New submission from William Chargin <wchar...@gmail.com>:
The `gzip` module properly uses the user-specified compression level to
control the underlying zlib stream compression level, but always writes
metadata that indicates that the maximum compression level was used.
Repro:
```
import gzip
blob = b"The quick brown fox jumps over the lazy dog." * 32
with gzip.GzipFile("fast.gz", mode="wb", compresslevel=1) as outfile:
outfile.write(blob)
with gzip.GzipFile("best.gz", mode="wb", compresslevel=9) as outfile:
outfile.write(blob)
```
Run this script, then run `wc -c *.gz` and `file *.gz`:
```
$ wc -c *.gz
82 best.gz
84 fast.gz
166 total
$ file *.gz
best.gz: gzip compressed data, was "best", last modified: Sun Jan 19 20:15:23
2020, max compression
fast.gz: gzip compressed data, was "fast", last modified: Sun Jan 19 20:15:23
2020, max compression
```
The file sizes correctly reflect the difference, but `file` thinks that
both archives are written at max compression.
The error is that the ninth byte of the header in the output stream is
hard-coded to `\002` at Lib/gzip.py:260 (as of 558f07891170), which
indicates maximum compression. The correct value to indicate maximum
speed is `\004`. See RFC 1952, section 2.3.1:
<https://tools.ietf.org/html/rfc1952>
Using GNU `gzip(1)` with `--fast` creates the same output file as the
one emitted by the `gzip` module, except for two bytes: the metadata and
the OS (the ninth and tenth bytes).
----------
components: Library (Lib)
files: repro.py
messages: 360268
nosy: wchargin
priority: normal
severity: normal
status: open
title: gzip metadata fails to reflect compresslevel
versions: Python 2.7, Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9
Added file: https://bugs.python.org/file48853/repro.py
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue39389>
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