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Question regarding __class__ closure

According to from

class C(A, B, metaclass=meta):
    def f(self):
        return __class__

To be compiled approximately like this:

def _outer_C(*__args__, **__kw__):
    class _inner_C(*__args__, **__kw__):
        def f(self):
            return __class__
    __class__ = _inner_C
    return _inner_C
C = _outer_C(A, B, metaclass=meta)

My question is:

class A:
    def foo(a):
        class X:
            x = __class__
            def fun(self):

        return X

Why does this work? While the following doesn't?

def bar():
    class Y:
        def fun(a):
            class X:
                m = n
                def foo(a):
            n = 1
            return X
    n = 0
    return Y
if __name__ == '__main__':
  y = bar()
  x = y().fun()

If the free variable is assigned in the outscope, it's turned to a cell var, and because it's assigned after the class body is evaluated, NameError is raised.

If the __class__ closure trick is really what it said to be, it should also raise a NameError, what's the trick use to make it work?

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