Add support for `dict.{keys,values,items}.mapping` by KotlinIsland · Pull Request #6039 · python/typeshed

KotlinIsland · 2021-09-17T00:11:46Z

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 · 2021-09-17T19:54:27Z

The SinbadCogs mypy-primer error looks concerning.

KotlinIsland · 2021-09-18T07:31:09Z

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 · 2021-09-19T01:51:46Z

python/mypy#11138

JelleZijlstra · 2021-09-19T03:18:58Z

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 · 2021-09-19T05:42:09Z

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 · 2021-09-19T05:44:53Z

-class _OrderedDictKeysView(KeysView[_KT], Reversible[_KT]):
+class _OrderedDictKeysView(_dict_keys[_KT, _VT], Reversible[_KT]):
    def __reversed__(self) -> Iterator[_KT]: ...

-class _OrderedDictItemsView(ItemsView[_KT, _VT], Reversible[Tuple[_KT, _VT]]):
+class _OrderedDictItemsView(_dict_items[_KT, _VT], Reversible[Tuple[_KT, _VT]]):
    def __reversed__(self) -> Iterator[Tuple[_KT, _VT]]: ...

-class _OrderedDictValuesView(ValuesView[_VT], Reversible[_VT]):
+class _OrderedDictValuesView(_dict_values[_VT, _KT], Reversible[_VT], Generic[_KT, _VT]):


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

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 · 2021-09-20T00:42:38Z

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 · 2021-09-20T01:49:09Z

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 · 2021-09-20T03:15:58Z

I think this is ready to pull.

Source commit: python/typeshed@a9227ed Fixes for python/typeshed#6039 Co-authored-by: hauntsaninja <>