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GeezerVison is showing github.com — json.dumps: single circle markers hash lookup · Issue #161661 · python/cpython

json.dumps: single circle markers hash lookup #161661

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json.dumps: single circle markers hash lookup#161661
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extension-modulesC modules in the Modules dirC modules in the Modules dirperformancePerformance or resource usagePerformance or resource usagetype-featureA feature request or enhancementA feature request or enhancement

Description

@aivarsk
aivarsk
opened on Nov 9, 2026
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Feature or enhancement

Right now the code does Contains + SetItem for circular reference checks. That can be done as a single hash lookup with PyDict_SetDefault and checking the dictionary size (memory read).

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

  • gh-161661: json.dumps use a single dict probe for circular-reference markers #161662

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  1. aivarsk
    added
    type-featureA feature request or enhancementA feature request or enhancement
    on Nov 9, 2026

  2. picnixz commented on Nov 9, 2026

    @picnixz
    picnixz
    on Nov 9, 2026
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    How much perf do we gain?

  3. aivarsk commented on Nov 9, 2026

    @aivarsk
    aivarsk
    on Nov 9, 2026
    ContributorAuthor
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    Updated to PyDict_SetDefaultRef for a cleaner approach. On a synthetic case with a lot of containers it gives me ~4.5% improvement. For pyperformance it's insignificant 0.x% change.
    Now with SetDefaultRef I think it's also a cleaner code regardless of perf gains or not.

  4. aisk
    added
    performancePerformance or resource usagePerformance or resource usage
    on Nov 9, 2026
  5. aisk
    added this to JSON issueson Nov 9, 2026
  6. aisk
    added
    extension-modulesC modules in the Modules dirC modules in the Modules dir
    on Nov 9, 2026
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