with-syntax makes mypy confuse int with Any

[Hi-Angel]

Bug Report

When an object is declared with with syntax, and then a method of that object is called, mypy thinks that method returns Any, even if the method is properly annotated to return something else. Removing with and declaring the object explicitly makes it work.

To Reproduce (in terms of terminal commands)

 λ cat test.py
class Connection:
    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        pass

    def register_method(self) -> int:
        return 1

def foo() -> int:
    with Connection() as conn:
        return conn.register_method()
 λ mypy test.py --warn-return-any
test.py:13: error: Returning Any from function declared to return "int"
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

mypy would return no errors.

Actual Behavior

mypy returned errors.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: --warn-return-any
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.6
• Operating system and version: Archlinux

[hauntsaninja]

You need to annotate __enter__ for mypy to know what type conn is. If you do that, things work. --warn-return-any is a little finicky, and I don't recommend using it without --strict / --disallow-untyped-defs

[Hi-Angel]

You need to annotate __enter__ for mypy to know what type conn is.

Thanks! Could you please elaborate here, I don't see any way to annotate __enter__. I mean, I can do this:

[…]
class Connection:
    def __enter__(self) -> Connection:
        return self
[…]

…but it makes python complain: NameError: name 'Connection' is not defined

[hauntsaninja]

from __future__ import annotations if on Python 3.7 or better, or -> "Connection" otherwise.

[Hi-Angel]

Oh, cool, that does the trick, thank you!