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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Specialized `STORE_SUBSCR_DICT` allows silent mutation of `frozendict` after adaptive specialization","articleBody":"# Bug report\n\n### Bug description:\n\nCommit 3e2f5c133f broadened `_GUARD_NOS_DICT` from `PyDict_CheckExact()` to `PyAnyDict_CheckExact()` for read operations (`BINARY_OP_SUBSCR_DICT`, `CONTAINS_OP_DICT`). However, the same guard is also used by `STORE_SUBSCR_DICT`, which still calls `_PyDict_SetItem_Take2()` directly; bypassing frozendict's immutability checks entirely.\n\nWhen a function containing `d[key] = value` is first specialized with `dict`, subsequent calls with `frozendict` pass the widened guard and mutate the frozendict at the C level. In debug builds this is an assertion crash; in release builds it's silent mutation with a stale cached hash.\n\n```python\ndef store(d, key, val):\n    d[key] = val\n\n# specialize\nfor i in range(100):\n    store({}, 'k', i)\n\n# now frozendict passes the widened guard and is mutated\nfd = frozendict({'a': 1})\nh_before = hash(fd)\n\nstore(fd, 'b', 2)  # should raise TypeError\n\nprint(fd)       # frozendict({'a': 1, 'b': 2}),    mutated\nprint(hash(fd) == h_before)  # True, stale hash\n```\n\n**Before specialization**, `store(frozendict(...), 'b', 2)` correctly raises `TypeError: 'frozendict' object does not support item assignment`.\n\n**After specialization**, it silently mutates the frozendict and leaves the cached hash stale.\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-145039\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/abdoulrasheed","@type":"Person","name":"abdoulrasheed"},"datePublished":"2026-02-22T16:48:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/145122/cpython/issues/145122"}