Covariance error inferred for lambda has invalid line number associated with it #8592

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1. kprzybyla commented on Mar 25, 2020

   

   kprzybyla

   on Mar 25, 2020

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   Also, for anyone that faces similar issue when using this recipe from hypothesis documentation, here's a working solution that avoids any mypy errors and do not require # type: ignore

   from typing import TypeVar
   
   from hypothesis import strategies as st
   
   _T = TypeVar("_T")
   _Ex = TypeVar("_Ex", covariant=True)
   
   def everything_except(*types: type) -> st.SearchStrategy[_Ex]:
       def filter_everything_except(value: _T) -> bool:
           return not isinstance(value, types)
   
       return st.from_type(type).flatmap(st.from_type).filter(filter_everything_except)

   👍React with 👍1Reacted by Walter Cloete
2. ilevkivskyi commented on Mar 27, 2020

   

   ilevkivskyi

   on Mar 27, 2020

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   Wrong line number for error is a pretty serious issue, hard to say what is a possible root cause however.
3. ilevkivskyi

   added

   bugmypy got something wrongmypy got something wrong

   priority-0-high

   on Mar 27, 2020
4. pranavrajpal commented on Aug 12, 2021

   

   pranavrajpal

   on Aug 12, 2021

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   I think the problem is that the check for whether a covariant type variable is used as the argument to a function reports the line number that the type variable is bound to a variable (which makes sense when that type variable is an argument for a FuncDef but doesn't make sense for a Callable variable or a lambda).

   The wrong line number (which was line 884 in my case) seems to be pointing here.

   I managed to create a somewhat simpler example:

   from typing import Any, Callable, TypeVar
   
   Ex = TypeVar("Ex")
   T = TypeVar("T", covariant=True)
   
   def takes_lambda(func: Callable[[Ex], Any]) -> None:
       pass
   
   def test(a: T) -> None: # E: Cannot use a covariant type variable as a parameter
       func: Callable[[T], Any]
       takes_lambda(func) # type: ignore
5. AlexWaygood

   added

   topic-error-reportingHow we report errorsHow we report errors

   on Mar 31, 2022
6. ilevkivskyi

   closed this as completedin 69dcb38on Oct 12, 2026
7. ilevkivskyi

   added a commit that references this issue on Oct 12, 2026

   Do not use types as error contexts during type checking (python#22156)

   ...

   69dcb38