Cannot infer type argument when mixing int and float

[JukkaL]

Mypy complains about this code, even though it looks fine to me:

from typing import Sequence, TypeVar

_T = TypeVar('_T')

def bisect_left(a: Sequence[_T], x: _T) -> int: ...

bisect_left([1, 2, 3], 5.391)  #  Cannot infer type argument 1 of "bisect_left"

This doesn't seem to be related to promotions, as it also happens with user-defined types:

class A: pass
class B(A): pass

bisect_left([B()], A())   # same error

[matthiaskramm]

Just out of curiosity: What is mypy supposed to do, when having

def f(x: _T, y: _T)

and a call

f(A(), B())

Should it try to "promote" either A to B or B to A? Or should it try to find a common base class between the two?

Or would the magic that makes bisect_left work not actually apply for the non-sequence case?

[JukkaL]

It looks for a value for _T that is compatible for all arguments. There are some additional rules, such as that we generally don't infer Any types, even though they would be valid everywhere, and we favor narrower types as results. It tries to look for common base classes and it also considers promotions such as int -> float. In your example, if B is a subclass of A, so we'd infer _T = A. object would also be a valid result, but since A is narrower we prefer that.

The non-sequence case should work already -- it's the sequence case that is broken.

[graingert]

any way of specifying type arguments to a function?

class A: pass
class B(A): pass

bisect_left[A]([B()], A())   # same error

[JukkaL]

Not directly, but you can use cast within the arguments to make all argument types match exactly, so it's obvious what the value of a type argument will be. Example:

from typing import cast

class A: pass
class B(A): pass

bisect_left([cast(A, B())], A())  # ok

[graingert]

@JukkaL here's an issue about what I mean #3096

[JukkaL]

Increasing priority since there was a user question that boils down to this issue and the fix seems simple.