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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Python 3.13.3 runs __iter__ twice in a list comprehension","articleBody":"# Bug report\n\n### Bug description:\n\nI can't upload comprehensionTestbed2.py because I am told the .py type is not allowed so I here is the script I used to reproduce this issue:\n\n===========================================\n\n```python\n# TestClass: Class that interacts with the DVD data file.\n\nclass TestClass:\n    # Initialize the file timestamp.\n    def __init__(self):\n        pass\n    # The iterator method.\n    def __iter__(self):\n        print('S1')\n        self._index = 0\n        return self\n    # The next method.\n    def __next__(self):\n        print('S5')\n        if self._index == 5:\n            raise StopIteration\n        value = self._index\n        self._index += 1\n        return value\n\n\n# Test code.\n\nprint('#1')\nfor n in TestClass():\n    print(n)\n\nprint('#2')\nnumbers = [n for n in TestClass()]\nprint(numbers)\n```\n==============================================\n\nHere is the output of the Python script above:\n\n```\nD:\\Files\\Testing\u003e comprehensionTestbed2.py                \n#1                                                        \nS1                                                        \nS5                                                        \n0                                                        \nS5                                                        \n1                                                        \nS5                                                        \n2                                                        \nS5                                                        \n3                                                        \nS5                                                        \n4                                                        \nS5                                                        \n#2                                                        \nS1                                                        \nS1  \u003c-----  The __iter__ method is invoked twice in the list comprehension when using 3.13.3 (and 3.13.2).                                            \nS5                                                        \nS5                                                        \nS5                                                        \nS5                                                        \nS5                                                        \nS5                                                        \n[0, 1, 2, 3, 4]                                           \n```\nThis does not happen with python 3.12.9 and is a real problem when the __iter__ method creates a mutex.\n\nEdward\n\n\n### CPython versions tested on:\n\n3.13\n\n### Operating systems tested on:\n\nWindows","author":{"url":"https://github.com/fb2078","@type":"Person","name":"fb2078"},"datePublished":"2025-04-18T22:24:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":10},"url":"https://github.com/132711/cpython/issues/132711"}