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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Trying to iterate over typing.Union results in hang","articleBody":"If you accidentally iterate over a `Union`, you get a hang:\r\n\r\n```python\r\nfrom typing import Union\r\nlist(Union)\r\n# ...hang\r\n```\r\n\r\nIt looks like it's an infinite loop:\r\n\r\n```python\r\nfrom typing import Union\r\nfor i in Union:\r\n    print(i)\r\n# 0, 1, 2, ...\r\n```\r\n\r\nAs 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.","author":{"url":"https://github.com/mrahtz","@type":"Person","name":"mrahtz"},"datePublished":"2022-05-03T20:09:04.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/92261/cpython/issues/92261"}