possibly-undefined false positive when assigning variable for every case in match statement for all possible values #16992

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1. DetachHead

   added

   bugmypy got something wrongmypy got something wrong

   on Mar 6, 2024
2. Andrej730 commented on Jun 27, 2024

   

   Andrej730

   on Jun 27, 2024

   Last edited by Andrej730

   More actions

   Interesting that adding case _ kind of resolves the issue though mypy does detect that case _ code is unreachable.

   # mypy: enable-error-code=possibly-undefined
   
   from typing import Literal
   
   value: Literal[1, 2]
   
   match value:
       case 1:
           result = 1
       case 2:
           result = 2
       case _:
           # error: Statement is unreachable  [unreachable]
           result = 3
   
   result

   I've also met the similar issue with if isinstance checks. In the last case adding redudant else block makes the case false negative.

   # mypy: enable-error-code="possibly-undefined"
   
   class A: ...
   class B: ...
   
   # False positive: Name "new_name" may be undefined  [possibly-undefined]
   def test_1(obj: A | B) -> None:
       if isinstance(obj, A):
           new_name = "test"
       elif isinstance(obj, B):
           new_name = "test"
       a = new_name
   
   # + adding "else: reveal_type(obj)" somehow resolves it
   def test_1b(obj: A | B) -> None:
       if isinstance(obj, A):
           new_name = "test"
       elif isinstance(obj, B):
           new_name = "test"
       else:
           reveal_type(obj)
       a = new_name
   
   # False positive: Name "new_name" may be undefined  [possibly-undefined]
   def get_obj() -> A | B: ...
   def test_2() -> None:
       obj = get_obj()
       if isinstance(obj, A):
           new_name = "test"
       elif isinstance(obj, B):
           new_name = "test"
       a = new_name
   # but adding "else: reveal_type(obj)" somehow resolves it
   def test_2b() -> None:
       obj = get_obj()
       if isinstance(obj, A):
           new_name = "test"
       elif isinstance(obj, B):
           new_name = "test"
       else:
           reveal_type(obj)
       a = new_name
   
   # Name "new_name" may be undefined  [possibly-undefined]
   # and it's correct
   def test_3(obj: str | float) -> None:
       if isinstance(obj, str):
           new_name = "test"
       elif isinstance(obj, float):
           new_name = "test"
       a = new_name
   
   # but adding "else: reveal_type(obj)" makes it false negative
   def test_3b(obj: str | float) -> None:
       if isinstance(obj, str):
           new_name = "test"
       elif isinstance(obj, float):
           new_name = "test"
       else:
           reveal_type(obj)
       a = new_name
3. sgaist

   added a commit that references this issue on Oct 6, 2025

   fix(web_protocol): ignore false positive on possibly undefined

   ...

   7db3345
4. tamird commented on May 1, 2026

   

   tamird

   on May 1, 2026

   Contributor

   More actions

   Another simple case that demonstrates the inconsistency with unreachable code:

   def takes_list_problem[T](stuff: list[T]) -> None:
       match stuff:
           case [one]:
               pass
           case [*_]:
               raise AssertionError("nope")
               
       print(one) # error: Name "one" may be undefined  [possibly-undefined]
       
   def takes_list_no_proble[T](stuff: list[T]) -> None:
       def one() -> T:
           match stuff:
               case [one]:
                   return one
               case [*_]:
                   raise AssertionError("nope")
       
       print(one())

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