gh-144386: Update equivalent code for "with", "async with" and "async for"

[serhiy-storchaka]

They use special method lookup for special methods.

• Issue: Behavioral differences between the with-statement and ExitStack.enter_context #144386

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📚 Documentation preview 📚: https://cpython-previews--144472.org.readthedocs.build/

[tanloong]

I happened to notice that there might be a small point to adjust. The updated equivalent code manager.__enter__ could look up __enter__ in the instance attributes, but the with-statement does not.

import sys
class CM:
    def __init__(self):
        def __enter__():
            pass
        def __exit__(*exc_details):
            pass
        self.__enter__ = __enter__
        self.__exit__ = __exit__


# Not works: TypeError: 'CM' object does not support the context manager protocol (missed __exit__ method)
with CM():
    pass

# Works
manager = CM()
enter = manager.__enter__
exit_ = manager.__exit__
value = enter()
hit_except = False
try:
    TARGET = value
except:
    hit_except = True
    if not exit(*sys.exc_info()):
        raise
finally:
    if not hit_except:
        exit_(None, None, None)

[serhiy-storchaka]

Yes, this is what the added note for.

[tanloong]

Oops, I missed that.