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gh-154675: Use lazy imports in json package by brittanyrey · Pull Request #156801 · python/cpython · GitHub

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gh-154675: Use lazy imports in json package - #156801

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brittanyrey wants to merge 2 commits into
python:mainfrom
brittanyrey:b-perf-json-lazy-imports
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gh-154675: Use lazy imports in json package#156801
brittanyrey wants to merge 2 commits into
python:mainfrom
brittanyrey:b-perf-json-lazy-imports

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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)

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 Bot mentioned this pull request Sep 1, 2026
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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.

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?

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

picnixz added the pending The issue will be closed if no feedback is provided label Sep 2, 2026
Update Misc/NEWS.d/next/Library/2026-09-01-14-20-00.gh-issue-154675.K…
b227c65 
…q7vZ2.rst

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

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.

johnslavik commented Sep 2, 2026

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I'll close then 👍🏽

johnslavik closed this Sep 2, 2026
brittanyrey deleted the b-perf-json-lazy-imports branch September 2, 2026 20:08
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