br>            print ex_value.__class__<br>            return True<br>        if ex_type is TypeError:<br>            print ex_value.__class__<br>            return  # return None<br>with DemoManager() as nothing:<br>    data = [1, 2, 3]<br>    data[4]  # raise IndexError, 该异常被__exit__处理了<br>with DemoManager() as nothing:<br>    data = [1, 2, 3]<br>    data['a']  # raise TypeError, 该异常没有被__exit__处理<br>输出:<br><type 'exceptions.IndexError'><br><type 'exceptions.TypeError'><br>Traceback (most recent call last):<br>  ...</p> <h2>对象的序列化</h2> <p>Python对象的序列化操作是pickling进行的。pickling非常的重要，以至于Python对此有单独的模块</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pickle</pre><div class="contentsignin">Copy after login</div></div> <p>，还有一些相关的魔术方法。使用pickling, 你可以将数据存储在文件中，之后又从文件中进行恢复。<br>下面举例来描述pickle的操作。从该例子中也可以看出,如果通过pickle.load 初始化一个对象, 并不会调用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法。<br># -*- coding: utf-8 -*-<br>from datetime import datetime<br>import pickle<br>class Distance(object):<br>    def __init__(self, meter):<br>        print 'distance __init__'<br>        self.meter = meter<br>data = {<br>    'foo': [1, 2, 3],<br>    'bar': ('Hello', 'world!'),<br>    'baz': True,<br>    'dt': datetime(2016, 10, 01),<br>    'distance': Distance(1.78),<br>}<br>print 'before dump:', data<br>with open('data.pkl', 'wb') as jar:<br>    pickle.dump(data, jar)  # 将数据存储在文件中<br>del data<br>print 'data is deleted!'<br>with open('data.pkl', 'rb') as jar:<br>    data = pickle.load(jar)  # 从文件中恢复数据<br>print 'after load:', data<br>值得一提，从其他文件进行pickle.load操作时，需要注意有恶意代码的可能性。另外，Python的各个版本之间,pickle文件可能是互不兼容的。<br>pickling并不是Python的內建类型，它支持所有实现pickle协议(可理解为接口)的类。pickle协议有以下几个可选方法来自定义Python对象的行为。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getinitargs__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>如果你希望unpickle时，</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法能够调用，那么就需要定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getinitargs__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, 该方法需要返回一系列参数的元组，这些参数就是传给</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__init__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>的参数。<br>该方法只对</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">old-style class</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>有效。所谓</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">old-style class</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>,指的是不继承自任何对象的类，往往定义时这样表示: </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">class A:</pre><div class="contentsignin">Copy after login</div></div> <p>, 而非</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">class A(object):</pre><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getnewargs__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>跟</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getinitargs__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>很类似，只不过返回的参数元组将传值给</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__new__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getstate__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>在调用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pickle.dump</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时，默认是对象的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__dict__</pre><div class="contentsignin">Copy after login</div></div> <p>属性被存储，如果你要修改这种行为，可以在</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getstate__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法中返回一个state。state将在调用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pickle.load</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时传值给</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setstate__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setstate__(self, state)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>一般来说,定义了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getstate__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>,就需要相应地定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__setstate__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>来对</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getstate__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>返回的state进行处理。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>如果pickle的数据包含了自定义的扩展类（比如使用C语言实现的Python扩展类）时，就需要通过实现</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法来控制行为了。由于使用过于生僻，这里就不展开继续讲解了。<br>令人容易混淆的是，我们知道, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">reduce()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>是Python的一个內建函数, 需要指出</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>并非定义了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">reduce()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>的行为，二者没有关系。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce_ex__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce_ex__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p> 是为了兼容性而存在的, 如果定义了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce_ex__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, 它将代替</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__reduce__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p> 执行。<br>下面的代码示例很有意思，我们定义了一个类Slate(中文是板岩的意思)。这个类能够记录历史上每次写入给它的值,但每次</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pickle.dump</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时当前值就会被清空，仅保留了历史。<br># -*- coding: utf-8 -*-<br>import pickle<br>import time<br>class Slate:<br>    '''Class to store a string and a changelog, and forget its value when pickled.'''