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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"compression.zstd: non-determinist with zstd_dict","articleBody":"# Bug report\n\n### Bug description:\n\nIt seems that in some cases, compressing with `compress(... , zstd_dict=ZstdDict(...).as_digested_dict)` is not deterministic, even when the parameters are the same.\n\nInitially reported at https://github.com/Rogdham/pyzstd/issues/66.\n\n### Reproducer\n\nThe following should print `1` but prints `2` most of the times.\n\nNote: the issue seems somewhat less frequent on free-threaded builds.\n\n```python\nfrom compression import zstd\n\nDICT = b\"7\\xa40\\xec/\u003c\\xabJ\\t\\x10\\x10\\xdf033\\xb3w\\n3\\xf1x\u003c\\x1e\\x8f\\xc7\\xe3\\xf1x\u003c\\xcf\\xf3\\xbc\\xf7\\xd4BAAAAAAAAAAAAAAAAAAAAAAAAA\\xa1P(\\x14\\n\\x85B\\xa1P(\\x14\\n\\x85\\xa2(\\x8a\\xa2(J)}t\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xe1\\xf1x\u003c\\x1e\\x8f\\xc7\\xe3\\xf1x\\x9e\\xe7y\\xef\\x01\\x01\\x00\\x00\\x00\\x04\\x00\\x00\\x00\\x08\\x00\\x00\\x00helloworld test data sample foo bar baz hello w\"\nDATA = b\"hello world test\"\n\nZSTD_DICT = zstd.ZstdDict(DICT).as_digested_dict\n\nvalues = {\n    zstd.compress(DATA, zstd_dict=ZSTD_DICT)\n    for _ in range(100_000)\n}\nprint(len(values))\n```\n\n### CPython versions tested on:\n\n3.14, 3.15, CPython main branch,\n\n### Operating systems tested on:\n\nLinux\n\n---\n\n\u003c!-- gh-linked-prs -->\n### Linked PRs\n* gh-150586\n* gh-153883\n* gh-153884\n\u003c!-- /gh-linked-prs -->\n","author":{"url":"https://github.com/Rogdham","@type":"Person","name":"Rogdham"},"datePublished":"2026-05-29T17:36:57Z","url":"https://github.com/python/cpython/issues/150583"}