Skip to content

Trying to iterate over typing.Union results in hang #92261

Closed
Closed
Trying to iterate over typing.Union results in hang#92261
Labels
topic-typingtype-bugAn unexpected behavior, bug, or error

Description

@mrahtz
mrahtz
opened on May 3, 2022

If you accidentally iterate over a Union, you get a hang:

from typing import Union
list(Union)
# ...hang

It looks like it's an infinite loop:

from typing import Union
for i in Union:
    print(i)
# 0, 1, 2, ...

As far as I can tell, this happens because typing.Union is implemented using typing._SpecialForm, and _SpecialForm is implemented using __getitem__ to support parameterisation like Union[int]. Unfortunately, because _SpecialForm doesn't implement __iter__, __getitem__ is also used for iteration - meaning that when trying to iterate, we effectively do Union[0], Union[1], etc.

Activity

  1. mrahtz
    added
    type-bugAn unexpected behavior, bug, or error
    on May 3, 2022

  2. JelleZijlstra commented on May 3, 2022

    @JelleZijlstra
    JelleZijlstra
    on May 3, 2022
    Member

    We should deprecate itertools.count because it's now redundant.

    More seriously, agree that we should disallow this.

  3. serhiy-storchaka
    added a commit that references this issue on May 8, 2022

    gh-92261: Disallow iteration of Union (and other special forms) (GH-9…

    4739997

  4. mrahtz commented on May 8, 2022

    @mrahtz
    mrahtz
    on May 8, 2022
    ContributorAuthor

    Fixed in 4739997.

  5. mrahtz
    closed this as completedon May 8, 2022
  6. AlexWaygood
    added
    topic-typing
    on May 8, 2022
  7. miss-islington
    added a commit that references this issue on May 9, 2022

    pythongh-92261: Disallow iteration of Union (and other special forms) (…

    ea6cf7e
  8. serhiy-storchaka
    added a commit that references this issue on May 10, 2022

    gh-92261: Disallow iteration of Union (and other special forms) (GH-9…

    7540a43
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Metadata

Metadata

Assignees

No one assigned

    Labels

    topic-typingtype-bugAn unexpected behavior, bug, or error

    Projects

    No projects

      Milestone

      No milestone

      Relationships

      None yet

      Development

      No branches or pull requests

      Issue actions