[Python-Dev] PEP 413: Faster evolution of the Python Standard Library

Nick Coghlan ncoghlan at gmail.com
Fri Feb 24 18:24:27 CET 2012 

To allow the PEP 407 authors to focus on making the case for doing
full CPython releases every 6 months (including language spec
updates), I've created PEP 413 as a competing proposal.

It leaves the current language versioning (and rate of change) alone,
while adding an additional date based standard library version number
and adopting a new development workflow that will allow "standard
library" releases to be made alongside each new maintenance release.

PEP text is below, or you can read it online:
http://www.python.org/dev/peps/pep-0413/

Cheers,
Nick.

PEP: 413
Title: Faster evolution of the Python Standard Library
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan at gmail.com>
Status: Draft
Type: Process
Content-Type: text/x-rst
Created: 2012-02-24
Post-History: 2012-02-24
Resolution: TBD


Abstract
========

This PEP proposes the adoption of a new date-based versioning scheme for
the standard library (distinct from, but coupled to, the existing language
versioning scheme) that allows accelerated releases of the Python standard
library, while maintaining (or even slowing down) the current rate of
change in the core language definition.

Like PEP 407, it aims to adjust the current balance between measured
change that allows the broader community time to adapt and being able to
keep pace with external influences that evolve more rapidly than the current
release cycle can handle (this problem is particularly notable for
standard library elements that relate to web technologies).

However, it's more conservative in its aims than PEP 407, seeking to
restrict the increased pace of development to builtin and standard library
interfaces, without affecting the rate of change for other elements such
as the language syntax and version numbering as well as the CPython
binary API and bytecode format.


Rationale
=========

To quote the PEP 407 abstract:

    Finding a release cycle for an open-source project is a delicate exercise
    in managing mutually contradicting constraints: developer manpower,
    availability of release management volunteers, ease of maintenance for
    users and third-party packagers, quick availability of new features (and
    behavioural changes), availability of bug fixes without pulling in new
    features or behavioural changes.

    The current release cycle errs on the conservative side. It is adequate
    for people who value stability over reactivity. This PEP is an attempt to
    keep the stability that has become a Python trademark, while offering a
    more fluid release of features, by introducing the notion of long-term
    support versions.

I agree with the PEP 407 authors that the current release cycle of the
*standard library* is too slow to effectively cope with the pace of change
in some key programming areas (specifically, web protocols and related
technologies, including databases, templating and serialisation formats).

However, I have written this competing PEP because I believe that the
approach proposed in PEP 407 of offering full, potentially binary
incompatible releases of CPython every 6 months places too great a burden
on the wider Python ecosystem.

Under the current CPython release cycle, distributors of key binary
extensions will often support Python releases even after the CPython branches
enter "security fix only" mode (for example, Twisted currently ships binaries
for 2.5, 2.6 and 2.7, NumPy and SciPy suport those 3 along with 3.1 and 3.2,
PyGame adds a 2.4 binary release, wxPython provides both 32-bit and 64-bit
binaries for 2.6 and 2.7, etc).

If CPython were to triple (or more) its rate of releases, the developers of
those libraries (many of which are even more resource starved than CPython)
would face an unpalatable choice: either adopt the faster release cycle
themselves (up to 18 simultaneous binary releases for PyGame!), drop
older Python versions more quickly, or else tell their users to stick to the
CPython LTS releases (thus defeating the entire point of speeding up the
CPython release cycle in the first place).

Similarly, many support tools for Python (e.g. syntax highlighters) can take
quite some time to catch up with language level changes.

At a cultural level, the Python community is also accustomed to a certain
meaning for Python version numbers - they're linked to deprecation periods,
support periods, all sorts of things. PEP 407 proposes that collective
knowledge all be swept aside, without offering a compelling rationale for why
such a course of action is actually *necessary* (aside from, perhaps, making
the lives of the CPython core developers a little easier at the expense of
everyone else).

