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GeezerVison is showing github.com — gh-154675: Use lazy imports in json package by brittanyrey · Pull Request #156801 · python/cpython

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@brittanyrey

@brittanyrey brittanyrey commented Sep 1, 2026 •
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summary

Apply lazy imports to json package to lift nested imports and optimize the modules.

perf

before after
python -c "import json.tool" 41.09 ms 21.24 ms (−48%)
python -c "import json" 20.98 ms 20.81 ms (−0.8%, neutral)
python -m json.tool f.json 46.06 ms 45.87 ms (−0.4%, neutral)
  • Issue: Improve import time of various stdlib modules #154675

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@brittanyrey
pythongh-154675: Use lazy imports in json.tool
db14e0d 
`argparse` and `_colorize` are only needed once `main()` runs, so import
them lazily. Also hoist the nested `warnings._deprecated` import in
`json.__getattr__` up to a module-level `lazy from`.

`python -c "import json.tool"` drops from 41.1 ms to 21.2 ms (-48%):
interleaved A/B, median of 9 rounds of min-of-25 runs, same binary with
`Lib/json` swapped via `PYTHONPATH`; an unrelated `import csv` control
moved +0.2%.

Running the CLI is unchanged -- `main()` always builds an `ArgumentParser`
and always calls `can_colorize()`, so both modules still get imported.
@bedevere-app bedevere-app Bot mentioned this pull request Sep 1, 2026
Improve import time of various stdlib modules #154675
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@bedevere-app bedevere-app Bot added the awaiting review label Sep 1, 2026
johnslavik
johnslavik approved these changes Sep 1, 2026
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@bedevere-app bedevere-app Bot added awaiting core review and removed awaiting review labels Sep 1, 2026
eendebakpt
eendebakpt reviewed Sep 2, 2026
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Comment thread Misc/NEWS.d/next/Library/2026-09-01-14-20-00.gh-issue-154675.Kq7vZ2.rst Resolved

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@eendebakpt

eendebakpt commented Sep 2, 2026

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Are there any use cases where the improved import time matters? Users importing json.tool will run main() and that will trigger imports of the lazy packages.
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@johnslavik

johnslavik commented Sep 2, 2026 •
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Are there any use cases where the improved import time matters? Users importing json.tool will run main() and that will trigger imports of the lazy packages.

Don't think so. And json.tool has no import-only use cases. Maybe drop this and news entry and leave only the json lazy import?
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@picnixz

picnixz commented Sep 2, 2026 •
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AFAIR, the PEP explicitly said that we do not want to mechanically convert everything to lazy imports. Considering the noise for json itself I wonder about this change. json.tool is likely to be used as a CLI and there is no reason to do "import json.tool" as the public API is empty IIRC
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@picnixz picnixz added the pending The issue will be closed if no feedback is provided label Sep 2, 2026
@brittanyrey @eendebakpt
Update Misc/NEWS.d/next/Library/2026-09-01-14-20-00.gh-issue-154675.K…
b227c65 
…q7vZ2.rst

Co-authored-by: Pieter Eendebak <[email protected]>
@brittanyrey

brittanyrey commented Sep 2, 2026

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Solid call out. Dug in and I completely see what you mean...argparse and colorize will immediately be reified, so this is quite a wash.
I'm happy to abandon this.
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@johnslavik

johnslavik commented Sep 2, 2026

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I'll close then 👍🏽
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@johnslavik johnslavik closed this Sep 2, 2026
@brittanyrey
brittanyrey deleted the b-perf-json-lazy-imports branch September 2, 2026 20:08
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@johnslavik johnslavik johnslavik approved these changes

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@brittanyrey @eendebakpt @johnslavik @picnixz
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