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PEP 413 -- Faster evolution of the Python Standard Library

PEP: 413
Title: Faster evolution of the Python Standard Library
Author: Nick Coghlan <ncoghlan at gmail.com>
Status: Withdrawn
Type: Process
Created: 2012-02-24
Post-History: 2012-02-24, 2012-02-25
Resolution: TBD

PEP Withdrawal

With the acceptance of PEP 453 meaning that pip will be available to most new Python users by default, this will hopefully reduce the pressure to add new modules to the standard library before they are sufficiently mature.

The last couple of years have also seen increased usage of the model where a standard library package also has an equivalent available from the Python Package Index that also supports older versions of Python.

Given these two developments and the level of engagement throughout the Python 3.4 release cycle, the PEP author no longer feels it would be appropriate to make such a fundamental change to the standard library development process.

Abstract

This PEP proposes the adoption of a separate 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 support 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).

However, 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 introduction of a new kind of CPython release: "standard library releases". As with PEP 407 , this will give CPython 3 kinds of release:

  • Language release: "x.y.0"
  • Maintenance release: "x.y.z" (where z > 0)
  • Standard library release: "x.y (xy.z)" (where z > 0)

Under this scheme, an unqualified version reference (such as "3.3") would always refer to the most recent corresponding language or maintenance release. It will never be used without qualification to refer to a standard library release (at least, not by python-dev - obviously, we can only set an example, not force the rest of the Python ecosystem to go along with it).

Language releases will continue as they are now, as new versions of the Python language definition, along with a new version of the CPython interpreter and the Python standard library. Accordingly, a language release may contain any and all of the following changes:

  • new language syntax
  • new standard library changes (see below)
  • 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 core interpreter eval loop
  • binary incompatible changes to the C ABI (although the PEP 384 stable ABI must still be preserved)
  • bug fixes

Maintenance releases will also continue as they do today, being strictly limited to bug fixes for the corresponding language release. No new features or radical internal changes are permitted.

The new standard library releases will occur in parallel with each maintenance release and will be qualified with a new version identifier documenting the standard library version. Standard library releases may include the following 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)
  • bug fixes from the corresponding maintenance release

Standard library version identifiers are constructed by combining the major and minor version numbers for the Python language release into a single two digit number and then appending a sequential standard library version identifier.

Release Cycle

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       # Maintenance release
3.3 (33.1)  # Standard library release

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

3.3.2       # Maintenance release
3.3 (33.2)  # 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 and standard library releases:

3.3.3       # Maintenance release
3.3 (33.3)  # Standard library release
3.4.0       # Language release

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

3.4.1       # Maintenance release
3.4 (34.1)  # Standard library release

3.4.2       # Maintenance release
3.4 (34.2)  # Standard library release

3.4.3       # Maintenance release
3.4 (34.3)  # Standard library release
3.5.0       # Language release

Programmatic Version Identification

To expose the new version details programmatically, this PEP proposes the addition of a new sys.stdlib_info attribute that records the new standard library version above and beyond the underlying interpreter version. Using the initial Python 3.3 release as an example:

sys.stdlib_info(python=33, version=0, releaselevel='final', serial=0)

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

Python 3.3.0 (33.0, default, Feb 17 2012, 23:03:41)
[GCC 4.6.1]

Security Fixes and Other "Out of Cycle" Releases

For maintenance releases the process of handling out-of-cycle releases (for example, to fix a security issue or resolve a critical bug in a new release), remains the same as it is now: the minor version number is incremented and a new release is made incorporating the required bug fixes, as well as any other bug fixes that have been committed since the previous release.

For standard library releases, the process is essentially the same, but the corresponding "What's New?" document may require some tidying up for the release (as the standard library release may incorporate new features, not just bug fixes).

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 choose between 3.3 (33.1), 3.3 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 release version numbers. I call this a tie.

Novice user, attempting to judge currency of third party documentation

Status quo: minor version differences indicate 18-24 months of language evolution

This PEP: same as status quo for language core, standard library version numbers indicate 6 months of standard library evolution.

PEP 407: minor version differences indicate 18-24 months of language evolution up to 3.3, then 6 months of language evolution thereafter.

Verdict: Since language changes and deprecations can have a much bigger effect on the accuracy of third party documentation than the addition of new features to the standard library, I'm calling this a win for the scheme in this PEP.

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 check 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 changed" markers in the documentation, check against sys.version_info

This PEP: look for "version added" or "version changed" 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 (33.1)", 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 it, and it is easy to request if needed.

