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Description
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.
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