Mypy could not do condition inference

Bug Report

To Reproduce

The min sample:

class MyClass:
    a_property: int = 1

    def do_something (
        self,
        source: Optional['MyClass'] = None
    ):
        is_not_none = source is not None

        if is_not_none:
            # My ass even knows that `source` must be a `MyClass` instance here.
            self.a_property = source.a_property

Expected Behavior

It should pass the test

Actual Behavior

Item "None" of "MyClass | None" has no attribute "a_property"  [union-attr]

The only way to avoid this error output is to replace each is_not_none with source is not None. It makes the code silly sometimes, and forces me to revamp the logic just for mypy which is a totally waste of my time.

Your Environment

Mypy version used: 1.15.0
Mypy command-line flags: mypy foo.py
Mypy configuration options from mypy.ini (and other config files): n/a
Python version used: 3.12.7

Hello, I want to solve this problem. Can I?

PRs are welcome but this is likely a complicated feature to add.

Also this issue is probably a duplicate but I haven't looked.

I think this is a duplicate of #9229.

This feature is formally called "aliased conditional expressions". The first time I saw it implemented was in the TypeScript compiler in 2021. I later implemented it in pyright.

As Jelle said, it's tricky to get this feature right because a static analyzer must prove that there is no statement that can invalidate the condition on any code path between the assignment of the condition variable and its later use for narrowing.

Consider the following three examples:

def func1(x: int | None):
    is_int = x is not None

    for _ in range(2):
        if is_int: # Safe to narrow x to `int`
            reveal_type(x) # int

def func2(x: int | None):
    is_int = x is not None

    for _ in range(2):
        if is_int: # Not safe to narrow x to `int`
            reveal_type(x) # int | None
        x = None

def func3(x: int | None):
    is_int = x is not None

    for _ in range(2):
        if is_int: # Not safe to narrow x to `int`
            reveal_type(x) # int | None
        is_int = True

Thanks

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Mypy could not do condition inference #19029

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Mypy could not do condition inference #19029

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USEONGEE commented on May 5, 2025

JelleZijlstra commented on May 5, 2025

erictraut commented on May 5, 2025

USEONGEE commented on May 5, 2025

kaelzhang commented on May 6, 2025

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