protocolbuffers/protobuf

Python: Use multi-phase initialisation (PEP 489)

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#21995 opened on May 28, 2025

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Description

What language does this apply to?

Python

Describe the solution you'd like

Hello,

Please consider adopting multi-phase init for your Python extension modules. In short, multi-phase makes extension modules behave more like pure-Python modules and means that the import system can determine what a module supports before executing it.

Multi-phase initialisation was introduced in PEP 489 for Python 3.5. It replaces the previous 'single-phase' mechanism by splitting the module creation process into creation and execution phases. Quoting from the documentation:

Extension modules created this way behave more like Python modules: the initialization is split between the creation phase, when the module object is created, and the execution phase, when it is populated. The distinction is similar to the __new__() and __init__() methods of classes.

It is the preferred mechanism for extension module initialisation, and we are considering explicitly deprecating single phase init.

Describe alternatives you've considered

The status quo.

A

Contributor guide

Tech stack
pythoncpp
Domain
backendapi
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Python C extension developmentPEP 489 multiphase initializationprotobuf build system for Python
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
The issue requests adopting multi phase initialisation (PEP 489) for Python extension modules in protobuf. The current single phase init should be refactored into separate creation and execution phases, as described in the linked PEP and documentation. The implementation involves modifying the Python C extension files (likely in the python/ directory) and ensuring backward compatibility with Python 3.5+. Since the issue is assigned and has comments, review the assignee's progress and any prior discussion before starting work.