PEP 649: class __annotate__ is shadowed by compiler-generated one

[luigig44]

Bug report

Bug description:

I'm playing around with PEP 649, PEP 749 annotations and I found this surprising behaviour:

class C:
    x: str
    def __annotate__(format):
        if format != 1:
            raise NotImplementedError()
        return {'x': int}

assert C.__annotate__(1) == {'x': int} # This returns {'x': str} and fails

I'm not yet sure why I would write code with annotations and then also write an explicit __annotate__, but I would expect the explicit method to prevail over the compiler-generated one. If I explicitly set C.__annotate__ = some_func afterwards it works as I would expect.

Calling dis on the provided example shows both __annotate__s getting compiled, and one overwriting the other in the class.

Related to #119180.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-133037: Add test for shadowing __annotate__ #133084

[sobolevn]

See related #122285

[JelleZijlstra]

This behavior is pretty inconsistent for other attributes set in the class namespace:

>>> class X:
...     __name__ = 42
...     __module__ = 42
...     __qualname__ = "42"
...     __firstlineno__ = 42
...     __static_attributes__ = 42
...     
>>> X.__name__
'X'
>>> X.__module__
42
>>> X.__qualname__
'42'
>>> X.__firstlineno__
42
>>> X.__static_attributes__
()

Still, it's probably nicer to make this work the way this issue asks for. This can be fixed with the approach in #131550 if we do the same thing in class namespaces as we already do there for modules (assign __annotate__ early in class body execution). I'd wait for that one to be merged first and then apply the same change to class bodies.

[JelleZijlstra]

This was actually already fixed by #132345, because now the compiler-generated __annotate__ function gets stored at a different key in the class dict. I will send a PR with a test case.

(You'll still get your name overwritten if you use the name __annotate_func__ in user code. Don't do that.)