class Meta(type) : def __init__(cls, name, bases, dct) : cls.attr = 'name' super().__init__(name, bases, dct) class A(metaclass = Meta) : attr = 'new_name' print(A.attr) # name

How, then, to override the class attribute?

  • you ask: how in class A define the attr attribute if A uses the meta metaclass that already sets the attr attribute on the class? That is, how to change the behavior provided by the metaclass in this case? Do you want A.attr == 'new_name' or A().attr == 'new_name' or both both the class attribute and the object attribute? - jfs
  • @jfs, yes, you need to override the class attribute (change the behavior provided by the metaclass). A class, not an object. (A.attr = 'new_name'). Is it possible Those. This is possible, of course, but the option, for example, after class declaration, write then A.attr = 'new_attr' - does not fit. - user203925
  • If you don’t like how the class is being created, then another metaclass can be passed. - jfs

1 answer 1

This can be explained by adding a few extra print'ov:

 class Meta(type): print("Meta body") def __init__(cls, name, bases, dct): print("Meta init for {name}".format(name=name)) cls.attr = 'name' super().__init__(name, bases, dct) def __new__(cls, name, bases, classdict): print("Meta new for {name}".format(name=name)) print(classdict) return type.__new__(cls, name, bases, classdict) class A(metaclass = Meta): print("A body") attr = 'new_name' def __init__(self): # Очевидно, не вызывается, потому что объект не создается print("A init") print(A.attr) # name

Conclusion:

 Meta body A body Meta new for A {'__qualname__': 'A', 'attr': 'new_name', '__module__': '__main__', '__init__': <function A.__init__ at 0x0000000004AB4D90>} Meta init for A name

Apparently, the metaclass is not the parent of the class, but what this class creates. One of the goals of metaclasses is to change classes at the stage of their creation. What happened.

For more, perhaps, it is better to refer to the classic explanation: https://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python