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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.
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Gregory P. Smith 2024-06-16 22:47:10 -07:00 committed by GitHub
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Doc/library/gzip.rst
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@ -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

15
Doc/whatsnew/3.11.rst
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@ -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