确定python中的类是否为元类

Question

Is there a way in python to determine if a class object is a metaclass?

I know that you could check using the equality operator. metaclass == type

But that wouldn't cover for user defined metaclasses.

Answer 1

Test if the object is a subclass of type :

issubclass(MetaClass, type)

This returns True for all metaclasses, including type itself.

Demo:

>>> class Meta(type): pass
...
>>> class Foo(object): pass
...
>>> issubclass(Meta, type)
True
>>> issubclass(Foo, type)
False

Answer 2

Most metaclasses are type subclasses ( issubclass(metaklass, type) ) but not all :

>>> def logging_meta(name, bases, namespace, **kwd):
...     print(name, bases, namespace, kwds)
...     return type(name, bases, namespace, **kwds)
... 
>>> class C(metaclass=logging_meta):
...     a = 1
... 
C () {'__module__': '__main__', '__qualname__': 'C', 'a': 1} {}
>>> issubclass(logging_meta, type)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: issubclass() arg 1 must be a class

ie, the answer is "any callable eg, a class with a __call__() method that accepts the same arguments as type() and returns a class object will do":

>>> class C(metaclass=lambda *a: lambda *a: None): pass
... 
>>> C()
>>> type(C)
<class 'function'>