Weird behavior in Python 3 frames

Question

I defined this function:

def caller_class():
    loc = inspect.stack()[1][0].f_locals
    cls = loc.get("__class__", None)
    return cls

Then tested it:

class A:
    def __init__(self):
        print(caller_class())

A()

The output was:

None
<A object at 0x0350BCE8>

However, I tweaked the test:

class A:
    def __init__(self):
        __class__
        print(caller_class())

A()

And the result was so different:

<class '__main__.A'>
<__main__.A object at 0x0350BEE0>

The only change was I added a __class__ mention in the __init__ function. Yet, the frame all of a sudden had a __class__ attribute. Why does this strange behavior occur?