René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Add `__class_getitem__` to `map` builtin, PEP 585 ","articleBody":"# Feature or enhancement\r\n\r\nRight now it is impossible to subscribe `map`:\r\n\r\n```python\r\n» ./python.exe\r\nPython 3.13.0a0 (heads/main-dirty:79823c103b6, Aug 31 2023, 15:46:08) [Clang 14.0.3 (clang-1403.0.22.14.1)] on darwin\r\nType \"help\", \"copyright\", \"credits\" or \"license\" for more information.\r\n\u003e\u003e\u003e map[str]\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\nTypeError: type 'map' is not subscriptable\r\n\u003e\u003e\u003e \r\n```\r\n\r\nI propose adding `__class_getitem__` support to `map`, because it makes sense in some cases.\r\nHowever, https://peps.python.org/pep-0585/ does not say anything about `map`.\r\n\r\nFirst of all, `map` is defined as `Generic` in `typeshed`:\r\n\r\n```python\r\nclass map(Iterator[_S], Generic[_S]):\r\n    def __next__(self) -\u003e _S: ...\r\n```\r\n\r\nFull def: https://github.com/python/typeshed/blob/9552ec7f72f3190eb529aec096783c624b43bd29/stdlib/builtins.pyi#L1473-L1514\r\n\r\nSo, any person that wants to, say, return `map[str]` right now has to use `'map[str]'` (via `''` or via `from __future__ import annotations`).\r\n\r\nSecond, while we can treat `map[T]` as just `Iterator[T]` in parameter types, we always want to as specific as possible in return types.\r\n\r\nThird, it is a builtin, so we assume that it is widely used. And it actually is, even in CPython's source itself we can find usages that can be annotated as `map[whatever]`:\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Lib/zipfile/_path/__init__.py#L368\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Doc/library/itertools.rst#L808\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Doc/library/itertools.rst#L964\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Doc/library/itertools.rst#L1022\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Lib/importlib/resources/readers.py#L76\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Lib/logging/config.py#L112\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Lib/importlib/metadata/__init__.py#L540\r\n- https://github.com/python/cpython/blob/3047f09490ae63f25d57efe1d14a9a65d9b5f6db/Lib/importlib/metadata/__init__.py#L787\r\n\r\n\r\nLastly, it is a one-line change (+ tests) and many other builtins and stdlib objects support that already.\r\n\r\nI cannot really think of any major cons.\r\n\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-108762\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/sobolevn","@type":"Person","name":"sobolevn"},"datePublished":"2023-09-01T12:59:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":16},"url":"https://github.com/108761/cpython/issues/108761"}