Библиотека Python разработчика(@BookPython). The creation of a class consists of two big steps. First, the class body is evaluated, just like any

The creation of a class consists of two big steps. First, the class body is evaluated, just like any function body. Second, the resulted namespace (the one that is returned by locals()) is used by a metaclass (type by default) to construct an actual class object.

class Meta(type):
    def __new__(meta, name, bases, ns):
        print(ns)
        return super().__new__(
            meta, name,
            bases, ns
        )


class Foo(metaclass=Meta):
    B = 2

The above code prints {'__module__': '__main__', '__qualname__': 'Foo', 'B': 3}. Obviously, if you do something like B = 2; B = 3, then the metaclass only knows about B = 3, since only that value is in ns. This limitation is based on the fact, that a metaclass works after the body evaluation. However, you can interfere in the evaluation by providing custom namespace. By default, a simple dictionary is used but you can provide a custom dictionary-like object using the metaclass __prepare__ method.

class CustomNamespace(dict):
    def __setitem__(self, key, value):
        print(f'{key} -> {value}')
        return super().__setitem__(key, value)


class Meta(type):
    def __new__(meta, name, bases, ns):
        return super().__new__(
            meta, name,
            bases, ns
        )

    @classmethod
    def __prepare__(metacls, cls, bases):
        return CustomNamespace()


class Foo(metaclass=Meta):
    B = 2
    B = 3

The output is the following:

__module__ -> __main__
__qualname__ -> Foo
B -> 2
B -> 3

And this is how enum.Enum is protected from duplicates.