Expose dict view types in ``types``

[AA-Turner]

Feature or enhancement

Proposal:

Expose the items, keys, and values types in the types module. See dictionary view objects. These correspond to the ItemsView, KeysView, and MappingView view types in collections.abc.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

• bpo-46399: Expose DictKeysType, DictValuesType, and DictItemsType in … #30629
• bpo-46399: Add GenericAlias support to dict_keys, dict_values and dict_items types  #30630

Linked PRs

• GH-132760: Expose dict view types in types #132761

[AlexWaygood]

Hmm — there was a lengthy discussion about this a while back in #90557, and the conclusion was that this would end up exposing implementation details without actually helping any known use case. What's changed since then?

[AA-Turner]

I saw that but (mistakenly) thought it was more to do with type-checkers rather than runtime semantics, so skipped it. I will read the discussion tomorrow.

My thinking for proposing this is that dictionary views are important enough to get a full section in stdtypes, with somewhat different semantics to the collections.abc types (e.g. the ABCs can never grow new attributes), so it is worthwhile exposing them, even if they are uncommon. I'll review the linked discussion, sorry again for missing it.

A

[ZeroIntensity]

this would end up exposing implementation details without actually helping any known use case.

To be fair, types is already chock-full of implementation details; CellType, CapsuleType, and any of the descriptor types, are all pretty CPython-specific.

[AlexWaygood]

To be fair, types is already chock-full of implementation details; CellType, CapsuleType, and any of the descriptor types, are all pretty CPython-specific.

For sure. But for each of those, there was a compelling use case for exposing these symbols in the types module, that outweighed the cost of exposing these implementation details. I thought I had a compelling use case when I filed #90557 several years ago, but by the end of the discussion I was persuaded that the costs outweighed the benefits for these types. @AA-Turner has given a slightly different use case now, but it wasn't given in the original issue description (and I'm not yet sure I agree that it meets the bar, personally, though I'm open to be persuaded!).

[serhiy-storchaka]

Iterator types are not exposed, and I think that dict view types should not be exposed for the same reason. Use protocols instead of concrete type checks.

[ZeroIntensity]

What protocol fits for DictItemsType?

[AlexWaygood]

collections.abc.ItemsView

[serhiy-storchaka]

Iterable and Set are most used.