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