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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Improve error message from `os.fspath` if `__fspath__` is set to `None`","articleBody":"# Feature or enhancement\r\n\r\nA common Python idiom is to set a magic method to `None` in a subclass if you want to disable the behaviour that the magic method enables. If you do so, a nice error message will be given if a user tries to \"call\" the magic method that's been set to `None`. For example:\r\n\r\n```pycon\r\n\u003e\u003e\u003e class Foo(list):\r\n...     __iter__ = None\r\n...\r\n\u003e\u003e\u003e iter(Foo())\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\nTypeError: 'Foo' object is not iterable\r\n\u003e\u003e\u003e class Bar(int):\r\n...     __hash__ = None\r\n...\r\n\u003e\u003e\u003e hash(Bar(42))\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\nTypeError: unhashable type: 'Bar'\r\n```\r\n\r\nHowever, Python doesn't currently do the same thing for `__fspath__`. If a subclass of an `os.PathLike` sets `__fspath__` to `None`, Python gives a bad error message when `os.fspath` is called on instances of that subclass:\r\n\r\n```pycon\r\n\u003e\u003e\u003e from pathlib import Path\r\n\u003e\u003e\u003e import os\r\n\u003e\u003e\u003e class Baz(Path):\r\n...     __fspath__ = None\r\n...\r\n\u003e\u003e\u003e os.fspath(Baz())\r\nTraceback (most recent call last):\r\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\r\nTypeError: 'NoneType' object is not callable\r\n```\r\n\r\n# Pitch\r\n\r\nIt would be nice if Python handled `__fspath__` being set to `None` the same way it handles magic methods such as `__iter__` or `__hash__` being set to `None`. The error message in such cases should be the `expected str, bytes or os.PathLike object, not Baz` error message that the interpreter gives when `os.fspath` is called on an object that has no `__fspath__` method at all.\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-106082\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/AlexWaygood","@type":"Person","name":"AlexWaygood"},"datePublished":"2023-06-23T19:03:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/106046/cpython/issues/106046"}