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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"struct: non-ASCII bytes format raises UnicodeEncodeError instead of struct.error on 3.13/3.14","articleBody":"# Bug report\n\nhttps://github.com/python/cpython/pull/153835 (backport of gh-85943) introduces a regression on 3.13 and 3.14. A bytes format with a non-ASCII byte now raises `UnicodeEncodeError` instead of `struct.error`:\n\n```python\n>>> import struct\n>>> struct.unpack(b'\\xc5', b'')\nUnicodeEncodeError: 'ascii' codec can't encode character '\\udcc5' in position 0: ordinal not in range(128)\n```\n\n3.14.0 raises `struct.error: bad char in struct format`. `main` is not affected since gh-145850 already rejects non-ASCII formats with `ValueError`.\n\nThis aborts `fuzz_struct_unpack`, so CIFuzz fails on any 3.14 PR that touches C files, e.g. gh-158162: https://github.com/python/cpython/actions/runs/36136424084/job/108075564322\n\n```\nUnicodeEncodeError: 'ascii' codec can't encode character '\\udcc5' in position 0: ordinal not in range(128)\n==75== ERROR: libFuzzer: deadly signal\n    #6 0x55d07f8c7884 in _run_fuzz /src/cpython3/Modules/_xxtestfuzz/fuzzer.c:575:9\n    #7 0x55d07f8c7884 in LLVMFuzzerTestOneInput /src/cpython3/Modules/_xxtestfuzz/fuzzer.c:640:11\nBase64: xQA=\n```\n\n\n\u003c!-- gh-linked-prs -->\n### Linked PRs\n* gh-158185\n\u003c!-- /gh-linked-prs -->\n","author":{"url":"https://github.com/corona10","@type":"Person","name":"corona10"},"datePublished":"2026-09-25T14:44:25Z","url":"https://github.com/python/cpython/issues/158172"}