8000 gh-112346: Document the OS byte in `gzip.compress` output change in 3.11 by gpshead · Pull Request #120480 · python/cpython · GitHub
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gh-112346: Document the OS byte in gzip.compress output change in 3.11 #120480

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Merged
merged 2 commits into from
Jun 17, 2024
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gh-112346: Document the OS byte in gzip.compress output change in 3.11 #120480

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3b63de2
gh-112346: Describe the "os" byte in gzip output change.
gpshead Jun 14, 2024
acf01da
Merge branch 'main' into docs/gh112346
gpshead Jun 15, 2024
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4 changes: 3 additions & 1 deletion Doc/library/gzip.rst
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Expand Up @@ -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
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15 changes: 15 additions & 0 deletions Doc/whatsnew/3.11.rst
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Expand Up @@ -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
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