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Mark classes as not instantiable #2066

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Mark classes as not instantiable#2066
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topic: featureDiscussions about new features for Python's type annotations

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@srittau
srittau
opened on Aug 11, 2025

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.

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  1. srittau
    added
    topic: featureDiscussions about new features for Python's type annotations
    on Aug 11, 2025

  2. RBerga06 commented on Aug 11, 2025

    @RBerga06
    RBerga06
    on Aug 11, 2025

    In a project of mine, I have a Rust extension (built with pyo3) where some classes cannot be instantiated from Python. I'm currently using this approach, which seems to be working (with pyright, I have not tried mypy yet):

    # mymodule.pyi
from typing import Never

class Foo:
   def __init__(self: Never, /) -> None: ...  # valid because `Never <: Foo`

x = Foo()  # error because `Foo <: Never` is false

    However, I would really appreciate it if there was an official way to do this.

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