Mixing python functions and predicate objects raises error

[Achimh3011]

A Callable and a predicate object implementing the same behavior should be assignable to the same variable without mypy raising an error.

• Are you reporting a bug, or opening a feature request?
  bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

https://gist.github.com/Achimh3011/262acb8bf6d1b61ee7da6c574b662bd1

• What is the actual behavior/output?

sample.py:39: error: Incompatible types in assignment (expression has type "Callable[[Path], bool]", variable has type "matching_suffix")

• What is the behavior/output you expect?

no error

• What are the versions of mypy and Python you are using?

mypy-extensions==0.4.1
typed-ast==1.1.0

• Do you see the same issue after installing mypy from Git master?

yes

• What are the mypy flags you are using? (For example --strict-optional)

none

[JelleZijlstra]

You probably need to pre-declare file_filter (e.g., with file_filter: Callable[[Path], bool]). Mypy basically goes with the first assignment otherwise.

[ilevkivskyi]

This is not a bug. matching_suffix is a narrower type than just Callable[[Path], bool] (for example you can access suffixes on it). You therefore need to either give an annotation or switch order of branches.

To clarify, this is an error from mypy's point of view:

if cond():
    x = 'hi'
else:
    x = 42

[ilevkivskyi]

Also FYI there is an existing proposal to allow this and automatically infer a union #2611

[Achimh3011]

Thx for the hint with the pre-declaration and the link to #2611.