Exponential operator results in typing.Any type

[isaacto]

Bug Report

A regression: in the current version of mypy, float exponential results in typing.Any. For int exponential it works, only float exponential fails.

(A clear and concise description of what the bug is.)

To Reproduce

$ cat test.py
reveal_type(2.0 ** 1.0)

Expected Behavior

It should say that the revealed type is builtins.float when run.

Actual Behavior

$ mypy test.py
test.py:1: note: Revealed type is "Any"

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: Nothing special.
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9.5
• Operating system and version: Ubuntu 20.04.3

[JelleZijlstra]

You're hitting this case: https://github.com/python/typeshed/blob/98afaa4c76d4279846f278eb5be8b99c391e6a99/stdlib/builtins.pyi#L1348

This returns Any because it could be a complex.

[isaacto]

So why (-2) ** 0.5 should be of type float? And if this is the reason, why it can't simply say the return type is complex instead (which would easily tell the user what are the considerations)?

Personally I think the best user experience is for it to be simply a float, but it is just a preference.

[JelleZijlstra]

So why (-2) ** 0.5 should be of type float?

That does look like a mypy bug (it should be complex but mypy thinks it returns float). I'll report it over at typeshed.

why it can't simply say the return type is complex

A lot of users who expect floats will be very confused. Our general policy so far has been to return Any in cases like this.