<br>    def __init__(self, value):<br>        self.value = value<br>        self.last_change = time.time()<br>        self.history = []<br>    def change(self, new_value):<br>        # 修改value, 将上次的valeu记录在history<br>        self.history.append((self.last_change, self.value))<br>        self.value = new_value<br>        self.last_change = time.time()<br>    def print_changes(self):<br>        print 'Changelog for Slate object:'<br>        for k, v in self.history:<br>            print '%s    %s' % (k, v)<br>    def __getstate__(self):<br>        # 故意不返回self.value和self.last_change,<br>        # 以便每次unpickle时清空当前的状态，仅仅保留history<br>        return self.history<br>    def __setstate__(self, state):<br>        self.history = state<br>        self.value, self.last_change = None, None<br>slate = Slate(0)<br>time.sleep(0.5)<br>slate.change(100)<br>time.sleep(0.5)<br>slate.change(200)<br>slate.change(300)<br>slate.print_changes()  # 与下面的输出历史对比<br>with open('slate.pkl', 'wb') as jar:<br>    pickle.dump(slate, jar)<br>del slate  # delete it<br>with open('slate.pkl', 'rb') as jar:<br>    slate = pickle.load(jar)<br>print 'current value:', slate.value  # None<br>print slate.print_changes()  # 输出历史记录与上面一致</p> <h2>运算符相关的魔术方法</h2> <p>运算符相关的魔术方法实在太多了，也很好理解，不打算多讲。在其他语言里，也有重载运算符的操作，所以我们对这些魔术方法已经很了解了。</p> <h3>比较运算符</h3> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__cmp__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>如果该方法返回负数，说明</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">self < other</pre><div class="contentsignin">Copy after login</div></div><p>; 返回正数，说明</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">self > other</pre><div class="contentsignin">Copy after login</div></div> <p>; 返回0说明</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">self == other</pre><div class="contentsignin">Copy after login</div></div> <p>。<br>强烈不推荐来定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__cmp__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, 取而代之, 最好分别定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__lt__</pre><div class="contentsignin">Copy after login</div></div> <p>等方法从而实现比较功能。</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__cmp__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>在Python3中被废弃了。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__eq__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>定义了比较操作符</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">==</pre><div class="contentsignin">Copy after login</div></div> <p>的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ne__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>定义了比较操作符</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">!=</pre><div class="contentsignin">Copy after login</div></div> <p>的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__lt__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>定义了比较操作符</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><</pre><div class="contentsignin">Copy after login</div></div><p>的行为.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__gt__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>定义了比较操作符</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">></pre><div class="contentsignin">Copy after login</div></div> <p>的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__le__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>定义了比较操作符</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><=</pre><div class="contentsignin">Copy after login</div></div><p>的行为.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ge__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>定义了比较操作符</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">>=</pre><div class="contentsignin">Copy after login</div></div> <p>的行为.<br>下面我们定义一种类型Word, 它会使用单词的长度来进行大小的比较, 而不是采用str的比较方式。<br>但是为了避免 </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">Word('bar') == Word('foo')</pre><div class="contentsignin">Copy after login</div></div> <p> 这种违背直觉的情况出现,并没有定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__eq__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, 因此Word会使用它的父类(str)中的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__eq__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>来进行比较。<br>下面的例子中也可以看出: 在编程语言中, 如果</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a >=b and a <= b</pre><div class="contentsignin">Copy after login</div></div><p>, 并不能推导出</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a == b</pre><div class="contentsignin">Copy after login</div></div><p>这样的结论。<br/># -*- coding: utf-8 -*-<br/>class Word(str):<br/> '''存储单词的类，定义比较单词的几种方法'''<br/> def __new__(cls, word):<br/> # 注意我们必须要用到__new__方法，因为str是不可变类型<br/> # 所以我们必须在创建的时候将它初始化<br/> if ' ' in word:<br/> print "Value contains spaces. Truncating to first space."<br/> word = word[:word.index(' ')] # 单词是第一个空格之前的所有字符<br/> return str.__new__(cls, word)<br/> def __gt__(self, other):<br/> return len(self) > len(other)<br>    def __lt__(self, other):<br>        return len(self) < len(other)<br/> def __ge__(self, other):<br/> return len(self) >= len(other)<br>    def __le__(self, other):<br>        return len(self) <= len(other)<br/>print 'foo < fool:', Word('foo') < Word('fool') # True<br/>print 'foolish > fool:', Word('foolish') > Word('fool')  # True<br>print 'bar >= foo:', Word('bar') >= Word('foo')  # True<br>print 'bar <= foo:', Word('bar') <= Word('foo') # True<br/>print 'bar == foo:', Word('bar') == Word('foo') # False, 用了str内置的比较方法来进行比较<br/>print 'bar != foo:', Word('bar') != Word('foo') # True</p><h3>一元运算符和函数</h3><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__pos__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了’+'号一元运算符(比如</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">+some_object</pre><div class="contentsignin">Copy after login</div></div><p>)</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__neg__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了’-'号一元运算符(比如</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">-some_object</pre><div class="contentsignin">Copy after login</div></div><p>)</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__invert__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">~</pre><div class="contentsignin">Copy after login</div></div><p>号一元运算符(比如</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">~some_object</pre><div class="contentsignin">Copy after login</div></div><p>)</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__abs__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">abs()</pre><div class="contentsignin">Copy after login</div></div><p>內建函数.