Bug report

Bug description:

python -m gzip --fast FILE and python -m gzip --best FILE compress at the default level. main() passes compresslevel to GzipFile only when reading stdin; the file-argument branch calls gzip.open(arg + ".gz", "wb") without it.

head -c 3000000 /dev/urandom | base64 > data
for opt in --fast --best; do cp data data$opt; python -m gzip $opt data$opt; done
ls -l data--fast.gz data--best.gz   # identical sizes, XFL header byte is 0 in both
python -m gzip --fast < data | wc -c   # differs from --best < data, as expected

Expected --fast and --best to produce different output (and XFL 4 and 2) for files, as they do for stdin. The options were added in gh-79150 (GH-9833), which wired them up only for stdin.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-161086: Honor --fast and --best for file arguments in gzip CLI #161087
• [3.15] gh-161086: Honor --fast and --best for file arguments in gzip CLI (GH-161087) #161118
• [3.14] gh-161086: Honor --fast and --best for file arguments in gzip CLI (GH-161087) #161119