dict not subscriptable despite using __future__ typing annotations #89280

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1. stefanistrate commented on Sep 6, 2021

   

   stefanistratemannequin

   on Sep 6, 2021

   MannequinAuthor

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   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}
2. stefanistrate

   added

   3.8 (EOL)end of lifeend of life

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Sep 6, 2021
3. sweeneyde commented on Sep 7, 2021

   

   sweeneyde

   on Sep 7, 2021

   Member

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   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.
4. sweeneyde

   closed this as completedon Sep 7, 2021
5. sweeneyde

   added

   invalid

   on Sep 7, 2021
6. sweeneyde

   closed this as completedon Sep 7, 2021
7. sweeneyde

   added

   invalid

   on Sep 7, 2021
8. ezio-melotti

   transferred this issue fromon Apr 10, 2022