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__round__(self, n)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">round()</pre><div class="contentsignin">Copy after login</div></div><p>内建函数. 参数n表示四舍五进的精度.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__floor__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">math.round()</pre><div class="contentsignin">Copy after login</div></div><p>, 向下取整.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ceil__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">math.ceil()</pre><div class="contentsignin">Copy after login</div></div><p>, 向上取整.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__trunc__(self)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">math.trunc()</pre><div class="contentsignin">Copy after login</div></div><p>, 向0取整.</p><h3>算术运算符</h3><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__add__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了加号运算.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__sub__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了减号运算.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__mul__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了乘法运算.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__floorp__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">//</pre><div class="contentsignin">Copy after login</div></div><p>运算符.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__p__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">/</pre><div class="contentsignin">Copy after login</div></div><p>运算符. 该方法在Python3中废弃. 原因是Python3中，pision默认就是true pision.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__truep__</pre><div class="contentsignin">Copy after login</div></div>(self, other)</h3><p>实现了true pision. 只有你声明了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">from __future__ import pision</pre><div class="contentsignin">Copy after login</div></div><p>该方法才会生效.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__mod__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">%</pre><div class="contentsignin">Copy after login</div></div><p>运算符, 取余运算.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__pmod__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pmod()</pre><div class="contentsignin">Copy after login</div></div><p>內建函数.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__pow__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">**</pre><div class="contentsignin">Copy after login</div></div><p>操作. N次方操作.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__lshift__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了位操作</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><<</pre><div class="contentsignin">Copy after login</div></div><p>.</p><h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rshift__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3><p>实现了位操作</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">>></pre><div class="contentsignin">Copy after login</div></div> <p>.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__and__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了位操作</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">&</pre><div class="contentsignin">Copy after login</div></div> <p>.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__or__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了位操作</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">|</pre><div class="contentsignin">Copy after login</div></div> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__xor__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了位操作</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">^</pre><div class="contentsignin">Copy after login</div></div> <h3>反算术运算符</h3> <p>这里只需要解释一下概念即可。假设针对some_object这个对象:<br>some_object + other<br>上面的代码非常正常地实现了some_object的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__add__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法。那么如果遇到相反的情况呢?<br>other + some_object<br>这时候，如果other没有定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__add__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法，但是some_object定义了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__radd__</pre><div class="contentsignin">Copy after login</div></div> <p>, 那么上面的代码照样可以运行。<br>这里的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__radd__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <p>就是</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__add__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <p>的反算术运算符。<br>所以，类比的，我们就知道了更多的反算术运算符, 就不一一展开了:</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rsub__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rmul__(self, other)</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rmul__(self, other)</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rfloorp__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rp__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rtruep__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rmod__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rpmod__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rpow__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rlshift__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rrshift__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rand__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ror__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__rxor__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <h3>增量赋值</h3> <p>这也是只要理解了概念就容易掌握的运算。举个例子:<br>x = 5<br>x += 1  # 这里的+=就是增量赋值，将x+1赋值给了x<br>因此对于</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a += b</pre><div class="contentsignin">Copy after login</div></div> <p>, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__iadd__</pre><div class="contentsignin">Copy after login</div></div> <p> 将返回</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a + b</pre><div class="contentsignin">Copy after login</div></div> <p>, 并赋值给a。