CPython release managers, handling a security fix

Status quo: create a new maintenance release incorporating the security fix and any other bug fixes under source control. Also create source releases for any branches open solely for security fixes.

This PEP: same as the status quo for maintenance branches. Also create a new standard library release (potentially incorporating new features along with the security fix). For security branches, create source releases for both the former maintenance branch and the standard library update branch.

PEP 407: same as the status quo for maintenance and security branches, but handling security fixes for non-LTS releases is currently an open question.

Verdict: until PEP 407 is updated to actually address this scenario, a clear win for this PEP.

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 a 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 .

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 (33.1)".

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

The 3.3-compat branch would be closed to normal development at the same time as the 3.3 maintenance branch. The 3.3-compat branch would remain open for security fixes for the same period of time as the 3.3 maintenance branch.

Effect on bugfix cycle

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

If critical bugs are found in a maintenance release, then new maintenance and standard library releases will be created to resolve the problem. The final part of the version number will be incremented for both the language version and the standard library version.

If critical bugs are found in a standard library release that do not affect the associated maintenance release, then only a new standard library release will be created and only the standard library's version number will be incremented.

Note that in these circumstances, the standard library release may include additional features, rather than just containing the bug fix. It is assumed that anyone that cares about receiving only bug fixes without any new features mixed in will already be relying strictly on the maintenance releases rather than using the new standard library releases.

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 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 6-monthly installments rather than delaying them all until the next language definition update, even those changes that are backwards compatible with the previously released version of Python.

The community benefits listed 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. So, during the 3.3 release cycle, we would see:

  • What's New in Python 3.3?
  • What's New in the Python Standard Library 33.1?
  • What's New in the Python Standard Library 33.2?
  • What's New in the Python Standard Library 33.3?

And then finally, we would see the next language release:

  • What's New in Python 3.4?

For the benefit of users that ignore standard library releases, the 3.4 What's New would link back to the What's New documents for each of the standard library releases in the 3.3 series.

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 may 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 ].

Given that the current manually updated NEWS file will be used for the 3.3.0 release, one possible layout for such an approach might look like:

Misc/
  NEWS  # Now autogenerated from news_entries
  news_entries/
    3.3/
      NEWS # Original 3.3 NEWS file
      maint.1/ # Maintenance branch changes
        core/
          <news entries>
        builtins/
          <news entries>
        extensions/
          <news entries>
        library/
          <news entries>
        documentation/
          <news entries>
        tests/
          <news entries>
      compat.1/ # Compatibility branch changes
        builtins/
          <news entries>
        extensions/
          <news entries>
        library/
          <news entries>
        documentation/
          <news entries>
        tests/
          <news entries>
      # Add maint.2, compat.2 etc as releases are made
    3.4/
      core/
        <news entries>
      builtins/
        <news entries>
      extensions/
        <news entries>
      library/
        <news entries>
      documentation/
        <news entries>
      tests/
        <news entries>
      # Add maint.1, compat.1 etc as releases are made

Putting the version information in the directory hierarchy 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.

Other benefits of reduced version coupling

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 features (most notably standard library updates) into user's hands more 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 problems or 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 a simple example, if a full two years was allowed between 3.3 and 3.4, the 3.3 release cycle would end up looking like:

3.2.4       # Maintenance release
3.3.0       # Language release

3.3.1       # Maintenance release
3.3 (33.1)  # Standard library release

3.3.2       # Maintenance release
3.3 (33.2)  # Standard library release

3.3.3       # Maintenance release
3.3 (33.3)  # Standard library release

3.3.4       # Maintenance release
3.3 (33.4)  # Standard library release
3.4.0       # Language release

The elegance of the proposed branch structure and NEWS entry layout is that this decision wouldn't really need to be made until shortly before the planned 3.4 release date. At that point, the decision could be made to postpone the 3.4 release and keep the 3.3 and 3.3-compat branches open after the 3.3.3 maintenance release and the 3.3 (33.3) standard library release, thus adding another standard library release to the cycle. The choice between another standard library release or a full language release would then be available every 6 months after that.

Further increasing the pace of standard library development

As noted in the previous section, one benefit of the scheme proposed in this PEP is that it largely decouples the language release cycle from the standard library release cycle. The standard library could be updated every 3 months, or even once a month, without having any flow on effects on the language version numbering or the perceived stability of the core language.

While that pace of development isn't practical as long as the binary installer creation for Windows and Mac OS X involves several manual steps (including manual testing) and for as long as we don't have separate "<branch>-release" trees that only receive versions that have been marked as good by the stable buildbots, it's still a useful criterion to keep in mind when considering proposed new versioning schemes: what if we eventually want to make standard library releases even faster than every 6 months?

