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raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__ #78997
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lfriedri commented
on Sep 27, 2018 lfriedrimannequinMannequinAuthorThe following creates an OSError:
import ctypes hasattr(ctypes.windll, 'test')
The expected behavior would be to return "False"
ctypes.windll is an instance of ctypes.LibraryLoader, which has a __getattr__ method that calls ctypes.WinDLL(name) and caches the result as an instance attribute. I suppose with chained exceptions it's reasonable to handle OSError in __getattr__ by raising AttributeError. For example:
class A: def __init__(self, name): raise OSError class B: def __getattr__(self, name): try: A(name) except OSError: raise AttributeError
Demo:
>>> b = B() >>> b.test Traceback (most recent call last): File "<stdin>", line 4, in __getattr__ File "<stdin>", line 3, in __init__ OSError During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 6, in __getattr__ AttributeError >>> hasattr(b, 'test') FalseFYI, I recommend avoiding the cdll and windll LibraryLoader instances. I wish they were deprecated because globally caching CDLL and WinDLL instances leads to conflicts between projects that use the same shared libraries.
lfriedri commented
on Sep 27, 2018 lfriedrimannequinMannequinAuthorThank you for your reply.
I am not sure if I understood correctly:
Do you suggest to modify ctypes.__init__.py so that the __getattr__ method of LibraryLoader catches the OSError and raises an AttributeError instead, as in your example?__getattr__ method of LibraryLoader catches the OSError and
raises an AttributeErrorYes. It seems no one was keen to work on this. I think it's relatively easy, so I'll add that flag in case someone is looking for an easy issue.
- changed the title
[-]ctypes + hasattr[/-][+]raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__[/+]on Mar 28, 2021 farfella commented
on Apr 4, 2021 farfellamannequinMannequinFirst patch fixing only the issue at hand on master. LibraryLoader now catches OSError for FileNotFoundError and raises AttributeError.
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on Dec 28, 2022 Thanks, looks like this was fixed!
For the record, why was a backwards incompatible change made in just a point release without any deprecation warnings? We can work around this change from throwing OSError to AttrbteError in cdll but why on earth was this change added in a minor release?
This change was really communicated badly in the windows section of the changelog:
[bpo-34816](https://bugs.python.org/issue?@action=redirect&bpo=34816): hasattr(ctypes.windll, 'nonexistant') now returns False instead of raising [OSError](https://docs.python.org/3/library/exceptions.html#OSError).
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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