Repository navigation
False negative when using the power (**) operator #7765
Description
When checking the following function with mypy 0.730 (Python version 3.7.4):
def foo(x: int) -> str:
return 2 ** xmypy reports no errors:
Success: no issues found in 1 source file
With the --strict option turned on, mypy reports:
foo.py:2: error: Returning Any from function declared to return "str"
The expected output would be something like
foo.py:2: error: Incompatible return value type (got "int", expected "str")
Activity
srittau commentedon Oct 21, 2019
This is an issue with typeshed due to a limitation in the type system.
foocan return either anintor afloat, depending on whetherxis a positive or negative number. While we could change typeshed in a way that2 ** xwill always returnfloat(which is correct, since for typing purposesintis a subtype iffloat), this would be rather annoying for users that know that it will return anint, for example because they know thatxis always non-negative. Therefore we returnAny, because there is no way to properly express this.We would need a range type to be able to express this.
nikola-benes commentedon Oct 21, 2019
Wouldn't
Union[int, float]be a better option instead ofAny? The users can then assert that the result is anintif they are sure thatxis non-negative.nikola-benes commentedon Oct 21, 2019
Also, a related problem is that
fails with
mypy --strict. What is the recomended way to deal with this? Explicit type conversion (float(2 ** x))?msullivan commentedon Oct 21, 2019
We try to avoid that sort of union return in typeshed stubs because they produce a pretty bad user experience.
msullivan commentedon Oct 21, 2019
Yeah, probably.
--disallow-return-anymaybe shouldn't be in --strict? See #7767 which I just openedFix mypy Any return
Fix mypy Any return