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Title: Partial type that is "initialised" via assignment is only temporarily narrowed by `isinstance` · Issue #18238 · python/mypy · GitHub

Open Graph Title: Partial type that is "initialised" via assignment is only temporarily narrowed by `isinstance` · Issue #18238 · python/mypy

X Title: Partial type that is "initialised" via assignment is only temporarily narrowed by `isinstance` · Issue #18238 · python/mypy

Description: In the following example, the last revealed type could/should be Union[builtins.int, None]: def f() -> bool: return True u: int | tuple[int] x = None if f(): x = u if isinstance(x, tuple): x = x[0] reveal_type(x) # note: "builtins.int" r...

Open Graph Description: In the following example, the last revealed type could/should be Union[builtins.int, None]: def f() -> bool: return True u: int | tuple[int] x = None if f(): x = u if isinstance(x, tuple): x = x[0]...

X Description: In the following example, the last revealed type could/should be Union[builtins.int, None]: def f() -> bool: return True u: int | tuple[int] x = None if f(): x = u if isinstance(x, tuple): x = x...

Opengraph URL: https://github.com/python/mypy/issues/18238

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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Partial type that is \"initialised\" via assignment is only temporarily narrowed by `isinstance`","articleBody":"In the following example, the last revealed type could/should be `Union[builtins.int, None]`:\r\n\r\n```python\r\ndef f() -\u003e bool: \r\n    return True\r\n\r\nu: int | tuple[int]\r\nx = None\r\nif f():\r\n    x = u\r\n    if isinstance(x, tuple):\r\n        x = x[0]\r\n    reveal_type(x)  # note: \"builtins.int\"\r\nreveal_type(x)  # note: Revealed type is \"Union[builtins.int, Tuple[builtins.int], None]\"\r\n```\r\n\r\nSee [this comment](https://github.com/python/mypy/pull/18180#issuecomment-2496238375) for background information.\r\n","author":{"url":"https://github.com/tyralla","@type":"Person","name":"tyralla"},"datePublished":"2024-12-03T21:00:32.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/18238/mypy/issues/18238"}

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