raise AttributeError if loading fails in ctypes.LibraryLoader.__getattr__

[lfriedri]

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

[lfriedri]

The following creates an OSError:

import ctypes
hasattr(ctypes.windll, 'test')

The expected behavior would be to return "False"

[eryksun]

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.

[lfriedri]

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]

__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.

[farfella]

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

[hauntsaninja]

Thanks, looks like this was fixed!

[kevinhendricks]

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?