dict not subscriptable despite using __future__ typing annotations

[stefanistrate]

BPO	45117
Nosy	@sweeneyde, @stefanistrate

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-09-07.04:48:42.150>
created_at = <Date 2021-09-06.16:11:44.909>
labels = ['invalid', 'type-bug', '3.8']
title = '`dict` not subscriptable despite using `__future__` typing annotations'
updated_at = <Date 2021-09-07.04:48:42.146>
user = 'https://github.com/stefanistrate'

bugs.python.org fields:

activity = <Date 2021-09-07.04:48:42.146>
actor = 'Dennis Sweeney'
assignee = 'none'
closed = True
closed_date = <Date 2021-09-07.04:48:42.150>
closer = 'Dennis Sweeney'
components = []
creation = <Date 2021-09-06.16:11:44.909>
creator = 'stefanistrate'
dependencies = []
files = []
hgrepos = []
issue_num = 45117
keywords = []
message_count = 2.0
messages = ['401149', '401205']
nosy_count = 2.0
nosy_names = ['Dennis Sweeney', 'stefanistrate']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45117'
versions = ['Python 3.8']

[stefanistrate]

According to PEP-585 (https://www.python.org/dev/peps/pep-0585/#implementation), I expect that typing aliases could simply use dict instead of typing.Dict. This works without problems in Python 3.9, but in Python 3.8, despite using from __future__ import annotations, I get the following error:

$ /usr/local/Cellar/python@3.8/3.8.11/bin/python3.8
Python 3.8.11 (default, Jun 29 2021, 03:08:07)
[Clang 12.0.5 (clang-1205.0.22.9)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from __future__ import annotations
>>> from typing import Any
>>> JsonDictType = dict[str, Any]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'type' object is not subscriptable

However, dict is subscriptable when not used in an alias:

$ /usr/local/Cellar/python@3.8/3.8.11/bin/python3.8
Python 3.8.11 (default, Jun 29 2021, 03:08:07)
[Clang 12.0.5 (clang-1205.0.22.9)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from __future__ import annotations
>>> from typing import Any
>>> def f(d: dict[str, Any]):
...     print(d)
...
>>> f({"abc": 1, "def": 2})
{'abc': 1, 'def': 2}

[sweeneyde]

Hi Stefan,

from __future__ import annotations only affects annotations -- just the things after the colon. It makes it so that annotations are never evaluated, so things like this work:

>>> from __future__ import annotations
>>> x: nonsense()()()()[other_nonsense](1<2>3)

The __future__ import is not a wholesale opt-in-to-all-new-typing-features, it's just an opt-in-to-not-evaluate-annotations.

dict.__class_getitem__ (which is what gets called when you type dict[str, Any]) was not added at all until Python 3.9 (GH-18239), so if you want to *evaluate* such expressions, you have to upgrade to 3.9+. In 3.8, use typing.Dict instead -- 3.8 is no longer accepting new features.

Thanks for the report, but I'm closing this for now.