Mapping.get overload has covariant type as argument

[randolf-scholz]

typeshed/stdlib/typing.pyi

Lines 576 to 589 in 2b9f200

	class Mapping(Collection[_KT], Generic[_KT, _VT_co]):
	# TODO: We wish the key type could also be covariant, but that doesn't work,
	# see discussion in https://github.com/python/typing/pull/273.
	@abstractmethod
	def __getitem__(self, __key: _KT) -> _VT_co: ...
	# Mixin methods
	@overload
	def get(self, __key: _KT) -> _VT_co | None: ...
	@overload
	def get(self, __key: _KT, default: _VT_co | _T) -> _VT_co | _T: ...
	def items(self) -> ItemsView[_KT, _VT_co]: ...
	def keys(self) -> KeysView[_KT]: ...
	def values(self) -> ValuesView[_VT_co]: ...
	def __contains__(self, __o: object) -> bool: ...

Shouldn't the second overload simply be

@overload 
def get(self, __key: _KT, default: _T) -> _VT_co | _T: ... 

Why is there a covariant variable being used as a function input? Is there any case where this simplified signature would be incompatible with the current signature?

[srittau]

This was previously tried in #2810, but then reverted in #2817, which has extensive discussion about this.

[randolf-scholz]

@srittau Looking at the example given in the PR

m: Mapping[str, List[str]]
x = m.get('x', [])
reveal_type(x)  # list[<nothing>] instead of list[str]

it seems that the issue is that there is a reasonable expectation that for Mapping.get, if _T is a subtype of _VT_co, then the return value should be interpreted as _VT_co. It seems that to do this correctly, some kind of conditional @overload would be required

@overload  # if _T ≤ _VT_co
def get(self, __key: _KT, default: _T) -> _VT_co: ... 
@overload  # else
def get(self, __key: _KT, default: _T) -> _T: ... 

[srittau]

@randolf-scholz If you want to write an exploratory PR that would certainly be welcome. We have a much better test infrastructure nowadays that allows us to judge the impact of any changes.

[randolf-scholz]

@srittau I think with the current typing system it's unfortunately impossible. One would need something like TypeScript's Conditional Types