Lambda with exponentiation return any warning

[alanhdu]

Using mypy 0.660 and Python 3.7, running mypy --warn-return-any on:

output = filter(lambda x: 2**x < 6, [1, 2, 3, 4])

returns

lambda.py:1: warning: Returning Any from function declared to return "object"

This only occurs when there's the exponentiation (**) in the lambda: if I change it to a + or *.

[alanhdu]

Looking at typeshed, it looks like the line responsible for the Any is:

https://github.com/python/typeshed/blob/6f09ccb63dfce0924e34fa04644ea77b63747310/stdlib/2and3/builtins.pyi#L170-L171

    def __pow__(self, x: int) -> Any: ...  # Return type can be int or float, depending on x.
    def __rpow__(self, x: int) -> Any: ...

But I'm not sure where "return object" comes into play. Should that Any just be a float?

[JukkaL]

float return type would be technically true, but that would cause a lot of issues with code that (correctly) wants to assume that x ** y is an integer. The convention in typeshed is to use Any as the return type if the return type depends on the value (not the type) of an argument. If you want a precise type, you can create a helper function (or alias pow) and explicitly annotate it.

[jonasrauber]

This also happens for map(lambda x: x.shape, xs) (where xs is a list of numpy arrays).