False positive for lambda within a function

[michaelbrewer]

Related Bug

Could be a duplicate of:

• Narrowed down types sometimes not propagated to lambda #4297

Bug Report

When running mypy with a function returning an Optional the contains of a lambda expression is interpreted as optional and a type error is raised

To Reproduce

Run mypy against the following file:

from typing import Optional

animals = ["cow"]


def find1(name: str) -> Optional[str]:
 return next(
 filter(
 lambda animal: animal.upper() == name,
 animals,
 ),
 None,
 )

Expected Behavior

No type errors should be returned

Actual Behavior

$ mypy bug.py
bug.py:9: error: Item "None" of "Optional[str]" has no attribute "upper"

NOTE: Same error with returning "Union[str, None]"
NOTE: Removing the Optional[str] stops the error, but that defaults the point

def find1(name: str):
 return next(
 filter(
 lambda animal: animal.upper() == name,
 animals,
 ),
 None,
 )

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9.4
• Operating system and version: MacOS 10.15.7

[hauntsaninja]

From the title I thought it was going to be a dupe of #2608, but it's not!

The dumb workaround is:

from typing import Optional

animals = ["cow"]


def find1(name: str) -> Optional[str]:
 ret = next(
 filter(
 lambda animal: animal.upper() == name,
 animals,
 ),
 None,
 )
 return ret

I think this boils down to the fact that mypy does complicated things to infer types for lambdas. If there's an annotation, it takes into account what type the expression is expected to be to try and understand the lambda. E.g., if you were to change ret = next(...) to ret: Optional[str] = next(...) you'd also get an error.

[michaelbrewer]

Thanks @hauntsaninja , my lint tools would have suggested to immediately return :). So i guess it is a coin flip.