python-peps/pep-0413.txt

PEP: 413
Title: Faster evolution of the Python Standard Library
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>
Status: Draft
Type: Process
Content-Type: text/x-rst
Created: 2012-02-24
Post-History: 2012-02-24, 2012-02-25
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


User Scenarios
==============

The versioning scheme proposed above is based on a number of user scenarios
that are likely to be encountered if this scheme is adopted. In each case,
the scenario is described for both the status quo (i.e. slow release cycle)
the versioning scheme in this PEP and the free wheeling minor version number
scheme proposed in PEP 407.

To give away the ending, the point of using a separate version number is that
for almost all scenarios, the important number is the *language* version, not
the standard library version. Most users won't even need to care that the
standard library version number exists. In the two identified cases where
it matters, providing it as a separate number is actually clearer and more
explicit than embedding the two different kinds of number into a single
sequence and then tagging some of the numbers in the unified sequence as
special.


Novice user, downloading Python from python.org in March 2013
-------------------------------------------------------------

**Status quo:** must choose between 3.3 and 2.7

**This PEP:** must first choose between 3.3 (13.02), 3.3 (12.08) and 2.7.

**PEP 407:** must choose between 3.4, 3.3 (LTS) and 2.7.

**Verdict:** explaining the meaning of a Long Term Support release is about as
complicated as explaining the meaning of the proposed standard library
version numbers. I call this a tie.


Novice user, looking for an extension module binary release
-----------------------------------------------------------

**Status quo:** look for the binary corresponding to the Python version you are
running.

**This PEP:** same as status quo.

**PEP 407 (full releases):** same as status quo, but corresponding binary version
is more likely to be missing (or, if it does exist, has to be found amongst
a much larger list of alternatives).

**PEP 407 (ABI updates limited to LTS releases):** all binary release pages will
need to tell users that Python 3.3, 3.4 and 3.5 all need the 3.3 binary.

**Verdict:** I call this a clear win for the scheme in this PEP. Absolutely
nothing changes from the current situation, since the standard library
version is actually irrelevant in this case (only binary extension
compatibility is important).


Extension module author, deciding whether or not to make a binary release
-------------------------------------------------------------------------

**Status quo:** unless using the PEP 384 stable ABI, a new binary release is
needed every time the minor version number changes.

**This PEP:** same as status quo.

**PEP 407 (full releases):** same as status quo, but becomes a far more
frequent occurrence.

**PEP 407 (ABI updates limited to LTS releases):** before deciding, must first
look up whether the new release is an LTS release or an interim release. If
it is an LTS release, then a new build is necessary.

**Verdict:** I call this another clear win for the scheme in this PEP. As with
the end user facing side of this problem, the standard library version is
actually irrelevant in this case. Moving that information out to a
separate number avoids creating unnecessary confusion.


Python developer, deciding priority of eliminating a Deprecation Warning
------------------------------------------------------------------------

**Status quo:** code that triggers deprecation warnings is not guaranteed to
run on a version of Python with a higher minor version number.

**This PEP:** same as status quo

**PEP 407:** unclear, as the PEP doesn't currently spell this out. Assuming the
deprecation cycle is linked to LTS releases, then upgrading to a non-LTS
release is safe but upgrading to the next LTS release may require avoiding
the deprecated construct.

**Verdict:** another clear win for the scheme in this PEP since, once again, the
standard library version is irrelevant in this scenario.


Alternative interpreter implementor, updating with new features
---------------------------------------------------------------

**Status quo:** new Python versions arrive infrequently, but are a mish-mash of
standard library updates and core language definition and interpreter
changes.

**This PEP:** standard library updates, which are easier to integrate, are
made available more frequently in a form that is clearly and explicitly
compatible with the previous version of the language definition. This means
that, once an alternative implementation catches up to Python 3.3, they
should have a much easier time incorporating standard library features as
they happen (especially pure Python changes), leaving minor version number
updates as the only task that requires updates to their core compilation and
execution components.

**PEP 407 (full releases):** same as status quo, but becomes a far more
frequent occurrence.

**PEP 407 (language updates limited to LTS releases):** unclear, as the PEP
doesn't currently spell out a specific development strategy. Assuming a
3.3 compatibility branch is adopted (as proposed in this PEP), then the
outcome would be much the same, but the version number signalling would be
slightly less clear (since you would have to look up to see if a particular
release was an LTS release or not).