But, if we go back to the primary rationale for increasing the pace of change
(i.e. more timely support for web protocols and related technologies), we can
note that those only require *standard library* changes. That means many
(perhaps even most) of the negative effects on the wider community can be
avoided by explicitly limiting which parts of CPython are affected by the
new release cycle, and allowing other parts to evolve at their current, more
sedate, pace.


Proposal
========

This PEP proposes the addition of a new ``sys.stdlib_info`` attribute that
records a date based standard library version above and beyond the underlying
interpreter version::

    sys.stdlib_info(year=2012, month=8, micro=0, releaselevel='final', serial=0)

This information would also be included in the ``sys.version`` string::

    Python 3.3.0 (12.08.0, default:c1a07c8092f7+, Feb 17 2012, 23:03:41)
    [GCC 4.6.1]

When maintenance releases are created, *two* new versions of Python would
actually be published on python.org (using the first 3.3 maintenance release,
planned for February 2013 as an example)::

    3.3.1 + 12.08.1  # Maintenance release
    3.3.1 + 13.02.0  # Standard library release

A standard library release would just be the corresponding maintenance
release, with the following additional, backwards compatible changes:

* new features in pure Python modules
* new features in C extension modules (subject to PEP 399 compatibility
  requirements)
* new features in language builtins (provided the C ABI remains unaffected)

A further 6 months later, the next 3.3 maintenance release would again be
accompanied by a new standard library release::

    3.3.2 + 12.08.2  # Maintenance release
    3.3.2 + 13.08.1  # Standard library release

Again, the standard library release would be binary compatible with the
previous language release, merely offering additional features at the
Python level.

Finally, 18 months after the release of 3.3, a new language release would
be made around the same time as the final 3.3 maintenance release:

    3.3.3 + 12.08.3  # Maintenance release
    3.4.0 + 14.02.0  # Language release

Language releases would then contain all the features that are not
permitted in standard library releases:

* new language syntax
* new deprecation warnings
* removal of previously deprecated features
* changes to the emitted bytecode
* changes to the AST
* any other significant changes to the compilation toolchain
* changes to the C ABI

The 3.4 release cycle would then follow a similar pattern to that for 3.3::

    3.4.1 + 14.02.1  # Maintenance release
    3.4.1 + 14.08.0  # Standard library release
    3.4.2 + 14.02.2  # Maintenance release
    3.4.2 + 15.02.0  # Standard library release
    3.4.3 + 14.02.3  # Maintenance release
    3.5.0 + 15.08.0  # Language release


Effects
=======

Effect on development cycle
---------------------------

Similar to PEP 407, this PEP will break up the delivery of new features into
more discrete chunks. Instead of whole raft of changes landing all at once
in a language release, each language release will be limited to 6 months
worth of standard library changes, as well as any changes associated with
new syntax.

Effect on workflow
------------------

This PEP proposes the creation of a single additional branch for use in the
normal workflow. After the release of 3.3, the following branches would be
in use::

  2.7         # Maintenance branch, no change
  3.3         # Maintenance branch, as for 3.2
  3.3-compat  # New branch, backwards compatible changes
  default     # Language changes, standard library updates that depend on them

When working on a new feature, developers will need to decide whether or not
it is an acceptable change for a standard library release. If so, then it
should be checked in on ``3.3-compat`` and then merged to ``default``.
Otherwise it should be checked in directly to ``default``.


Effect on bugfix cycle
----------------------

The effect on the bug fix cycle is essentially the same as that on the
workflow for new features - there is one additional branch to pass through
before the change reaches default branch.


Effect on the community
-----------------------

PEP 407 has this to say about the effects on the community:

    People who value stability can just synchronize on the LTS releases which,
    with the proposed figures, would give a similar support cycle (both in
    duration and in stability).

I believe this statement is just plain wrong. Life isn't that simple. Instead,
developers of third party modules and frameworks will come under pressure to
support the full pace of the new release cycle with binary updates, teachers
and book authors will receive complaints that they're only covering an "old"
version of Python ("You're only using 3.3, the latest is 3.5!"), etc.

As the minor version number starts climbing 3 times faster than it has in the
past, I believe perceptions of language stability would also fall (whether
such opinions were justified or not).

