详解python单例模式与metaclass
单例模式的实现方式
将类实例绑定到类变量上
class Singleton(object):
_instance = None
def __new__(cls, *args):
if not isinstance(cls._instance, cls):
cls._instance = super(Singleton, cls).__new__(cls, *args)
return cls._instance
但是子类在继承后可以重写__new__以失去单例特性
class D(Singleton):
def __new__(cls, *args):
return super(D, cls).__new__(cls, *args)
使用装饰器实现
def singleton(_cls):
inst = {}
def getinstance(*args, **kwargs):
if _cls not in inst:
inst[_cls] = _cls(*args, **kwargs)
return inst[_cls]
return getinstance
@singleton
class MyClass(object):
pass
问题是这样装饰以后返回的不是类而是函数，当然你可以singleton里定义一个类来解决问题，但这样就显得很麻烦了
使用__metaclass__，这个方式最推荐
class Singleton(type):
_inst = {}
def __call__(cls, *args, **kwargs):
if cls not in cls._inst:
cls._inst[cls] = super(Singleton, cls).__call__(*args)
return cls._inst[cls]
class MyClass(object):
__metaclass__ = Singleton
metaclass
元类就是用来创建类的东西，可以简单把元类称为“类工厂”，类是元类的实例。type就是Python的内建元类，type也是自己的元类，任何一个类
>>> type(MyClass) type >>> type(type) type
python在创建类MyClass的过程中，会在类的定义中寻找__metaclass__，如果存在则用其创建类MyClass，否则使用内建的type来创建类。对于类有继承的情况，如果当前类没有找到，会继续在父类中寻找__metaclass__，直到所有父类中都没有找到才使用type创建类。
如果模块里有__metaclass__的全局变量的话，其中的类都将以其为元类，亲自试了，没这个作用，无任何影响
查看type的定义，
type(object) -> the object's type
type(name, bases, dict) -> a new type
所以利用type定义一个类的元类，可以用函数返回一个上面第二种定义的对象，也可以继承type并重写其中的方法。
直接使用type生成的对象作为元类，函数作用是使属性变为大写
def update_(name, bases, dct):
attrs = ((name, value) for name, value in dct.items() if not name.startswith('__'))
uppercase_attr = {name.upper(): value for name, value in attrs}
return type(name, bases, uppercase_attr)
class Singleton(object):
__metaclass__ = update_
abc = 2
d = Singleton()
print d.ABC
# 2
上一节中，单例模式元类实现用的是类继承方式，而对于第一种__new__的方式，本质上调用的是type.__new__，不过使用super能使继承更清晰一些并避免一些问题
这里简单说明一下，__new__是在__init__前调用的方法，会创建对象并返回，而__init__则是用传入的参数将对象初始化。看一下type中这两者以及__call__的实现
def __init__(cls, what, bases=None, dict=None): # known special case of type.__init__
"""
type(object) -> the object's type
type(name, bases, dict) -> a new type
# (copied from class doc)
"""
pass
@staticmethod # known case of __new__
def __new__(S, *more): # real signature unknown; restored from __doc__
""" T.__new__(S, ...) -> a new object with type S, a subtype of T """
pass
def __call__(self, *more): # real signature unknown; restored from __doc__
""" x.__call__(...) <==> x(...) """
pass
前面提到类相当于元类的实例化，再联系创建单例模式时使用的函数，用的是__call__，其实用三种magic method中任何一种都是可以的，来看一下使用元类时各方法的调用情况
class Basic(type):
def __new__(cls, name, bases, newattrs):
print "new: %r %r %r %r" % (cls, name, bases, newattrs)
return super(Basic, cls).__new__(cls, name, bases, newattrs)
def __call__(self, *args):
print "call: %r %r" % (self, args)
return super(Basic, self).__call__(*args)
def __init__(cls, name, bases, newattrs):
print "init: %r %r %r %r" % (cls, name, bases, newattrs)
super(Basic, cls).__init__(name, bases, dict)
class Foo:
__metaclass__ = Basic
def __init__(self, *args, **kw):
print "init: %r %r %r" % (self, args, kw)
a = Foo('a')
b = Foo('b')
结果
new: <class '__main__.Basic'> 'Foo' () {'__module__': '__main__', '__metaclass__': <class '__main__.Basic'>, '__init__': <function init at 0x106fd5320>}
init: <class '__main__.Foo'> 'Foo' () {'__module__': '__main__', '__metaclass__': <class '__main__.Basic'>, '__init__': <function init at 0x106fd5320>}
call: <class '__main__.Foo'> ('a',)
init: <__main__.Foo object at 0x106fee990> ('a',) {}
call: <class '__main__.Foo'> ('b',)
init: <__main__.Foo object at 0x106feea50> ('b',) {}
元类的__init__和__new__只在创建类Foo调用了一次，而创建Foo的实例时，每次都会调用元类的__call__方法
以上就是本文的全部内容，对python单例模式与metaclass进行了描述，希望对大家的学习有所帮助。
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