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(🐞) false narrowing with reassignment in loop #11612

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

Description

@KotlinIsland
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
    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
    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
    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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