I believe isolating the increased pace of change to the standard library,
and clearly delineating it with a separate date-based version number will
greatly reassure the rest of the community that no, we're not suddenly
asking them to triple their own rate of development. Instead, we're merely
going to ship standard library updates for the next language release in
three 6-monthly installments rather than delaying them all, even those that
are backwards compatible with the previously released version of Python.

The community benefits list in PEP 407 are equally applicable to this PEP,
at least as far as the standard library is concerned:

    People who value reactivity and access to new features (without taking the
    risk to install alpha versions or Mercurial snapshots) would get much more
    value from the new release cycle than currently.

    People who want to contribute new features or improvements would be more
    motivated to do so, knowing that their contributions will be more quickly
    available to normal users.

If the faster release cycle encourages more people to focus on contributing
to the standard library rather than proposing changes to the language
definition, I don't see that as a bad thing.


Handling News Updates
=====================


What's New?
-----------

The "What's New" documents would be split out into separate documents for
standard library releases and language releases. If the major version
number only continues to increase once every decade or so, resolving the
eventual numbering conflict can be safely deemed somebody elses problem :)


NEWS
----

Merge conflicts on the NEWS file is already a hassle. Since this PEP
proposes introduction of an additional branch into the normal workflow,
resolving this becomes even more critical. While Mercurial phases will
help to some degree, it would be good to eliminate the problem entirely.

One suggestion from Barry Warsaw is to adopt a non-conflicting
separate-files-per-change approach, similar to that used by Twisted [2_].

For this PEP, one possible layout for such an approach (adopted following
the release of 3.3.0+12.8.0 using the existing NEWS process) might look
like::

  Misc/
    lang_news/
      3.3.1/
        <files for core language changes>
      3.4.0/
        <files for core language changes>
    stdlib_news/
      12.08.1/
        builtins/
          <files for builtin changes>
        extensions/
          <files for extension module changes>
        library/
          <files for pure Python module changes>
        documentation/
          <files for documentation changes>
        tests/
          <files for testing changes>
      13.02.0/
        builtins/
          <files for builtin changes>
        extensions/
          <files for extension module changes>
        library/
          <files for pure Python module changes>
        documentation/
          <files for documentation changes>
        tests/
          <files for testing changes>
    NEWS  # Now autogenerated from lang_news and stdlib_news

Putting the version information in the directory heirarchy isn't strictly
necessary (since the NEWS file generator could figure out from the version
history), but does make it easy for *humans* to keep the different versions
in order.


Why isn't PEP 384 enough?
=========================

PEP 384 introduced the notion of a "Stable ABI" for CPython, a limited
subset of the full C ABI that is guaranteed to remain stable. Extensions
built against the stable ABI should be able to support all subsequent
Python versions with the same binary.

This will help new projects to avoid coupling their C extension modules too
closely to a specific version of CPython. For existing modules, however,
migrating to the stable ABI can involve quite a lot of work (especially for
extension modules that define a lot of classes). With limited development
resources available, any time spent on such a change is time that could
otherwise have been spent working on features that are offer more direct
benefits to end users.


Why not separate out the standard library entirely?
===================================================

Because it's a lot of work for next to no pay-off. CPython without the
standard library is useless (the build chain won't even finish). You
can't create a standalone pure Python standard library, because too many
"modules" are actually tightly linked in to the internal details of the
respective interpreters (e.g. ``weakref``, ``gc``, ``sys``).

Creating a separate development branch that is kept compatible with the
previous feature release should provide most of the benefits of a
separate standard library repository with only a fraction of the pain.


Acknowledgements
================

Thanks go to the PEP 407 authors for starting this discussion, as well as
to those authors and Larry Hastings for initial discussions of the proposal
made in this PEP.

References
==========

.. [1] PEP 407: New release cycle and introducing long-term support versions
   http://www.python.org/dev/peps/pep-0407/

.. [2] Twisted's "topfiles" approach to NEWS generation
   http://twistedmatrix.com/trac/wiki/ReviewProcess#Newsfiles

Copyright
=========

This document has been placed in the public domain.


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-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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