def foo(x: str | None):
    if x is None:
        x = "asdf"

    def inner():
        return x + "foo"

pyright looks for reassignments following the closure definition in the flow graph. mypy does something similar (but its implementation is a little sketchier). This heuristic seems to work fairly well in practice. Was a very common cause of user confusion and duplicate reports before mypy added support for this.