Steve, the PR that you pushed has the wrong error and error message. I told Philippe in msg377335 that ctypes raises FileNotFoundError with no error code. For example:

    >>> try: ctypes.CDLL('spam')
    ... except OSError as e: err = e
    ...
    >>> err
    FileNotFoundError("Could not find module 'spam' (or one of its dependencies). Try using the full path with constructor syntax.")
    >>> err.winerror is None
    True

The advice to use dumpbin is fine and works well in simple cases. I wouldn't use it generally since recursiveley parsing through the dependency graph of every dependent DLL could be tedious.