I just encountered code like this:

if hasattr(x, 'initialize'):
     x.initialize()

The type of x was an ABC but it doesn't include initialize. This can be easily worked around by rewriting it like this:

if hasattr(x, 'initialize'):
     cast(Any, x).initialize()

However, mypy could do better, plausibly. For example:

• If x has a union type, infer only union item types with attribute initialize after the hasattr check. So if type of x is Union[str, X] and X has initialize, infer type of x to be X in the if body.
• Allow specifying "potentially undefined" attributes in types. Accessing these requires a hasattr check (a little like Optional[...] requiring something like an is not None check). Not sure what the syntax for this would be like. It would be nice to support these in ABCs as well.