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shutil: avoid importing bz2, lzma and compression.zstd just to probe availability #154904
Description
shutil imports bz2/lzma/zstd at startup just to check they exist
shutil imports bz2, lzma and compression.zstd at module scope purely to discover whether those formats are available, then throws the bindings away:
try:
import bz2
del bz2
_BZ2_SUPPORTED = True
except ImportError:
_BZ2_SUPPORTED = FalseEvery program importing shutil pays for that, even if it never creates or extracts an archive and shutil is imported transitively by much of the stdlib: on main, each of urllib.request, tempfile, zipfile, tarfile, http.server and venv ends up with shutil in sys.modules.
Net import cost (median of 40 fresh interpreters):
| net cost | |
|---|---|
| import shutil | 12.75 ms |
| import urllib.request | 36.64 ms |
This matters most for short-lived processes on constrained hardware: CLI tools and serverless cold starts, where a few ms per process is worth removing.
I have a patch with tests that cuts this by ~2.5 ms (−19.6%) on import shutil, and will open a PR.
Activity
StanFromIreland commented on Jul 30, 2026
(JFYI @aisk, perf improvements are features)
maxday commented on Aug 6, 2026
@StanFromIreland / @aisk
It seems that the PR has been approved but we're missing a review from the core team, do you know who should I ping?
(Not sure about the SLA, so let me know if there is process/policy) Thanks!
gh-154904: Speed up import of shutil by probing compression extension…
pythongh-154904: Speed up import of shutil by probing compression ext…