If the practical issues were ever resolved, then the separate standard library versioning scheme in this PEP could handle it. The tagged version number approach proposed in PEP 407 could not (at least, not without a lot of user confusion and uncertainty).

Other Questions

Why not use the major version number?

The simplest and most logical solution would actually be to map the major.minor.micro version numbers to the language version, stdlib version and maintenance release version respectively.

Instead of releasing Python 3.3.0, we would instead release Python 4.0.0 and the release cycle would look like:

4.0.0  # Language release

4.0.1  # Maintenance release
4.1.0  # Standard library release

4.0.2  # Maintenance release
4.2.0  # Standard library release

4.0.3  # Maintenance release
4.3.0  # Standard library release
5.0.0  # Language release

However, the ongoing pain of the Python 2 -> Python 3 transition (and associated workarounds like the python3 and python2 symlinks to refer directly to the desired release series) means that this simple option isn't viable for historical reasons.

One way that this simple approach could be made to work is to merge the current major and minor version numbers directly into a 2-digit major version number:

33.0.0  # Language release

33.0.1  # Maintenance release
33.1.0  # Standard library release

33.0.2  # Maintenance release
33.2.0  # Standard library release

33.0.3  # Maintenance release
33.3.0  # Standard library release
34.0.0  # Language release

Why not use a four part version number?

Another simple versioning scheme would just add a "standard library" version into the existing versioning scheme:

3.3.0.0  # Language release

3.3.0.1  # Maintenance release
3.3.1.0  # Standard library release

3.3.0.2  # Maintenance release
3.3.2.0  # Standard library release

3.3.0.3  # Maintenance release
3.3.3.0  # Standard library release
3.4.0.0  # Language release

However, this scheme isn't viable due to backwards compatibility constraints on the sys.version_info structure.

Why not use a date-based versioning scheme?

Earlier versions of this PEP proposed a date-based versioning scheme for the standard library. However, such a scheme made it very difficult to handle out-of-cycle releases to fix security issues and other critical bugs in standard library releases, as it required the following steps:

  1. Change the release version number to the date of the current month.
  2. Update the What's New, NEWS and documentation to refer to the new release number.
  3. Make the new release.

With the sequential scheme now proposed, such releases should at most require a little tidying up of the What's New document before making the release.

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 offer more direct benefits to end users.

There are also other benefits to separate versioning (as described above) that are not directly related to the question of binary compatibility with third party C extensions.

Why no binary compatible additions to the C ABI in standard library releases?

There's a case to be made that additions to the CPython C ABI could reasonably be permitted in standard library releases. This would give C extension authors the same freedom as any other package or module author to depend either on a particular language version or on a standard library version.

The PEP currently associates the interpreter version with the language version, and therefore limits major interpreter changes (including C ABI additions) to the language releases.

An alternative, internally consistent, approach would be to link the interpreter version with the standard library version, with only changes that may affect backwards compatibility limited to language releases.

Under such a scheme, the following changes would be acceptable in standard library releases:

  • Standard library updates
    • new features in pure Python modules
    • new features in C extension modules (subject to PEP 399 compatibility requirements)
    • new features in language builtins
  • Interpreter implementation updates
    • binary compatible additions to the C ABI
    • changes to the compilation toolchain that do not affect the AST or alter the bytecode magic number
    • changes to the core interpreter eval loop
  • bug fixes from the corresponding maintenance release

And the following changes would be acceptable in language releases:

  • new language syntax
  • any updates acceptable in a standard library release
  • new deprecation warnings
  • removal of previously deprecated features
  • changes to the AST
  • changes to the emitted bytecode that require altering the magic number
  • binary incompatible changes to the C ABI (although the PEP 384 stable ABI must still be preserved)

While such an approach could probably be made to work, there does not appear to be a compelling justification for it, and the approach currently described in the PEP is simpler and easier to explain.

Why not separate out the standard library entirely?

A concept that is occasionally discussed is the idea of making the standard library truly independent from the CPython reference implementation.

My personal opinion is that actually making such a change would involve a lot of work for next to no pay-off. CPython without the standard library is useless (the build chain won't even run, let alone the test suite). You also can't create a standalone pure Python standard library either, because too many "standard library modules" are actually tightly linked in to the internal details of their respective interpreters (for example, the builtins, weakref , gc , sys , inspect , ast ).

Creating a separate CPython development branch that is kept compatible with the previous language release, and making releases from that branch that are identified with a separate standard library version number 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
Source: https://github.com/python/peps/blob/master/pep-0413.txt