IDE confusion around functions calling each other?

[zsiegel92]

If modal function fn_a calls modal function fn_b, and fn_b also calls fn_a, pyright gets confused!

The screenshot is from what I think is a MWE of this bug:

import modal


app = modal.App()


@app.function()
def fn_a(call_b: bool = True):
    if call_b:
        fn_b.remote()
    return


@app.function()
def fn_b():
    fn_a.remote(call_b=False)


@app.local_entrypoint()
def main():
    fn_a.remote()

The program above ia valid and runs fine, but my IDE is having a problem with something. The issue is cryptic - fn_a.remote appears to the IDE to be undefined - but only when both functions call each other! If the functions don't both call each other, either one can call either one with .remote with no issues.

Using VS Code, Python 3.12.

[erikbern]

Python typing expert @freider – any idea what's going on?

[freider]

Weird! Since the issue only happens if both functions call each other, I would guess this has something to do with pyright/pylance internals rather than the Modal types.

Here is my best guess: pyright for some reason tries to determine the type of each function by looking inside the function body where there is a reference to the other function, which is then evaluated etc, causing a reference "loop" between the functions, which is probably terminated by pyright by just setting the type to "function".

I'm not sure why pyright would actually need to look at the function body to determine types here though, maybe it does so just to type the body... Maybe it would help to add return type annotations (-> None) so it can determine return type directly from the signature? I don't know anything about pyright internals I'm afraid

[freider]

I just tested it, and adding the -> None return type annotation in your example seems to fix the issue! :)

[zsiegel92]

@freider annotating one of the functions as -> X did indeed fix the bug, both in the toy example and in my actual problem code.

This was very weird and pretty hard to pin down. Thanks for validating and responding so quickly! 🙏 @erikbern @freider

FWIW I only experienced this with Python 3.12, I switched my IDE to 3.11 and did not have this problem (though can't just switch my project ofc). Probably not worth leaving an issue on the Pyright repo or the Pylance repo since I can't come up with a MWE without using modal.App.function-decorated functions.