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