有细微的差异,主要涉及到继承。当使用函数作为元类时,生成的类实际上是type
的一个实例,可以无限制地从其继承;但是,这样的子类将不会调用元类函数。当使用type
的子类作为元类时,生成的类将是该元类的实例,其任何子类也将是其元类的实例;然而,多重继承将受到限制。
举例说明这些差异:
>>> def m1(name, bases, atts):
>>> print "m1 called for " + name
>>> return type(name, bases, atts)
>>>
>>> def m2(name, bases, atts):
>>> print "m2 called for " + name
>>> return type(name, bases, atts)
>>>
>>> class c1(object):
>>> __metaclass__ = m1
m1 called for c1
>>> type(c1)
<type 'type'>
>>> class sub1(c1):
>>> pass
>>> type(sub1)
<type 'type'>
>>> class c2(object):
>>> __metaclass__ = m2
m2 called for c2
>>> class sub2(c1, c2):
>>> pass
>>> type(sub2)
<type 'type'>
请注意,在定义sub1和sub2时,没有调用元类函数。它们将被创建,就好像c1和c2没有元类一样,但是在创建后进行了操作。
>>> class M1(type):
>>> def __new__(meta, name, bases, atts):
>>> print "M1 called for " + name
>>> return super(M1, meta).__new__(meta, name, bases, atts)
>>> class C1(object):
>>> __metaclass__ = M1
M1 called for C1
>>> type(C1)
<class '__main__.M1'>
>>> class Sub1(C1):
>>> pass
M1 called for Sub1
>>> type(Sub1)
<class '__main__.M1'>
注意这里已经有了一些区别:在创建Sub1时调用了M1,而且两个类都是M1的实例。我在这里使用super()
进行实际创建,原因稍后会变得清晰。
>>> class M2(type):
>>> def __new__(meta, name, bases, atts):
>>> print "M2 called for " + name
>>> return super(M2, meta).__new__(meta, name, bases, atts)
>>> class C2(object):
>>> __metaclass__ = M2
M2 called for C2
>>> type(C2)
<class '__main__.M2'>
>>> class Sub2(C1, C2):
>>> pass
M1 called for Sub2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 23, in __new__
TypeError: Error when calling the metaclass bases
metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
这是在使用元类时多重继承的主要限制。Python不知道M1和M2是否兼容,因此它强制你创建一个新的元类来确保它执行你需要的操作。
>>> class M3(M1, M2):
>>> def __new__(meta, name, bases, atts):
>>> print "M3 called for " + name
>>> return super(M3, meta).__new__(meta, name, bases, atts)
>>> class C3(C1, C2):
>>> __metaclass__ = M3
M3 called for C3
M1 called for C3
M2 called for C3
>>> type(C3)
<class '__main__.M3'>
这就是为什么我在元类的
__new__
函数中使用了
super()
:这样每个函数都可以在 MRO 中调用下一个函数。
某些情况下,您的类可能需要是
type
类型,或者可能希望避免继承问题,在这种情况下,元类函数可能是最好的选择。在其他情况下,类的类型可能确实很重要,或者您可能希望操作所有子类,在这种情况下,继承自
type
会是更好的选择。请根据具体情况自由选择适合的风格。