Unneeded type widening of inner function call when using explicit return type

[knbk]

Using an explicit return type unnecessarily widens the types in filter():

def return_a(strings: list[str]) -> Optional[str]:
 # Item "None" of "Optional[str]" has no attribute "strip"
 return next(filter(lambda s: s.strip() == "a", strings), None)

and a comparable error when the filter function is explicitly typed:

def matches_a(s: str) -> bool:
 return s.strip() == "a"

def return_a(strings: list[str]) -> Optional[str]:
 # Argument 1 to "filter" has incompatible type "Callable[[str], Any]"; expected "Callable[[Optional[str]], Any]"
 return next(filter(matches_a, strings), None)

No error occurs when the explicit return type is removed:

# return_a is inferred to return str | None
def return_a(strings: list[str]):
 # No error
 return next(filter(lambda s: s.strip() == "a", strings), None)

Mypy 0.931, Python 3.8.6.

[hauntsaninja]

Note that mypy doesn't infer return types, so in your last example, mypy is actually treating the return type of return_a as Any.

But the issue is true in general, it's a somewhat commonly reported problem with mypy's type context, e.g. the following type checks:

def return_a(strings: list[str]) -> Optional[str]:
 ret = next(filter(lambda s: s.strip() == "a", strings), None)
 return ret