PEP 837: Extensible JSON serialization

[serhiy-storchaka]

New PEP proposing an extension mechanism for the json encoder at
three levels: the __json__()/__raw_json__() protocol (library
level), a copyreg.json() registry following the copyreg.pickle()
precedent (application level), and a per-encoder dispatch_table
attribute mirroring pickle.Pickler (call level).

Reference implementation: python/cpython branch json-customize4
(link: https://github.com/serhiy-storchaka/cpython/tree/json-customize4).

The idea has been discussed repeatedly since 2010; see the PEP's
Motivation for the tracker issues and the python-ideas /
discuss.python.org threads:
2010,
2019,
2020,
2022 (registry),
2022–2024 (__json__).

Basic requirements (all PEP Types)

• Read and followed PEP 1 & PEP 12
• File created from the latest PEP template
• PEP has next available number, & set in filename (pep-NNNN.rst), PR title (PEP 123: <Title of PEP>) and PEP header
• Title clearly, accurately and concisely describes the content in 79 characters or less
• Core dev/PEP editor listed as Author or Sponsor, and formally confirmed their approval
• Author, Status (Draft), Type and Created headers filled out correctly
• PEP-Delegate, Topic, Requires and Replaces headers completed if appropriate (none applicable)
• Required sections included
  • Abstract (first section)
  • Copyright (last section; exact wording from template required)
• Code is well-formatted (PEP 7/PEP 8) and is in code blocks, with the right lexer names if non-Python
• PEP builds with no warnings, pre-commit checks pass and content displays as intended in the rendered HTML
• Authors/sponsor added to .github/CODEOWNERS for the PEP

Standards Track requirements

• PEP topic discussed in a suitable venue with general agreement that a PEP is appropriate
• Suggested sections included (unless not applicable)
  • Motivation
  • Specification
  • Rationale
  • Backwards Compatibility
  • Security Implications
  • How to Teach This
  • Reference Implementation
  • Rejected Ideas
  • Open Issues

🤖 Generated with Claude Code

[read-the-docs-community]

Documentation build overview

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4 files changed

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± index.html
± numerical/index.html
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[serhiy-storchaka]

Discussion thread: https://discuss.python.org/t/pep-837-extensible-json-serialization/108124 (Discussions-To and Post-History headers added).

[hugovk]

I probably won't have time for a full review due to EuroPython, here's a couple of suggestions.

[hugovk]

Suggested change

	Created: 11-Jul-2026
	Created: 12-Jul-2026

[serhiy-storchaka]

I wrote it 11-Jul-2026, but submitted next morning 12-Jul-2026. What counts?

[serhiy-storchaka]

Thank you. Fixed rendering and modernized cpython-issue references.