Allow omitting initializer for Final attributes in dataclasses #5608

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1. ilevkivskyi

   added

   feature

   priority-1-normal

   topic-pluginsThe plugin API and ideas for new pluginsThe plugin API and ideas for new plugins

   on Sep 12, 2018
2. AlexWaygood

   added

   topic-finalPEP 591PEP 591

   topic-dataclasses

   and removed

   topic-pluginsThe plugin API and ideas for new pluginsThe plugin API and ideas for new plugins

   on Mar 24, 2022
3. jcrist commented on Mar 25, 2023

   

   jcrist

   on Mar 25, 2023

   More actions

   I just ran into this. pyright allows these type annotations as is (Final indicates an instance variable), and discussion in python/cpython#89547 seems to indicate this is what'd be most ergonomic to support. It also matches how dataclasses itself handles these annotations.

   For now, a workaround is to add # type: ignore on the annotation line. The dataclass definition still seems to be processed correctly, this just skips the error about a missing initializer.

   import dataclasses
   from typing import Final
   
   
   @dataclasses.dataclass
   class Test:
       # The `type: ignore` is needed here to avoid an error due to a missing
       # initializer
       x: Final[int]  # type: ignore
   
   
   t = Test(1)  # Signature still inferred properly
   reveal_type(t.x)
   t2 = Test("bad")  # Which means mypy marks this as an error

   $ mypy ex.py 
   ex.py:13: note: Revealed type is "builtins.int"
   ex.py:14: error: Argument 1 to "Test" has incompatible type "str"; expected "int"  [arg-type]
   Found 1 error in 1 file (checked 1 source file)
   

   For my own use cases, it'd be nice if the fix here also applied to the recent dataclass_tranform support, so this fix could work for other dataclass-like things as well.
4. tmke8 commented on May 13, 2024

   

   tmke8

   on May 13, 2024

   Contributor

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   The typing spec now specifies that this should be allowed: python/typing#1669

   👍React with 👍3Reacted by Pragy Agarwal, Randolf Scholz and Lukas Atkinson
5. ilevkivskyi

   closed this as completedin #21334on Apr 26, 2026
6. pull

   added a commit that references this issue on Apr 26, 2026

   Add support for Final[...] in dataclasses (python#21334)

   ...

   a850b66