René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"dict_set_fromkeys() calculates size of dictionary improperly","articleBody":"# Bug report\n\n### Bug description:\n\nThe function dict_set_fromkeys() in the file dictobject.c adds elements of an iterable to an existing dictionary. The size of the expanded dictionary is estimated as PySet_GET_SIZE(iterable) and the size of the existing dictionary is not considered.\n\nThis is unlogical.\n\nA more resonable estimation is \"mp-\u003ema_used + PySet_GET_SIZE(iterable)\". What turns this into a bug is the fact that a large dictionary plus a small iterable can cause the function to loop forever.\n\nThe following code is adopted from the official tests, file test_dict.py. Official binaries for Python 3.13.3 compiled with MSVC will loop forever when running this modified test (the official test has range(10)). Other platforms may be affected as well.\n\n```python\nclass baddict3(dict):\n    def __new__(cls):\n        return d\nd = {i : i for i in range(17)}\nres = d.copy()\nres.update(a=None, b=None, c=None)\nprint(res)\nq = baddict3.fromkeys({\"a\", \"b\", \"c\"})\nprint(q)\n```\nA minor error is in the function calculate_log2_keysize(minsize) which has three branches for calculating its result. However, when minsize \u003c 10 the results of the three branches can differ. Since _BitScanReverse64() is Windows-only other platforms might be unaffected.\n\nA final unrelated note. PEP 11 – CPython platform support states that aarch64-unknown-linux-gnu and x86_64-unknown-linux-gnu are Tier 1 AND Tier 2.\n\n### CPython versions tested on:\n\n3.13\n\n### Operating systems tested on:\n\nWindows\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-133627\n* gh-133685\n* gh-133686\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ThomasBr0","@type":"Person","name":"ThomasBr0"},"datePublished":"2025-04-21T00:17:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/132762/cpython/issues/132762"}