python-peps / pep-3000.txt

PEP: 3000
Title: Python 3000
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum <>
Status: Final
Type: Process
Content-Type: text/x-rst
Created: 05-Apr-2006


This PEP sets guidelines for Python 3000 development.  Ideally, we
first agree on the process, and start discussing features only after
the process has been decided and specified.  In practice, we'll be
discussing features and process simultaneously; often the debate about
a particular feature will prompt a process discussion.


Python 3000, Python 3.0 and Py3K are all names for the same thing.
The project is called Python 3000, or abbreviated to Py3k.  The actual
Python release will be referred to as Python 3.0, and that's
what "python3.0 -V" will print; the actual file names will use the
same naming convention we use for Python 2.x.  I don't want to pick a
new name for the executable or change the suffix for Python source

PEP Numbering

Python 3000 PEPs are numbered starting at PEP 3000.  PEPs 3000-3099
are meta-PEPs -- these can be either process or informational PEPs.
PEPs 3100-3999 are feature PEPs.  PEP 3000 itself (this PEP) is
special; it is the meta-PEP for Python 3000 meta-PEPs (IOW it describe
the process to define processes).  PEP 3100 is also special; it's a
laundry list of features that were selected for (hopeful) inclusion in
Python 3000 before we started the Python 3000 process for real.  PEP
3099, finally, is a list of features that will *not* change.


See PEP 361 [#pep361], which contains the release schedule for Python
2.6 and 3.0.  These versions will be released in lockstep.

Note: standard library development is expected to ramp up after 3.0a1
is released.

I expect that there will be parallel Python 2.x and 3.x releases for
some time; the Python 2.x releases will continue for a longer time
than the traditional 2.x.y bugfix releases.  Typically, we stop
releasing bugfix versions for 2.x once version 2.(x+1) has been
released.  But I expect there to be at least one or two new 2.x
releases even after 3.0 (final) has been released, probably well into
3.1 or 3.2.  This will to some extent depend on community demand for
continued 2.x support, acceptance and stability of 3.0, and volunteer

I expect that Python 3.1 and 3.2 will be released much sooner after
3.0 than has been customary for the 2.x series.  The 3.x release
pattern will stabilize once the community is happy with 3.x.

Compatibility and Transition

Python 3.0 will break backwards compatibility with Python 2.x.

**There is no requirement that Python 2.6 code will run unmodified on
Python 3.0.** Not even a subset.  (Of course there will be a *tiny*
subset, but it will be missing major functionality.)

Python 2.6 will support forward compatibility in the following two

* It will support a "Py3k warnings mode" which will warn dynamically
  (i.e. at runtime) about features that will stop working in Python
  3.0, e.g. assuming that range() returns a list.
* It will contain backported versions of many Py3k features, either
  enabled through __future__ statements or simply by allowing old and
  new syntax to be used side-by-side (if the new syntax would be a
  syntax error in 2.x).

Instead, and complementary to the forward compatibility features in
2.6, there will be a separate source code conversion tool [1]_.  This
tool can do a context-free source-to-source translation.  For example,
it can translate ``apply(f, args)`` into ``f(*args)``.  However, the
tool cannot do data flow analysis or type inferencing, so it simply
assumes that ``apply`` in this example refers to the old built-in

The recommended development model for a project that needs to support
Python 2.6 and 3.0 simultaneously is as follows:

0. You should have excellent unit tests with close to full coverage.
1. Port your project to Python 2.6.
2. Turn on the Py3k warnings mode.
3. Test and edit until no warnings remain.
4. Use the 2to3 tool to convert this source code to 3.0 syntax.
   **Do not manually edit the output!**
5. Test the converted source code under 3.0.
6. If problems are found, make corrections to the **2.6** version
   of the source code and go back to step 3.
7. When it's time to release, release separate 2.6 and 3.0 tarballs
   (or whatever archive form you use for releases).

It is recommended not to edit the 3.0 source code until you are ready
to reduce 2.6 support to pure maintenance (i.e. the moment when you
would normally move the 2.6 code to a maintenance branch anyway).

PS. We need a meta-PEP to describe the transitional issues in detail.

Implementation Language

Python 3000 will be implemented in C, and the implementation will be
derived as an evolution of the Python 2 code base. This reflects my
views (which I share with Joel Spolsky [2]_) on the dangers of complete
rewrites. Since Python 3000 as a language is a relatively mild
improvement on Python 2, we can gain a lot by not attempting to
reimplement the language from scratch. I am not against parallel
from-scratch implementation efforts, but my own efforts will be
directed at the language and implementation that I know best.


Suggestions for additional text for this PEP are gracefully accepted
by the author.  Draft meta-PEPs for the topics above and additional
topics are even more welcome!


.. [1] The 2to3 tool, in the subversion sandbox

.. [2] Joel on Software: Things You Should Never Do, Part I

.. [#pep361] PEP 361 (Python 2.6 and 3.0 Release Schedule)


This document has been placed in the public domain.

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