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.