Add support for dict.{keys,values,items}.mapping

[KotlinIsland]

I added _dict_keys, _dict_values, _dict_items classes to builtins.pyi and made dict return those instead of the base XView.

from: https://bugs.python.org/issue40890

[JelleZijlstra]

The SinbadCogs mypy-primer error looks concerning.

[KotlinIsland]

Hi @JelleZijlstra, I think this is an issue with mypy, not with this change:

from typing import TypeVar, Generic, ValuesView

_KT = TypeVar("_KT")
_VT = TypeVar("_VT")
_KT_co = TypeVar("_KT_co", covariant=True)  # Key type covariant containers.
_VT_co = TypeVar("_VT_co", covariant=True)  # Value type covariant containers.

class AValues(ValuesView[_VT_co], Generic[_KT_co, _VT_co]):
    pass

class A(dict[_KT, _VT], Generic[_KT, _VT]):
    def c(self) -> AValues[_KT, _VT]:
        return AValues(self)

a = A({1: [1]})
l = [*a.c()]
print(l)  # [[1]]
reveal_type(l)  # note: Revealed type is "builtins.list[builtins.int*]"

from typing import TypeVar, Generic, ValuesView

_KT = TypeVar("_KT")
_VT = TypeVar("_VT")
_KT_co = TypeVar("_KT_co", covariant=True)  # Key type covariant containers.
_VT_co = TypeVar("_VT_co", covariant=True)  # Value type covariant containers.

class AValues(ValuesView[_VT_co], Generic[_VT_co]):
    pass

class A(dict[_KT, _VT], Generic[_KT, _VT]):
    def c(self) -> AValues[_VT]:
        return AValues(self)

a = A({1: [1]})
l = [*a.c()]
print(l)  # [[1]]
reveal_type([*a.c()])  # note: Revealed type is "builtins.list[builtins.list*[builtins.int*]]"

[DetachHead]

python/mypy#11138

[JelleZijlstra]

Mypy does seem buggy there, but I think you can work around the bug by swapping the type arguments to _dict_values (make it Generic[_VT_co, _KT_co]). That's a bit confusing for people who want to instantiate a _dict_values, but that should be rare so it's probably fine.

[KotlinIsland]

Hi @JelleZijlstra, just a small note: are the generics on _dict_items redundant?

class _dict_items(ItemsView[_KT_co, _VT_co], Generic[_KT_co, _VT_co]):

# could be:
class _dict_items(ItemsView[_KT_co, _VT_co]):

[KotlinIsland]

Should the generics here be covariant? the base types are.

[Akuli]

That makes sense: they are immutable, so a _dict_items[int, str] should also be a valid _dict_items[object, object].

Nit: Please do not force-push or git commit --amend, so that we can see what changed after your previous review. We will squash the commits when merging, so the whole PR will appear as a single commit in the Git history anyway.

[JelleZijlstra]

And yes, the generics on _dict_items are redundant. I don't have a strong opinion on whether to keep them. Maybe it's nice to have them explicit because we're doing some unusual shenanigans with its sister class's Generic parameters.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

aioredis (https://github.com/aio-libs/aioredis.git)
- aioredis/client.py:3401: error: Incompatible types in assignment (expression has type "KeysView[Any]", variable has type "Union[Sequence[Union[bytes, str, memoryview]], Mapping[bytes, float]]")
+ aioredis/client.py:3401: error: Incompatible types in assignment (expression has type "_dict_keys[Any, Any]", variable has type "Union[Sequence[Union[bytes, str, memoryview]], Mapping[bytes, float]]")
- aioredis/client.py:3401: error: Incompatible types in assignment (expression has type "KeysView[Any]", variable has type "Union[Sequence[Union[bytes, str, memoryview]], Mapping[str, float]]")
+ aioredis/client.py:3401: error: Incompatible types in assignment (expression has type "_dict_keys[Any, Any]", variable has type "Union[Sequence[Union[bytes, str, memoryview]], Mapping[str, float]]")
- aioredis/client.py:3401: error: Incompatible types in assignment (expression has type "KeysView[Any]", variable has type "Union[Sequence[Union[bytes, str, memoryview]], Mapping[memoryview, float]]")
+ aioredis/client.py:3401: error: Incompatible types in assignment (expression has type "_dict_keys[Any, Any]", variable has type "Union[Sequence[Union[bytes, str, memoryview]], Mapping[memoryview, float]]")
- aioredis/client.py:3403: error: Incompatible types in assignment (expression has type "None", variable has type "ValuesView[Any]")
+ aioredis/client.py:3403: error: Incompatible types in assignment (expression has type "None", variable has type "_dict_values[Any, Any]")

[KotlinIsland]

I think this is ready to pull.