Inconsistent error when aliasing a type from a module without stubs

[frankpf]

test_mypy.py:

import pymongo # this can be any third-party module without stubs

Collection = pymongo.collection.Collection

def func(a: Collection) -> bool:
   return True

Here's the output of mypy test_mypy.py :

test_mypy.py:1: error: No library stub file for module 'pymongo'
test_mypy.py:1: note: (Stub files are from https://github.com/python/typeshed)
test_mypy.py: note: In function "func":
test_mypy.py:5: error: Invalid type "test_mypy.Collection"

However, if I change test_mypy.py to this:

import pymongo

def func(a: pymongo.collection.Collection) -> bool:
  return True

then mypy does not complain about pymongo.collection.Collection being an invalid type. This seems inconsistent/wrong, Collection is not an invalid type, it's just an alias for a type without a stub.

[frankpf]

This seems similar to https://github.com/python/mypy/issues/2075.

[gvanrossum]

Looks like it doesn't treat Collection = pymongo.collection.Collection as a type alias definition? In the second example, pymongo is an object of type Any and attributes thereof are also treated as Any.

Agreed this should be dealt with better.

[JukkaL]

I've hit this issue as well. It's not clear if Collection should be treated as a type alias, since it could also be an ordinary variable. Mypy should perhaps behave as if there was an explicit Any annotation. This doesn't generate an error:

...
Collection: Any = pymongo.collection.Collection  # Note type annotation!

def func(a: Collection) -> bool:  # OK
   return True