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Python 3


A packaged version of python 3.* will be provided within Fedora 13 as an optional component, parallel-installable with the Python 2 stack.

The critical system components that use Python 2 (yum, anaconda) shall continue to use Python 2.


Current status

  • Targeted release: Fedora 13
  • Last updated: 2009-10-23
  • Percentage of completion: 10%

Tracker bug: https://bugzilla.redhat.com/showdependencytree.cgi?id=530636

Package review for python3: NEW: https://bugzilla.redhat.com/show_bug.cgi?id=526126

Proposed packaging guidelines for python 3: PackagingDrafts/Python3

Detailed Description

Python 3 is intended by upstream to be the future of Python, but we have many critical components that use Python 2. Python 2 and Python 3 are sufficiently different that we need both (try writing "print" in each). Python 2 will be around for a long time.

An interesting summary of Python 3 adoption can be seen here: http://renesd.blogspot.com/2009/09/py3kpython3-more-than-one-year-on-096.html

How to do this? I propose that Fedora shall have separate, parallel-installable Python 2 and Python 3 stacks. I believe we can get things to the point where on a Fedora box you'd be able to install both stacks, and have some processes running python 2 code, and some running python 3, simultaneously.

Where I would draw the line is on having both python 2 and python 3 running within the same _process_: the two libraries share most of their symbol names, but with differing implementations, and the result of trying to dynamically link the two into the same address space would be highly unstable.

As an example, you'd be able to install both mod_python and mod_python3 rpms, but you wouldn't be able to (sanely) configure httpd to have both running simultaneously (I guess we should add a run-time warning for this case)

Benefit to Fedora

Fedora has long been a great platform for doing Python 2 development, but we don't yet have Python 3. Having Python 3 available via rpms will extend our Python coverage.

Note also:

  • Python 3.0 was released almost 10 months ago, on 2008-12-03, and the latest release of the 3.* branch is 3.1.1, released on 2009-08-17.
  • Other distros have python 3, though not necessarily with anything "on top" resembling the full python 2 stack.
  • We have a working, valuable python 2 stack, which is used by critical system components (yum and anaconda): we must not destabilize the python 2 stack.
  • Python 3 is sufficiently different from python 2 that we need them to be independent software stacks.


A plan for packaging Python 3 and Python 3 modules was posted to fedora-devel-list here: https://www.redhat.com/archives/fedora-devel-list/2009-October/msg00054.html

A rough plan for Fedora 13 might be:

  • get python3 packaged in a manner compatible with the above
  • (persuade /usr/lib/rpm/brp-python-bytecompile to use the correct python when building rpms containing .py files)
  • get rpm bindings working with python3
  • get some useful components working e.g. a web stack: Django, TurboGears etc (though e.g. Django's py3k support is a long way off IIRC); ideas?
    • Web stacks may not be the best thing to focus on since production largely wants to use mod_wsgi... which we won't be able to mix between py2 and py3 stacks. --abadger1999 01:52, 13 November 2009 (UTC)
  • solidify packaging guidelines for python 2 vs python 3 once we've got some experience with the above and hopefully proven the techniques
  • look at porting major components over to python3 (but probably don't actually do this for F13; leave python 2 as the critical component, I suspect): yum (rpm), anaconda
  • add the new packages to the comps file. It's not yet clear what changes to the comps file are required.

At a minimum, we'd want python 3 to be available as an rpm via yum.

Module status

(alphabetical by python module name)

Python Module Fedora Python 2 package Upstream status of Python 3 Fedora Python 3 package
coverage python-coverage Upstream releasing dual-purpose tarballs rhbz536948
gtk pygtk2 Not yet ready: https://bugzilla.gnome.org/show_bug.cgi?id=566641 Need to finish upstream work
lxml python-lxml Upstream releasing dual-purpose tarballs In CVS; ready to build rhbz533290
MySQLdb MySQL-python
psycopg2 python-psycopg2
PyKDE4 PyKDE4 (from the kdebindings srpm)
PyQt4 PyQt4
rpm rpm-python (subpackage of "rpm") dmalcolm and pmatilai ported the C extension for librpm to work with both python 2 and 3; fixes are in upstream git, waiting for a release: http://dmalcolm.livejournal.com/3340.html
sqlalchemy python-sqlalchemy Upstream aiming for py3k support in 0.6, from a separate tarball http://groups.google.com/group/sqlalchemy/browse_thread/thread/055f78bc78801c13 http://www.sqlalchemy.org/trac/browser/sqlalchemy/trunk/README.py3k http://www.sqlalchemy.org/download.html
sip sip added (single tarball)

Other python consumers

(alphabetical by package name)

Fedora Python 2 Package Upstream status of Python 3 Status in Fedora
blender Blender 2.5 migrated its embedded version of python from Python 2 to Python 3.

