Shouldn't __metaclass__ force the use of a metaclass in Python?

Question

I\'ve been trying to learn about metaclasses in Python. I get the main idea, but I can\'t seem to activate the mechanism. As I understand it, you can specify M to be as the

Answer 1

In Python 3 (which you are using) metaclasses are specified by a keyword parameter in the class definition:

class ClassMeta(metaclass=M):
  pass

Specifying a __metaclass__ class property or global variable is old syntax from Python 2.x and not longer supported. See also "What's new in Python 3" and PEP 2115.

Answer 2

The syntax of metaclasses has changed in Python 3.0. The __metaclass__ attribute is no longer special at either the class nor the module level. To do what you're trying to do, you need to specify metaclass as a keyword argument to the class statement:

p = print

class M(type):
    def __init__(*args):
        type.__init__(*args)
        print("The rain in Spain")

p(1)
class ClassMeta(metaclass=M): pass

Yields:

1
The rain in Spain

As you'd expect.

Answer 3

This works as you expect in Python 2.6 (and earlier), but in 3.0 metaclasses are specified differently:

class ArgMeta(metaclass=M): ...