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.

[quanghuynh10111-png]

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)

Originally posted by @tanloong in python/cpython#144472 (comment)

Originally posted by @quanghuynh10111-png in python/cpython#144671