Confusing error message for a missing classmethod decorator #11791

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1. wrobell

   added

   bugmypy got something wrongmypy got something wrong

   on Dec 18, 2021
2. sobolevn commented on Dec 18, 2021

   

   sobolevn

   on Dec 18, 2021

   Last edited by sobolevn

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   This works for me on latest master when I remove @unique decorator:

   import typing as tp
   import enum
   
   # @enum.unique
   class ColorIndex(enum.IntEnum):
       BLACK = 0
   
       def _missing_(cls, value: object) -> tp.Any:
           return ColorIndex.BLACK

   » mypy out/ex.py --strict-optional --warn-unreachable --show-traceback
   out/ex.py:7: error: Signature of "_missing_" incompatible with supertype "Enum"
   out/ex.py:7: note:      Superclass:
   out/ex.py:7: note:          @classmethod
   out/ex.py:7: note:          def _missing_(cls, value: object) -> Any
   out/ex.py:7: note:      Subclass:
   out/ex.py:7: note:          def _missing_(cls, value: object) -> Any
3. sobolevn commented on Dec 18, 2021

   

   sobolevn

   on Dec 18, 2021

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   No, I was wrong. It depends on the cache.
   
4. sobolevn commented on Dec 18, 2021

   

   sobolevn

   on Dec 18, 2021

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   This is very hard to solve, but I've got a working prototype. I am going to finish it tomorrow.
   
5. wrobell commented on Dec 19, 2021

   

   wrobell

   on Dec 19, 2021

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   @sobolevn Thanks for the fix.

   I wonder if it is also possible to improve the main error message?

   Replace

   Signature of "_missing_" incompatible with supertype "Enum"
   

   with

   Supertype "Enum" defines method "_missing_" as a class method
   

   ?
6. sobolevn commented on Dec 19, 2021

   

   sobolevn

   on Dec 19, 2021

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   I think it is better to keep the existing format. It is way more informative.
   When it works correctly, it shows everything we need to know.
7. wrobell commented on Dec 19, 2021

   

   wrobell

   on Dec 19, 2021

   Last edited by wrobell

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   Agreed on the format. Still, the error message is going to be confusing.

   Please consider:

   import typing as tp
   import enum
   
   @enum.unique
   class ColorIndex(enum.IntEnum):
       BLACK = 0
       GREY = 1
       WHITE = 2
   
       @classmethod
       def _missing_(cls, value: object) -> ColorIndex:
           return ColorIndex.BLACK
   

   then

   $ mypy t.py
   Success: no issues found in 1 source file
   

   Without classmethod decorator:

   $ mypy t.py
   t.py:10: error: Signature of "_missing_" incompatible with supertype "Enum"
   t.py:10: note:      Superclass:
   t.py:10: note:          @classmethod
   t.py:10: note:          def _missing_(cls, value: object) -> Any
   t.py:10: note:      Subclass:
   t.py:10: note:          def _missing_(cls, value: object) -> ColorIndex
   Found 1 error in 1 file (checked 1 source file)
   

   The error message suggests I should change ColorIndex to Any, while it is missing classmethod decorator.
8. 97littleleaf11

   added

   topic-enum

   on Feb 20, 2022
9. AlexWaygood

   added

   diagnostics

   topic-error-reportingHow we report errorsHow we report errors

   topic-inheritanceInheritance and incompatible overridesInheritance and incompatible overrides

   on Mar 27, 2022
10. GabrieleCalarota commented on Jul 18, 2022

    

    GabrieleCalarota

    on Jul 18, 2022

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    Any updates on this?