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Python中用Decorator来简化元编程的教程

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Jun 10, 2016 pm 03:15 PM

python

少劳多得

Decorator 与 Python 之前引入的元编程抽象有着某些共同之处：即使没有这些技术，您也一样可以实现它们所提供的功能。正如 Michele Simionato 和我在可爱的 Python 专栏的早期文章中指出的那样，即使在 Python 1.5 中，也可以实现 Python 类的创建，而不需要使用 “元类” 挂钩。

Decorator 根本上的平庸与之非常类似。Decorator 所实现的功能就是修改紧接 Decorator 之后定义的函数和方法。这总是可能的，但这种功能主要是由 Python 2.2 中引入的 classmethod() 和 staticmethod() 内置函数驱动的。在旧式风格中，您可以调用 classmethod()，如下所示：
清单 1. 典型的 “旧式” classmethod

class C:
  def foo(cls, y):
    print "classmethod", cls, y
  foo = classmethod(foo)

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虽然 classmethod() 是内置函数，但并无独特之处；您也可以使用自己的方法转换函数。例如：
清单 2. 典型的 “旧式” 方法的转换

def enhanced(meth):
  def new(self, y):
    print "I am enhanced"
    return meth(self, y)
  return new
class C:
  def bar(self, x):
    print "some method says:", x
  bar = enhanced(bar)

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decorator 所做的一切就是使您避免重复使用方法名，并且将 decorator 放在方法定义中第一处提及其名称的地方。例如：
清单 3. 典型的 “旧式” classmethod

class C:
  @classmethod
  def foo(cls, y):
    print "classmethod", cls, y
  @enhanced
  def bar(self, x):
    print "some method says:", x

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decorator 也可以用于正则函数，采用的是与类中的方法相同的方式。令人惊奇的是，这一切是如此简单（严格来说，甚至有些不必要），只需要对语法进行简单修改，所有东西就可以工作得更好，并且使得程序的论证更加轻松。通过在方法定义的函数之前列出多个 decorator，即可将 decorator 链接在一起；良好的判断可以有助于防止将过多 decorator 链接在一起，不过有时候将几个 decorator 链接在一起是有意义的：
清单 4. 链接 decorator

@synchronized
@logging
def myfunc(arg1, arg2, ...):
  # ...do something
# decorators are equivalent to ending with:
#  myfunc = synchronized(logging(myfunc))
# Nested in that declaration order

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Decorator 只是一个语法糖，如果您过于急切，那么它就会使您搬起石头砸了自己的脚。decorator 其实就是一个至少具有一个参数的函数 —— 程序员要负责确保 decorator 的返回内容仍然是一个有意义的函数或方法，并且实现了原函数为使连接有用而做的事情。例如，下面就是 decorator 两个不正确的用法：
清单 5. 没有返回函数的错误 decorator

>>> def spamdef(fn):
...   print "spam, spam, spam"
...
>>> @spamdef
... def useful(a, b):
...   print a**2 + b**2
...
spam, spam, spam
>>> useful(3, 4)
Traceback (most recent call last):
 File "<stdin>", line 1, in &#63;
TypeError: 'NoneType' object is not callable

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decorator 可能会返回一个函数，但这个函数与未修饰的函数之间不存在有意义的关联：
清单 6. 忽略传入函数的 decorator

>>> def spamrun(fn):
...   def sayspam(*args):
...     print "spam, spam, spam"
...   return sayspam
...
>>> @spamrun
... def useful(a, b):
...   print a**2 + b**2
...
>>> useful(3,4)
spam, spam, spam

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最后，一个表现更良好的 decorator 可以在某些方面增强或修改未修饰函数的操作：
清单 7. 修改未修饰函数行为的 decorator

>>> def addspam(fn):
...   def new(*args):
...     print "spam, spam, spam"
...     return fn(*args)
...   return new
...
>>> @addspam
... def useful(a, b):
...   print a**2 + b**2
...
>>> useful(3,4)
spam, spam, spam
25

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您可能会质疑，useful() 到底有多么有用？addspam() 真的是那样出色的增强吗？但这种机制至少符合您通常能在有用的 decorator 中看到的那种模式。

高级抽象简介

根据我的经验，元类应用最多的场合就是在类实例化之后对类中的方法进行修改。decorator 目前并不允许您修改类实例化本身，但是它们可以修改依附于类的方法。这并不能让您在实例化过程中动态添加或删除方法或类属性，但是它让这些方法可以在运行时根据环境的条件来变更其行为。现在从技术上来说，decorator 是在运行 class 语句时应用的，对于顶级类来说，它更接近于 “编译时” 而非 “运行时”。但是安排 decorator 的运行时决策与创建类工厂一样简单。例如：
清单 8. 健壮但却深度嵌套的 decorator

def arg_sayer(what):
  def what_sayer(meth):
    def new(self, *args, **kws):
      print what
      return meth(self, *args, **kws)
    return new
  return what_sayer

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