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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"csv: csv.reader does not detect re-entrant iterator advancement","articleBody":"## Feature\n\nWhen a `csv.reader` input iterator calls `next()` on the same reader from its\nown `__next__()` method, the inner call advances the reader. RustPython then\nallows the outer call to continue parsing with stale reader state instead of\nraising `csv.Error` like CPython.\n\n```python\nimport csv\n\n\nclass ReentrantIterator:\n    def __init__(self):\n        self.reader = None\n        self.index = 0\n\n    def __iter__(self):\n        return self\n\n    def __next__(self):\n        self.index += 1\n        if self.index == 1:\n            next(self.reader)\n            return \"a,b\"\n        if self.index == 2:\n            return \"x\"\n        raise StopIteration\n\n\niterator = ReentrantIterator()\nreader = csv.reader(iterator)\niterator.reader = reader\nprint(next(reader))\n\n# CPython main:\n# _csv.Error: iterator has already advanced the reader\n#\n# RustPython:\n# ['a', 'b']\n```\n\nCPython rejects the outer call once it detects that the re-entrant inner call\nhas already advanced the reader. This prevents the outer call from continuing\nwith parser state that no longer belongs to it.\n\n## Python Documentation or reference to CPython source code\n\n- https://github.com/python/cpython/issues/145105\n- https://github.com/python/cpython/pull/145106\n\n\u003csub\u003eDrafted with AI assistance (Codex).\u003c/sub\u003e","author":{"url":"https://github.com/widehyo1","@type":"Person","name":"widehyo1"},"datePublished":"2026-07-19T12:53:16.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/8323/RustPython/issues/8323"}