<br>所以很容易理解下面的魔术方法了:</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__iadd__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__isub__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__imul__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ifloorp__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ip__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__itruep__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__imod__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ipow__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ilshift__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__irshift__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__iand__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ior__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__ixor__(self, other)</pre><div class="contentsignin">Copy after login</div></div> <h3>类型转化</h3> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__int__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了类型转化为int的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__long__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了类型转化为long的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__float__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了类型转化为float的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__complex__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了类型转化为complex(复数, 也即1+2j这样的虚数)的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__oct__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了类型转化为八进制数的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__hex__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了类型转化为十六进制数的行为.</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__index__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>在切片运算中将对象转化为int, 因此该方法的返回值必须是int。用一个例子来解释这个用法。<br>class Thing(object):<br>    def __index__(self):<br>        return 1<br>thing = Thing()<br>list_ = ['a', 'b', 'c']<br>print list_[thing]  # 'b'<br>print list_[thing:thing]  # []<br>上面例子中, </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">list_[thing]</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>的表现跟</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">list_[1]</pre><div class="contentsignin">Copy after login</div></div> <p>一致，正是因为Thing实现了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__index__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法。<br>可能有的人会想，</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">list_[thing]</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>为什么不是相当于</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">list_[int(thing)]</pre><div class="contentsignin">Copy after login</div></div> <p>呢? 通过实现Thing的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__int__</pre><div class="contentsignin">Copy after login</div></div> <p>方法能否达到这个目的呢?<br>显然不能。如果真的是这样的话，那么</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">list_[1.1:2.2]</pre><div class="contentsignin">Copy after login</div></div> <p>这样的写法也应该是通过的。<br>而实际上，该写法会抛出TypeError: </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">slice indices must be integers or None or have an __index__ method</pre><div class="contentsignin">Copy after login</div></div> <p><br>下面我们再做个例子,如果对一个dict对象执行</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">dict_[thing]</pre><div class="contentsignin">Copy after login</div></div> <p>会怎么样呢?<br>dict_ = {1: 'apple', 2: 'banana', 3: 'cat'}<br>print dict_[thing]  # raise KeyError<br>这个时候就不是调用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__index__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>了。虽然</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">list</pre><div class="contentsignin">Copy after login</div></div> <p>和</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">dict</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>都实现了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__getitem__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法, 但是它们的实现方式是不一样的。<br>如果希望上面例子能够正常执行, 需要实现Thing的</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__hash__</pre><div class="contentsignin">Copy after login</div></div> <p> 和 </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__eq__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>方法.<br>class Thing(object):<br>    def __hash__(self):<br>        return 1<br>    def __eq__(self, other):<br>        return hash(self) == hash(other)<br>dict_ = {1: 'apple', 2: 'banana', 3: 'cat'}<br>print dict_[thing]  # apple</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__coerce__(self, other)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>实现了混合模式运算。<br>要了解这个方法,需要先了解</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">coerce()</pre><div class="contentsignin">Copy after login</div></div> <p>内建函数: 官方文档上的解释是, coerce(x, y)返回一组数字类型的参数, 它们被转化为同一种类型，以便它们可以使用相同的算术运算符进行操作。如果过程中转化失败，抛出TypeError。<br>比如对于</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">coerce(10, 10.1)</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>, 因为10和10.1在进行算术运算时，会先将10转为10.0再来运算。因此</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">coerce(10, 10.1)</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>返回值是(10.0, 10.1).</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__coerce__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>在Python3中废弃了。</p> <h2>其他魔术方法</h2> <p>还没讲到的魔术方法还有很多，但有些我觉得很简单，或者很少见，就不再累赘展开说明了。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__str__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">str()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时调用。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__repr__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">repr()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时调用。</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">str()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>和</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">repr()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>都是返回一个代表该实例的字符串，<br>主要区别在于: str()的返回值要方便人来看,而repr()的返回值要方便计算机看。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__unicode__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">unicode()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时调用。