8000 Different behavior for Optional with if condition `not Var` and `Var is not None` · Issue #7696 · python/mypy · GitHub
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actionless opened this issue Oct 11, 2019 · 2 comments
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Different behavior for Optional with if condition not Var and Var is not None #7696

actionless opened this issue Oct 11, 2019 · 2 comments
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false-positive mypy gave an error on correct code needs discussion topic-type-variables

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@actionless
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  • Are you reporting a bug, or opening a feature request?
    Bug
  • Please insert below the code you are checking with mypy,
    or a mock-up repro if the source is private. We would appreciate
    if you try to simplify your case to a minimal repro.
from typing import List, Optional, Any

class MyClass1:

    _result: Optional[List[str]] = None
    _foo: Any = None

    @classmethod
    def get(cls, some_arg: Optional[str]) -> List[str]:
        if cls._result is None:
            cls._result = [some_arg or 'a'] + (
                cls._foo.split(',') if cls._foo else []
            )
        return cls._result

class MyClass2:

    _result: Optional[List[str]] = None
    _foo: Any = None

    @classmethod
    def get(cls, some_arg: Optional[str]) -> List[str]:
        if not cls._result:
            cls._result = [some_arg or 'a'] + (
                cls._foo.split(',') if cls._foo else []
            )
        return cls._result
#       ^ [mypy] : Incompatible return value type (got "Optional[List[str]]", expected "List[str]")

  • What is the actual behavior/output?
    mypy_func.py:44: error: Incompatible return value type (got "Optional[List[str]]", expected "List[str]")
    Found 1 error in 1 file (checked 1 source file)

  • What are the versions of mypy and Python you are using?
    Do you see the same issue after installing mypy from Git master?

$ python -m mypy --version
mypy 0.740+dev.742d33a41271206d6eb7dd2e4b04659de4683597

$ python --version
Python 3.7.4
@actionless actionless mentioned this issue Oct 11, 2019
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@JukkaL
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JukkaL commented Oct 11, 2019

Yeah, this looks inconsistent. Here's a bit shorter example:

from typing import Optional, List, Any

a: Any

def f(x: Optional[List[int]]) -> None:
    if not x:
        x = a
    reveal_type(x)  # Optional[List[int]]

def g(x: Optional[List[int]]) -> None:
    if x is None:
        x = a
    reveal_type(x)  # Union[List[int], Any]

def h(x: Optional[List[int]]) -> None:
    x = a
    reveal_type(x)  # Optional[List[int]]

Maybe assigning Any to a variable with type Optional[X] should bind the type to Union[X, Any].

@JukkaL JukkaL added false-positive mypy gave an error on correct code needs discussion labels Oct 11, 2019
@github-actions github-actions bot mentioned this issue Aug 10, 2024
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@ilevkivskyi
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Mypy now consistently narrows to Union[X, Any] in all cases.

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@JukkaL @actionless @ilevkivskyi @AlexWaygood
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