However, Blender 2.6's schedule suggests this version might not be regarded as stable in time for Fedora 13

(Upstream ships with a bundled copy of 3.1; we would want to build against a system copy of Python 3)

mod_wsgi mod_wsgi version 3.0 and later supports Python 3.1 and later: http://code.google.com/p/modwsgi/wiki/SupportForPython3X http://code.google.com/p/modwsgi/wiki/ChangesInVersion0300

How To Test

Testing Python 3 functionality

You should be able to test the new python 3 stack using:

$ yum install python3

and then starting the python 3 interpreter using

$ python3

You should then see the Python 3 interpreter:

Python 3.1.1 (r311:74480, Sep 29 2009, 17:01:17) 
[GCC 4.1.2 20071124 (Red Hat 4.1.2-42)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

Follow the "Dive Into Python 3" tutorial

If you've installed the python3 rpm, it should be possible for you to follow the tutorial on Python 3 given here: http://diveintopython3.org/

This is an excellent tutorial, and working through it with our RPMs makes a good test that all is working as expected (obviously it would be a major task to work through the whole book, but at least you'd learn a lot in the process).

Ensuring non-brokenness of Python 2 stack

The Python 3 stack should not interfere with the Python 2 stack; the latter is used by many components. The most critical ones here are yum and anaconda.

Please verify that "yum" still works after installing python3.

If there are other python 2 rpms that you are familiar with, please check that they still work after installing the python3 rpm.

Uninstallability of Python 3

  • It ought to be possible for you to remove python3 without affecting other components
    • A better phrasing of this point would be: It ought to be possible to remove python3 without affecting any python2 components. If some app gets ported to python3 upstream in F13, I do think we should allow it.
  • Try uninstalling python 3 using "yum remove python3".
  • Verify that other components still work as expected

Upstream regression test suite

Run Python's regression test suite (within the python3-test subpackage)


as root and verify that only expected errors occur.

As of 2009-10-27, if you run the above command as root, the only failure should be in test_httpservers (appears to be a permissions issue with how we've packaged the test scripts). You may also see an error in test_socket if your machine's hostname isn't set up in DNS/hosts.

If you run using sudo, you may see an extra failure in test_distutils (test_check_environ).

If you run as a non-root user, you'll see additional failures due to tests that expect write access to certain directories

Static testing

Verify that for all subpackages, on all architectures, that the python and python3 packages are independent:

  • verify that there aren't any paths (files/dirs/links) owned by both packages
  • verify that there aren't any "Provides" provided by both packages

A script to automatically verify this can be seen at https://bugzilla.redhat.com/show_bug.cgi?id=526126#c17

User Experience

It should be possible to start a Python 3 interpreter by invoking:

$ python3
Python 3.1.1 (r311:74480, Sep 29 2009, 17:01:17) 
[GCC 4.1.2 20071124 (Red Hat 4.1.2-42)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

The python 3 install should be independent of the python 2 install.

It should be possible to have some processes on the system running python 2 code, and some processes on the system running python 3 code. However you won't be able to run both python 2 and python 3 within the same process.


Getting the core "python3" package into Fedora requires us:

  • to come up with packaging rules for Python 3.
  • to ensure our packaging tools can cope with multiple, parallel python installs:

Building out the python 3 stack beyond the core interpreter component will involve working closely with many different upstream projects. The more we can do this, the better, but it's not necessary for achieving the main goal of having the core python 3 interpreter available via rpm.

rpmbuild changes

rpmbuild currently automatically invokes /usr/lib/rpm/brp-python-bytecompile without arguments, thus using "/usr/bin/python" to byte-compile every .py file that's in a package payload, generating a bytecode file for the 2.6 ABI.

This breaks down if we're to deal with multiple python runtimes:

  • the magic ABI value stored in the .pyc/.pyo file needs to match that of the python binary. .pyc files below /usr/lib/python2.6 need to have the 2.6 magic value, whereas .pyc files below /usr/lib/python3.1 need to have the 3.1 magic value.
  • the syntax of the python language can change (e.g. 2 vs 3), and the compilation can fail with syntax errors if you use the wrong python runtime

Status: I work around this "by hand" in the python3.spec. Need to fix "properly" before building out python3- stack with further packages.

rpmlint changes

I've written a new test for rpmlint to do some of the .pyc checking. See: https://www.zarb.org/pipermail/rpmlint-discuss/2009-October/000775.html (I also uploaded the patch here: http://dmalcolm.fedorapeople.org/rpmlint/add-tests-for-python-bytecode-files.patch )

Status: patch sent upstream, not yet in our Fedora rpmlint rpms.

Contingency Plan

We will be adding all-new RPMs to Fedora. If there's a problem, we can simply choose to not include them without impacting the rest of the Fedora 13 release.

Note: In order to enact this contingency plan, we must do separate packages for python2 and python3. If we keep pursuing the one srpm to build both python2 and python3 route and we have to back out the python3 stack, we also have to backout the python3 changes in the srpms. The same is also true of bindings to C libraries. --abadger1999 02:04, 13 November 2009 (UTC)

Good point. I've conditionalized support throughout the shared specfile changes, adding a with_python3 boolean. Is this an acceptable compromise? --Dmalcolm 18:06, 13 November 2009 (UTC)


An excellent tutorial on Python 3 can be seen at http://diveintopython3.org/

Note: We may want to package this since we have the python2 version of diveintopython packaged already.

--Dmalcolm 23:24, 19 November 2009 (UTC) help packaging it would be appreciated! I tried grabbing this from Mercurial, but the toolchain/licensing appeared to have changed greatly since the python2 version of the book

Release Notes

  • Fedora now includes a Python 3 runtime, parallel-installable with our existing Python 2 runtime.

FIXME: obviously this will have to change based on exactly how much of the stack I manage to build out.

Comments and Discussion