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raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__ #78997Description
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2018-09-27.07:14:17.884>
labels = ['easy', 'type-bug', '3.8', '3.9', '3.10', 'ctypes']
title = 'raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__'
updated_at = <Date 2021-04-04.02:29:42.566>
user = 'https://github.com/lfriedri' bugs.python.org fields:
activity = <Date 2021-04-04.02:29:42.566>
actor = 'ateeq'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2018-09-27.07:14:17.884>
creator = 'lfriedri'
dependencies = []
files = ['49932']
hgrepos = []
issue_num = 34816
keywords = ['patch', 'easy']
message_count = 5.0
messages = ['326528', '326556', '326557', '389629', '390168']
nosy_count = 3.0
nosy_names = ['eryksun', 'lfriedri', 'ateeq']
pr_nums = ['25177']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34816'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10'] Linked PRs
Activitylfriedri commented on Sep 27, 2018 MannequinAuthor More actions The following creates an OSError:
import ctypes
hasattr(ctypes.windll, 'test') The expected behavior would be to return "False" eryksun commented on Sep 27, 2018 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 AttributeErrorDemo:
>>> 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 MannequinAuthor More actions Thank 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? eryksun commented on Mar 28, 2021
__getattr__ method of LibraryLoader catches the OSError and
raises an AttributeError
Yes. 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, 2021farfella commented on Apr 4, 2021 First patch fixing only the issue at hand on master. LibraryLoader now catches OSError for FileNotFoundError and raises AttributeError. added a commit that references this issue on Dec 15, 2022gh-78997: AttributeError if loading fails in LibraryLoader.__getattr__ hauntsaninja commented on Jan 29, 2023 Thanks, looks like this was fixed! kevinhendricks commented on Feb 28, 2024 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? fzimmermann89 commented on Jun 25, 2025 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).
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