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Mark classes as not instantiable

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#2,066 1 comment 3 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Feature
Clarity
Needs clarification
Activity status
Stale
Tech stack
python
Domain
tooling

Research direction

Start with the runtime example in the issue and the linked typeshed reference, python/typeshed#14558. Compare the proposed new annotations, setting new to None, and a decorator against existing typing behavior. Done means the project has agreed on a way to express that a class cannot be instantiated and has defined its expected semantics.

Written by the indexing model from the issue text.

Description

topic: feature

From python/typeshed#14558: Some classes (like re.Pattern) are not instantiable at runtime:

>>> import re
>>> re.Pattern()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create 're.Pattern' instances

Currently, there is no working way to mark classes as such. Ideas to mark classes:

  • Annotate the return type of __new__ as NoReturn/Never.
  • Set __new__ to None (similar to how __hash__ works).
  • A new decorator.
Dominant language
Python
Stars
1.8k
Forks
302
Avg merge
23h
Merged PRs (30d)
8

Contributor guide

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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