raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__

@eryksun

BPO	34816
Nosy	@eryksun, @lfriedri, @farfella
PRs	gh-78997: AttributeError if loading fails in LibraryLoader.__getattr__ #25177
Files	34816.patch

^{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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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

gh-78997: fix bad rebase of moved test file #100424

The 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')
    False

FYI, 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.

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?

__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.

First patch fixing only the issue at hand on master. LibraryLoader now catches OSError for FileNotFoundError and raises AttributeError.

gh-78997: AttributeError if loading fails in LibraryLoader.__getattr__

Merge branch 'main' into fastmock

gh-78997: fix bad rebase of moved test file (#100424)

Merged main into regmachine branch (#50)

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).

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raise AttributeError if loading fails in ctypes.LibraryLoader.getattr #78997

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raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__ #78997

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lfriedri commented on Sep 27, 2018

eryksun commented on Sep 27, 2018

lfriedri commented on Sep 27, 2018

eryksun commented on Mar 28, 2021

farfella commented on Apr 4, 2021

hauntsaninja commented on Jan 29, 2023

kevinhendricks commented on Feb 28, 2024

fzimmermann89 commented on Jun 25, 2025

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raise AttributeError if loading fails in ctypes.LibraryLoader.getattr #78997