[issue47136] The variable __module__ in the class body getting an undesirable value from __prepare__ of the metaclass

[Steven D'Aprano]

Steven D'Aprano <steve at pearwood.info> added the comment:

It would be nice if the class creation process was documented a bit 
better. Here is my experiment to see what is going on:

```
# the global `__name__` is normally the module's name.
__name__ = "something"

class Meta_default_prepare(type):
    def __new__(meta, name, bases, ns):
        print("ns for", name, "\n   ", ns)
        return super().__new__(meta, name, bases, ns)

class Meta_custom_prepare(Meta_default_prepare):
    def __prepare__(meta, *args):
        return {'__name__': 'another_name'}

class Spam(metaclass=Meta_default_prepare):
    pass

class Eggs(metaclass=Meta_custom_prepare):
    pass

print("Spam module and name:", Spam.__module__, Spam.__name__)
print("Eggs module and name:", Eggs.__module__, Eggs.__name__)

```

And the output in Python 3.10 is:

```
ns for Spam 
    {'__module__': 'something', '__qualname__': 'Spam'}
ns for Eggs 
    {'__name__': 'another_name', '__module__': 'another_name', '__qualname__': 'Eggs'}
Spam module and name: something Spam
Eggs module and name: another_name Eggs
```

My take on this is that if the key __name__ is not present, the value of 
the class __module__ is taken from the global variable. So far so good.

But if '__name__' is a key in the mapping returned by __prepare__, it 
gets left in the class dict, and gets used to set the class __module__ 
as well.

But in neither case does it get used to set the class __name__.

I cannot decide whether or not this makes sense to me.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue47136>
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