mypy throws error using min with a key function specified

[timabbott]

Reproducer: min([1, 2], key = lambda x: -x)

[JukkaL]

The stub handles this case, but mypy complains because it doesn't consider keyword argument names when picking an overload variant. This is a vaguely known issue that has been around for a long time, but I can't find a duplicate tracker issue.

[timabbott]

Looks like this also affects sorted; reproducer:
sorted([{'name': 'test'}], key=lambda x: x['name'])

[gvanrossum]

Tim: I don't believe sorted() has the same problem -- it's not overloaded. Can you file that (with the error you get) as a separate issue?

[gvanrossum]

The PY3 overloads for min() currently look like this:

@overload
def min(arg1: _T, arg2: _T, *args: _T) -> _T: ...
@overload
def min(iterable: Iterable[_T], key: Callable[[_T], Any] = None) -> _T: ...

(max() and PY2 are similar.)
I can make all cases work (I think) by swapping the two and adding the key: to the one that's currently missing it, giving this:

@overload
def min(iterable: Iterable[_T], key: Callable[[_T], Any] = None) -> _T: ...
@overload
def min(arg1: _T, arg2: _T, *args: _T, key: Callable[[_T], Any] = None) -> _T: ...

My tests are as follows:

assert min(1, 2) == 1
assert min(1, 2, key=lambda x: -x) == 2
assert min([1, 2]) == 1
assert min([1, 2], key=lambda x: -x) == 2

Jukka, are you okay if I commit that to typeshed?

[gvanrossum]

Jukka, can you look at python/typeshed#58 ? It fixes the issue for me.

[timabbott]

Ok created #1160 for the sorted issue

[gvanrossum]

Closed by 6a47ac7.

[gvanrossum]

Sorry, I'm going to have to revert the fix, I think Jukka's right and this requires a fix to mypy.
What broke with the "fix" was max(a, 10) where the type of a is Any.

[JukkaL]

I'm looking at fixing overload resolution.