False positive and negative with lambda narrowing

[md2468]

Bug Report

Type narrowing does not work correctly in function bodies. Even if the type is narrowed down in the context in which a function or a lambda is defined, that narrowing seems to be 'forgotten' within the function body.

To Reproduce

x: int | None = 1

# Either of these two type checks produces identical results:
if isinstance(x, int):
# if x is not None:

    # Revealed type is "builtins.int"
    reveal_type(x)
    
    def f() -> int: 
         # Revealed type is "Union[builtins.int, None]"
        reveal_type(x)
        
        # error: Unsupported operand types for + ("None" and "int")  [operator]
        # note: Left operand is of type "int | None"
        # return x + 2 
        
        # No errors here:
        return 1 + 2
    
    # error: Unsupported operand types for + ("None" and "int")  [operator]
    # note: Left operand is of type "int | None"
    # (lambda: x + 2)()
    
    # No errors here:
    (lambda: 1 + 2)()
    
    # However, defining a lambda without calling it does not cause errors:
    l = lambda: x + 2

Gist URL: https://gist.github.com/mypy-play/41141b62a73edc56b6561088adea8ebf
Playground URL: https://mypy-play.net/?mypy=latest&python=3.11&flags=strict&gist=41141b62a73edc56b6561088adea8ebf

Expected Behavior

No errors in the lines where errors are actually reported. Type narrowing should make all of these expressions valid.

Actual Behavior

The errors given in the comments.

Your Environment

• Mypy version used: 1.5.1.
• Mypy command-line flags: tested with no flags and --strict.
• Mypy configuration options from mypy.ini (and other config files): None.
• Python version used: 3.11.

[hauntsaninja]

Mypy is basically correct here, although it should probably also complain about the lambda (looks a little tricky to do). See https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions.

Note when not at top level you can't mutate locals from outside so it's sounder to have heuristics that avoid false positives: #15133 This is why if you put your entire code snippet inside a checked function, it will pass.

[md2468]

Thank you very much for the explanation, that was a pretty bad oversight on my part.

[erictraut]

If you put your entire code snippet inside a checked function, it will pass.

I agree that it should pass in this case, but it apparently does not. The following code still results in a type error for the lambda. This seems like a mypy bug.

def func(x: int | None) -> None:
    if isinstance(x, int):

        def f() -> int:
            return x + 2 # No Error

        (lambda: x + 2)() # Error

[hauntsaninja]

OP's example does work when put in a function as-is. The difference seems to be with immediately evaluated lambdas:

def func(x: int | None = 1) -> None:
    if isinstance(x, int):

        def f() -> int: 
            return x + 2

        l = lambda: x + 2
        l()

[hauntsaninja]

Reopened and retitled the issue. The false positive is Eric's case, the false negative is for the lambda in:

x: int | None = 1
if isinstance(x, int):
    def f() -> int: 
        return x + 2
    ff = lambda: x + 2
x = None

[hauntsaninja]

Both should be fixed by #16407