From bac4edad69bb20dd9460766e062637cae999e1e0 Mon Sep 17 00:00:00 2001 From: "Gregory P. Smith" Date: Sun, 16 Jun 2024 22:47:10 -0700 Subject: [PATCH] gh-112346: Document the OS byte in `gzip.compress` output change in 3.11 (#120480) gh-112346: Describe the "os" byte in gzip output change. --- Doc/library/gzip.rst | 4 +++- Doc/whatsnew/3.11.rst | 15 +++++++++++++++ 2 files changed, 18 insertions(+), 1 deletion(-) diff --git a/Doc/library/gzip.rst b/Doc/library/gzip.rst index 152cba4f653..6b6e158f6eb 100644 --- a/Doc/library/gzip.rst +++ b/Doc/library/gzip.rst @@ -196,7 +196,9 @@ The module defines the following items: .. versionchanged:: 3.11 Speed is improved by compressing all data at once instead of in a streamed fashion. Calls with *mtime* set to ``0`` are delegated to - :func:`zlib.compress` for better speed. + :func:`zlib.compress` for better speed. In this situation the + output may contain a gzip header "OS" byte value other than 255 + "unknown" as supplied by the underlying zlib implementation. .. versionchanged:: 3.13 The gzip header OS byte is guaranteed to be set to 255 when this function diff --git a/Doc/whatsnew/3.11.rst b/Doc/whatsnew/3.11.rst index 7a74df330a8..b601bd453f5 100644 --- a/Doc/whatsnew/3.11.rst +++ b/Doc/whatsnew/3.11.rst @@ -768,6 +768,21 @@ functools (Contributed by Yurii Karabas in :issue:`46014`.) +.. _whatsnew311-gzip: + +gzip +---- + +* The :func:`gzip.compress` function is now faster when used with the + **mtime=0** argument as it delegates the compression entirely to a single + :func:`zlib.compress` operation. There is one side effect of this change: The + gzip file header contains an "OS" byte in its header. That was traditionally + always set to a value of 255 representing "unknown" by the :mod:`gzip` + module. Now, when using :func:`~gzip.compress` with **mtime=0**, it may be + set to a different value by the underlying zlib C library Python was linked + against. + (See :gh:`112346` for details on the side effect.) + .. _whatsnew311-hashlib: hashlib