**Verdict:** while not as clear cut as some previous scenarios, I'm still
calling this one in favour of the scheme in this PEP. Explicit is better than
implicit, and the scheme in this PEP makes a clear split between the two
different kinds of update rather than adding a separate "LTS" tag to an
otherwise ordinary release number. Tagging a particular version as being
special is great for communicating with version control systems and associated
automated tools, but it's a lousy way to communicate information to other
humans.

Python developer, deciding their minimum version dependency
-----------------------------------------------------------

**Status quo:** look for "version added" or "version tagged" markers in the
documentation, check against ``sys.version_info``

**This PEP:** look for "version added" or "version tagged" markers in the
documentation. If written as a bare Python version, such as "3.3", check
against ``sys.version_info``. If qualified with a standard library version,
such as "3.3 (13.02)", check against ``sys.stdlib_info``.

**PEP 407:** same as status quo

**Verdict:** the scheme in this PEP actually allows third party libraries to be
more explicit about their rate of adoption of standard library features. More
conservative projects will likely pin their dependency to the language
version and avoid features added in the standard library releases. Faster
moving projects could instead declare their dependency on a particular
standard library version. However, since PEP 407 does have the advantage of
preserving the status quo, I'm calling this one for PEP 407 (albeit with a
slim margin).


Python developers, attempting to reproduce a tracker issue
----------------------------------------------------------

**Status quo:** if not already provided, ask the reporter which version of
Python they're using. This is often done by asking for the first two lines
displayed by the interactive prompt or the value of ``sys.version``.

**This PEP:** same as the status quo (as ``sys.version`` will be updated to
also include the standard library version), but may be needed on additional
occasions (where the user knew enough to state their Python version, but that
proved to be insufficient to reproduce the fault).

**PEP 407:** same as the status quo

**Verdict:** another marginal win for PEP 407. The new standard library version
*is* an extra piece of information that users may need to pass back to
developers when reporting issues with Python libraries (or Python itself,
on our own tracker). However, by including it in ``sys.version``, many
fault reports will already include, and it is easy to request if needed.


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.

If a release date slips by a month or two, I would keep the planned standard
library version number rather than updating it to reflect the actual release
date.

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``.

The ``3.3-compat`` branch would be closed after the 3.3+13.08 release, as
the next release at that time will be a full language release.

The "version added" and "version changed" markers for any changes made on
the ``3.3-compat`` branch would need to be flagged with both the language
version and the standard library version. For example: "3.3 (13.02)".

Any changes made directly on the ``default`` branch would just be flagged
with "3.4" as usual.


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 are 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/
    news_entries/
      3.3.1/ # Maintenance branch changes
        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>
      3.4.0/ # default branch changes
        language/
          <files for core language changes>
        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/ # 3.3 compatibility branch changes
        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 news_entries

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 easier for *humans* to keep the different versions
in order.


Option: Slowing down the language release cycle
===============================================

The current release cycle is a compromise between the desire for stability
in the core language definition and C extension ABI, and the desire to get
new feature (most notably standard library updates) into users hands quickly.

With the standard library release cycle decoupled (to some degree) from that
of the core language definition, it provides an opportunity to actually 
*slow down* the rate of change in the language definition. The language
moratorium for Python 3.2 effectively slowed that cycle down to *more than 3
years* (3.1: June 2009, 3.3: August 2012) without causing any major
complaints.

The NEWS file management scheme described above is actually designed to
allow us the flexibility to slow down language releases at the same time
as standard library releases become more frequent.

As simple example, if a full two years was allowed between 3.3 and 3.4,
the 3.3 release cycle would be up looking like::

    3.2.4            # Maintenance release
    3.3.0 + 12.08.0  # Language release

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

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

    3.3.3 + 12.08.3  # Maintenance release
    3.3.3 + 14.02.1  # Standard library release

    3.3.4 + 12.08.4  # Maintenance release
    3.4.0 + 14.08.0  # Language release

The elegance of the proposed NEWS entry layout is that this decision
wouldn't need to be made until after the 13.08 standard library release. At
that point, the ``3.3-compat`` branch could be kept open (thus adding
another standard library release to the cycle), or else it could be closed,
committing to the next release being a full language release. The choice
between another standard library release or a full language release would
then be available every 6 months after that.


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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