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gh-62944: Add performance note for out-of-order extraction from compressed archives - #151115
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… compressed archives Extracting members in a different order than they appear in a compressed tarfile requires re-decompressing from the beginning of the stream for each backward seek. Add a note to tarfile.open() documenting this and recommending in-order extraction or use of TarFile.extractall() for best performance.
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ethanfurman
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Jun 8, 2026
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When a compressed tarfile is opened with a block mode (
'r:gz','r:bz2','r:xz','r:zst'), the module wraps the compressed stream and emulatesseeking. Seeking forwards is cheap — decompress until you reach the target
offset. Seeking backwards is expensive — the stream must be re-decompressed
from the beginning, because compressed formats like gzip, bzip2, lzma, and
Zstandard are not random-accessible.
Accessing archive members out of their storage order therefore causes
repeated full re-decompressions: for a 500 MB gzip archive extracted in
reverse order the total bytes decompressed approaches O(n²/2). This came up
in the original bug report (bpo-18744 / gh-62944), where the reporter saw a
~60× slowdown compared to manually opening with
gzip.open().The current documentation is silent on this performance characteristic.
@serhiy-storchakaconfirmed in the original issue: "adding a warning looksreasonable."
Changes
Added a
.. note::block inside thetarfile.open()documentation, immediatelyafter the table of
r:*modes, explaining:TarFile.extractall()used for best performance
Test coverage
Documentation-only change; no test file modifications needed.
NEWS entry
Not required for documentation-only changes per the CPython developer guide.
CLA
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nahcmonmay not be signed. The PR will beheld by the CLA bot until it is signed at https://cla.python.org/.