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(🐞) false narrowing with reassignment in loop · Issue #11612 · python/mypy · GitHub

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(🐞) false narrowing with reassignment in loop #11612

Closed
#18433
Closed
(🐞) false narrowing with reassignment in loop#11612
#18433
Labels
bugmypy got something wrongtopic-reachabilityDetecting unreachable codetopic-type-narrowingConditional type narrowing / binder

Description

KotlinIsland
opened on Nov 25, 2021
a: int | None
a = None
for i in range(10):
    reveal_type(a)  # "None" and also "int | None"
    if a:
        print(a)  # error: unreachable
    a = i

Also:

a = None
for b in [1, 2]:
    if a is None:
        a = b
        continue
    print("hi")  # false unreachable error

Activity

  1. KotlinIsland
    added
    bugmypy got something wrong
    on Nov 25, 2021

  2. sobolevn commented on Dec 6, 2021

    sobolevn
    on Dec 6, 2021
    Member

    This can be fixed by:

    a: int | None = None
for b in [1, 2]:
    if a is None:
        a = b
        continue
    print("hi")  # ok

    The other question is: why a is not infered to be None | int?

  3. KotlinIsland commented on Dec 6, 2021

    KotlinIsland
    on Dec 6, 2021
    ContributorAuthor

    It is, if you put reveal_type after the loop it reveals as int | None

    Perhaps it is inferring as int | None, but only past line 4, not considering the loop.

  4. AlexWaygood
    added
    topic-reachabilityDetecting unreachable code
    on Mar 30, 2022
  5. KotlinIsland
    changed the title [-]false unreachable with conditional reassignment then continue in loop[/-] [+](🐞) false unreachable with conditional reassignment then continue in loop[/+] on Oct 31, 2023
  6. KotlinIsland
    changed the title [-](🐞) false unreachable with conditional reassignment then continue in loop[/-] [+](🐞) false unreachable with reassignment in loop[/+] on Oct 31, 2023

  7. KotlinIsland commented on Oct 31, 2023

    KotlinIsland
    on Oct 31, 2023
    ContributorAuthor

    @sobolevn It's not just the inference, I've updated the OP with a much better example.

  8. KotlinIsland
    changed the title [-](🐞) false unreachable with reassignment in loop[/-] [+](🐞) false narrowing with reassignment in loop[/+] on Oct 31, 2023
  9. hauntsaninja
    added
    topic-type-narrowingConditional type narrowing / binder
    on Nov 18, 2024
  10. tyralla
    added a commit that references this issue on Jan 9, 2025

    Avoid false `unreachable` and `redundant-expr` warnings in loops.

    53d894a
  11. hauntsaninja
    closed this as completedin #18433on Jan 12, 2025
  12. hauntsaninja
    closed this as completedin 9685171on Jan 12, 2025
  13. sterliakov
    added a commit that references this issue on Jan 12, 2025

    Avoid false `unreachable` and `redundant-expr` warnings in loops. (py…

    9685171
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