From Fedora Project Wiki

Python 3 as Default

Summary

Up until now, Fedora has used Python 2 as a default Python implementation. This change proposes switching to Python 3. The term "switching" is explained in detail in Scope section.

Owner

  • Name: Slavek Kabrda
  • Email: bkabrda@redhat.com
  • Release notes owner:

Current status

Detailed Description

Python 3 is the next generation of Python programming language. It is currently mature and stable - 3.0 was released in December 2008, current latest stable version is 3.3.2 released in May 2013. The reasons why to switch to Python 3 as a default are mentioned in Benefit to Fedora section, possible issues in Scope. This feature assumes that DNF will be the default package manager in F22, as Yum doesn't and never will work with Python 3. If DNF doesn't make it to Fedora 22, this feature has to be postponed accordingly.

Benefit to Fedora

Python 2.7 (latest Python 2 release, which we also have in Fedora) is currently in maintenance mode only, which means upstream only accepts bugfixes and security fixes, but no new features are being implemented. According to upstream, Python 2.7 is the last release of Python 2 (http://www.python.org/dev/peps/pep-0404/). Support for Python 2 is guaranteed by upstream until May 2015 (http://www.python.org/dev/peps/pep-0373/#maintenance-releases), then it may continue for some time or it may not. Python 3, on the other hand, is actively developed and new features are being added every release.

Fedora already has Python 3 stack, that is parallel to Python 2 stack. The are few benefits of switching the "primary" Python stack:

  • Getting upstream support is not limited by time.
  • Our system tools will be able to switch to Python 3, drop Python 2 support and use new features of Python 3.
  • As a distribution that stays close to upstream, Fedora should help Python community go forward by contributing patches and working closely with upstreams to get this accomplished. Thus this Change is meant to benefit not only Fedora, but also broad Python community.
  • Pushing Python 3 as a default in Fedora should improve perception of Python 3 as a mature language and speed up adoption by upstreams.

Scope

The main goal is switching to Python 3 as a default, in which state:

  • DNF is the default package manager instead of Yum, which only works with Python 2
  • Python 3 is the only Python in minimal buildroot
  • Python 3 is the only Python on LiveCD
  • Anaconda and all of its dependencies run on Python 3
  • cloud-init and all of its dependencies run on Python 3

(see Dependencies for the list of packages that need to be ported)

This will also require revisiting Python guidelines (broader discussion with community and FPC approval - TBD). The result of the discussion will reflect in this feature in further instructions for Fedora packagers.

Work needed to be done:

  • Proposal owners:
    • Discuss changes in Python packaging guidelines with Fedora community and FPC
    • Help various upstreams with migration to Python 3 (already in progress for some time)
    • Help other packagers with introduction of python3- subpackages
  • Other developers:
    • Packagers who own a package that uses Python in any way should make sure that it works with Python 3:
      • If the package works with Python 3, they should add python3- subpackage where appropriate.
      • For some packages that build with Python, like gdb or Rhythmbox, doing python3- subpackages makes no sense. Packagers should only make sure that the packages work with Python 3. There should a single point, at which all these packages will be rebuilt with Python 3, possibly in a Koji side tag.
      • If the package doesn't work with Python 3, packagers should encourage upstreams and send patches.
  • Release engineering:
    • We will use a side tag in Koji and merge it to rawhide only if everything works as expected.
  • Policies and guidelines:
    • As mentioned above, this will require a discussion with community and FPC and alteration of Python packaging guidelines.

Work that should be done for Fedora 21

For Fedora 21, we should focus on helping upstreams with porting to Python 3 and we should introduce python3- subpackages where appropriate. No other changes should be done during this timeframe.

Upgrade/compatibility impact

The Python 2 stack will stay in Fedora, it will just not be the default one. Depending on the modifications done to Python packaging guidelines, this probably means that Python 2 packages will stay the way they are and default tools will drag in python3- dependencies. Upstream recommends, that /usr/bin/python should point to Python 2 runtime for the time being, so if we go with that, there shouldn't be any serious compatibility impact:

  • Users will still be able to install Python 2 packages
  • Both Python 2 and 3 stacks will still live in parallel

Libraries that are built with Python only once (like gdb) may force users to rewrite their custom scripts (that use the bindings) to Python 3.

How To Test

TBD

User Experience

Users shouldn't notice any change in behaviour, except from installation of python3- packages by default.

Dependencies

See https://fedoraproject.org/wiki/User:Churchyard/python3 (our tracking page with notes) or https://bugzilla.redhat.com/show_bug.cgi?id=1014209 (tracking bug).

Contingency Plan

  • Contingency mechanism: (What to do? Who will do it?) Don't merge the koji side-tag into rawhide, revert changes in dist-git.
  • Contingency deadline: Development freeze
  • Blocks release? No

Documentation

http://docs.python.org/dev/howto/pyporting.html

http://docs.python.org/3/howto/cporting.html

https://wiki.gnome.org/PyGObject/IntrospectionPorting

Release Notes

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