False-negative when instantiating a class before its constructor is defined

[ffigiel]

I've found a small bug:

class Operation:

    @classmethod
    def randomize(cls, config: Config) -> 'Operation':
        return cls(config)

    def __init__(self, config: Config) -> None:
        self.config = config

This complains at line 5, Too many arugments for "Operation". Swapping these two methods around (init before randomize) fixed the issue.

Looks like mypy tried to type-check cls(config) before learning what Operation's constructor looks like.

[gvanrossum]

Oh, interesting. This could be dealt with by deferring the first method when the class signature isn't known yet. But we'd need some kind of flag to tell us whether we've processed __init__ yet (and maybe if there isn't one it should be set to True at the start, by the semantic analyzer).

Do you feel up to trying to submit a PR for this? No pressure!

[ffigiel]

I could work on that, any tips what module/function to begin from?

[gvanrossum]

Best place to get help would be https://gitter.im/python/mypy.

[JukkaL]

Bumping priority because this seem to happen with some frequency and the workaround is not easy to figure out.

[JelleZijlstra]

This is a dupe of #1727, which is older and has more discussion; going to close this one and make the other one high-pri.

[mckaysalisbury]

It seems as if any method can call any method, so a two-pass system might work best. Once to process all of the function signatures, then another to process the code.

[gvanrossum]

Thanks for the advice.