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Sum of an iterable of bools is considered to have a bool type #7579
Description
Consider the following code
foo = sum([True, False])
foo += 1mypyv0.931 is OK with it and reportsfootype asintmypyv0.940 warns
Incompatible types in assignment (expression has type "int", variable has type "Union[bool, Literal[0]]")
The stub does indeed state
@overload
def sum(__iterable: Iterable[_SumT]) -> _SumT | Literal[0]: ...Though it is somewhat unfortunate. I've got beaten by this after updating mypy to the latest version.
Activity
JelleZijlstra commentedon Apr 2, 2022
See #7578 where we recently experimented with ways to address this sort of thing.
ktbarrett commentedon Apr 2, 2022
@JelleZijlstra I'm not sure how that's relevant. The problem is that
bool + boolisintnotbool. I think the only way to fix this is to add an overload forboolto returnint.JelleZijlstra commentedon Apr 2, 2022
@ktbarrett it's relevant because currently we assume that
sum(Iterable[T])returnsT. We tried an alternative where we madesumreturn the return type of__add__instead, but backed it out because it led to false positives.Type checkers already understand that
bool + boolreturns an int.ktbarrett commentedon Apr 2, 2022
Ah, I see the middle commit now. No one seemed to comment on the failure.
jpy-git commentedon Apr 2, 2022
I think the reason this was passing in mypy 0.931 was that the return used to be typed as
_T | intand has since been changed to_T | Literal[0](the typevar change in #7578 won't have made it into mypy yet)https://github.com/python/mypy/blob/4486d764c07a1cee9792d691e8e85cedfed2a451/mypy/typeshed/stdlib/builtins.pyi#L1389-L1393
That
intin the return type would have been allowingfoo += 1but now it's been changed toLiteral[0](since I think it's intended to represent the defaultstartvalue).You can see an example here: https://mypy-play.net/?mypy=0.941&python=3.10&flags=strict&gist=4b96aaf8e1261db4bf484c0f76138638
pyrightdoesn't raise this error on the newer stubs so maybe mypy needs to be updated to account for this as well?Alternatively, if the
Literal[0]isn't providing any significant benefit, maybe it would just be simplest to revertLiteral[0]toint?AlexWaygood commentedon Apr 2, 2022
Not quite.
Literal[0]is there to represent the special case of the empty iterable being passed tosum.jpy-git commentedon Apr 2, 2022
yes because if you have an empty iterable you never add anything to the start value which is by default 0 😄
AlexWaygood commentedon Apr 2, 2022
Sorry, looks like we were saying the same thing in different ways there 😄
AlexWaygood commentedon Apr 2, 2022
Yeah, there's some unfortunate disagreement among type checkers at the moment on how to treat literal types 😕 see #7258 for an extended discussion
But while the immediate cause of the new error might have been the change from
inttoLiteral[0], I don't think that's really the underlying problem here.AlexWaygood commentedon May 28, 2022
The specific case of
bool + bool -> inthas been fixed in #7975. The general case can't be solved without introducing a significant number of false-positive errors, as we saw when experimenting in #7578. As such, I'm closing this issue as as-fixed-as-it-can-be-for-now.