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Title: gh-150583: Fix zstd compression with digested ZstdDict by Rogdham · Pull Request #150586 · python/cpython · GitHub

Open Graph Title: gh-150583: Fix zstd compression with digested ZstdDict by Rogdham · Pull Request #150586 · python/cpython

X Title: gh-150583: Fix zstd compression with digested ZstdDict by Rogdham · Pull Request #150586 · python/cpython

Description: Changes Force setting a compression_level of ZSTD_CLEVEL_DEFAULT when creating the ZstdCompressor object. Question for reviewers: should I use _zstd_set_c_level(ZSTD_CLEVEL_DEFAULT) instead? See reproducer in #150583. Analysis When creating a ZstdCompressor object (_zstd_ZstdCompressor_new_impl), the compression_level field is unset, and as a result keeps its value from whatever was in memory (not deterministic), assuming no level is passed in the constructor. Then, when set, the C dict is loaded into the compression context with _zstd_load_c_dict, which in turn calls _zstd_load_impl. In the case of a digested dictionary, _get_CDict(zd, self->compression_level) is now called, which in turns calls ZSTD_createCDict(..., compressionLevel). So an arbitrary compression level is passed to that call to libzstd. Bug impact Prerequisites: Compress data with compression.zstd Do not specify a compression level (neither with level nor through options) Pass a digested dictionary with zstd_dict (e.g. with ZstdDict(...).as_digested_dict) Consequence: the compression level used is arbitrary, and may change from one run to another. Misc Backport suggestion: to 3.15 and 3.14. AI disclosure: analysis was performed with help of an LLM. @emmatyping 👋 Issue: gh-150583

Open Graph Description: Changes Force setting a compression_level of ZSTD_CLEVEL_DEFAULT when creating the ZstdCompressor object. Question for reviewers: should I use _zstd_set_c_level(ZSTD_CLEVEL_DEFAULT) instead? See re...

X Description: Changes Force setting a compression_level of ZSTD_CLEVEL_DEFAULT when creating the ZstdCompressor object. Question for reviewers: should I use _zstd_set_c_level(ZSTD_CLEVEL_DEFAULT) instead? See re...

Opengraph URL: https://github.com/python/cpython/pull/150586

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dda4ab6 Fix zstd compression with digested ZstdDict Rogdham May 29, 2026 https://github.com/python/cpython/pull/150586/commits/dda4ab6f65d9ae37d245d6b9789fbc3d8718ad0e
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d713981 Update Misc/NEWS.d/next/Library/2026-06-03-19-35-32.gh-issue-150583.d… Rogdham Jul 6, 2026 https://github.com/python/cpython/pull/150586/commits/d713981c68877d3fbe0407f46d841679bb878533
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