</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">unicode()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>与</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">str()</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>的区别在于: 前者返回值是unicode, 后者返回值是str。unicode和str都是</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">basestring</pre><div class="contentsignin">Copy after login</div></div> <p>的子类。<br>当你对一个类只定义了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__str__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>但没定义</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__unicode__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>时,</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__unicode__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>会根据</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__str__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>的返回值自动实现,即</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">return unicode(self.__str__())</pre><div class="contentsignin">Copy after login</div></div> <p>;<br>但返回来则不成立。<br>class StrDemo2:<br>    def __str__(self):<br>        return 'StrDemo2'<br>class StrDemo3:<br>    def __unicode__(self):<br>        return u'StrDemo3'<br>demo2 = StrDemo2()<br>print str(demo2)  # StrDemo2<br>print unicode(demo2)  # StrDemo2<br>demo3 = StrDemo3()<br>print str(demo3)  # <__main__.StrDemo3 instance><br>print unicode(demo3)  # StrDemo3</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__format__(self, formatstr)</pre><div class="contentsignin">Copy after login</div></div></h3> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">"Hello, {0:abc}".format(a)</pre><div class="contentsignin">Copy after login</div></div> <p>等价于</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">format(a, "abc")</pre><div class="contentsignin">Copy after login</div></div> <p>, 等价于</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a.__format__("abc")</pre><div class="contentsignin">Copy after login</div></div> <p>。<br>这在需要格式化展示对象的时候非常有用，比如格式化时间对象。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__hash__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">hash()</pre><div class="contentsignin">Copy after login</div></div> <p>时调用, 返回值是数值类型。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__nonzero__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">bool()</pre><div class="contentsignin">Copy after login</div></div> <p>时调用, 返回True或者False。<br>你可能会问, 为什么不是命名为</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__bool__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>? 我也不知道。<br>我只知道该方法在Python3中改名为</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__bool__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>了。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__dir__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">dir()</pre><div class="contentsignin">Copy after login</div></div> <p>时调用。通常实现该方法是没必要的。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__sizeof__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">sys.getsizeof()</pre><div class="contentsignin">Copy after login</div></div> <p>时调用。返回对象的大小，单位是bytes。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__instancecheck__(self, instance)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例调用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">isinstance(instance, class)</pre><div class="contentsignin">Copy after login</div></div> <p>时调用。 返回值是布尔值。它会判断instance是否是该类的实例。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__subclasscheck__(self, subclass)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">issubclass(subclass, class)</pre><div class="contentsignin">Copy after login</div></div> <p>时调用。返回值是布尔值。它会判断subclass否是该类的子类。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__copy__(self)</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">copy.copy()</pre><div class="contentsignin">Copy after login</div></div> <p>时调用。返回”浅复制”的对象。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__deepcopy__(self, memodict={})</pre><div class="contentsignin">Copy after login</div></div></h3> <p>对实例使用</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">copy.deepcopy()</pre><div class="contentsignin">Copy after login</div></div> <p>时调用。返回”深复制”的对象。</p> <h3><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__call__(self, [args...])</pre><div class="contentsignin">Copy after login</div></div></h3> <p>该方法允许类的实例跟函数一样表现:<br>class XClass:<br>    def __call__(self, a, b):<br>        return a + b<br>def add(a, b):<br>    return a + b<br>x = XClass()<br>print 'x(1, 2)', x(1, 2)<br>print 'callable(x)', callable(x)  # True<br>print 'add(1, 2)', add(1, 2)<br>print 'callable(add)', callable(add)  # True</p> <h2>Python3中的差异</h2> <p>Python3中，str与unicode的区别被废除了,因而</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__unicode__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>没有了，取而代之地出现了</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__bytes__</pre><div class="contentsignin">Copy after login</div></div> <p>.<br>Python3中，pision默认就是true pision, 因而</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__p__</pre><div class="contentsignin">Copy after login</div></div> <p>废弃.</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__coerce__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>因存在冗余而废弃.</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__cmp__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <p>因存在冗余而废弃.</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__nonzero__</pre><div class="contentsignin">Copy after login</div></div> <p>改名为</p> <div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">__bool__</pre><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div><div class="contentsignin">Copy after login</div></div> <div></div> <!--<p class='doc-content-pic doc-pic'><img src="https://img.php.cn/upload/article/000/000/013/b418c5819b5ebecfe1f9367c41f9fb08-0.jpg" / alt="A closer look at magic methods in Python" > </p>-->

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