[REJECTED] PEP 605: A rolling feature release stream for CPython

Hi folks,

Prompted by Łukasz’s work on the annual cadence proposal in PEP 602, and Steve Dower’s initial write-up of a fast track/slow track proposal in the related discussion thread, Steve and I are happy to present PEP 605, a proposal to produce a more regular series of production ready releases from the master git branch: https://www.python.org/dev/peps/pep-0605/

I’ve withdrawn my previous proposal in PEP 598 in favour of this one.

The major difference between this PEP and its predecessors in PEP 598 and PEP 602 is that it focuses on the question of “What would we need to change in order to make our beta releases production ready for at least some environments, and reduce our feature delivery latency that way?”, whereas the other two PEPs both have a much larger potential impact on users that are actually happy with the way our final releases are currently managed.

(Edit: comments prior to [REJECTED] PEP 605: A rolling feature release stream for CPython mostly relate to an earlier draft of the proposal)

PEP: 605
Title: A rolling feature release stream for CPython
Author: Steve Dower steve.dower@python.org, Nick Coghlan ncoghlan@gmail.com


Rather than proposing more frequent full CPython releases (as PEP 602 does),
or a policy change to allow backwards compatible feature additions later in a
release series (as PEP 598 does), this PEP instead proposes that we create a
rolling stream of production-ready beta releases, together with alpha releases
that are specifically designed to be suitable as platforms for building extension modules
and wheel archives that are compatible with the subsequent beta releases.

The key desired outcome of this proposal is that the usage guidance given for
beta releases would become “suitable for production use only in environments
with sufficiently robust compatibility testing and operational monitoring
capabilities”, rather than current unqualified “not for production use”.

Similarly, the guidance given for alpha releases would be amended to state
“intended for library compatibility testing and the creation of ABI compatible
binary artifacts”, rather than simply saying “not for production use”.

The PEP authors believe these outcomes can be achieved by amending CPython’s
pre-release management process as described in the Proposal section below.

This PEP also proposes that the frequency of X.Y.0 releases be adjusted to
begin each new release series in August every two years (starting in 2021,
around two years after the release of Python 3.8.0).

Example Future Release Schedules

Under this proposal, Python 3.9.0a1 would be released in December 2019, two
months after the Python 3.8.0 baseline feature release in October 2019.

Assuming no further breaking changes were made to the full CPython ABI, the
3.9.0b2 release would then follow 2 months later in February 2020, continuing
through to 3.9.0b9 in April 2021.

Any time a breaking change to the full CPython ABI was introduced, the first
pre-release that included it would be marked as an alpha release.

3.9.0rc1 would be published in June 2021, 3.9.0rc2 in July 2021, and then
the full release published as 3.9.0 in August 2021.

The cycle would start over again in October 2021, with the publication
of 3.10.0a1 (4 months after the creation of the 3.9.x maintenance branch).

The exact schedule of maintenance releases would be up to the release team,
but assuming maintenance releases of 3.9.x were also to occur every other month
(offset from the 3.10.0 beta releases), the overall release timeline
would look like:

  • 2019-12: 3.9.0a1
  • 2020-02: 3.9.0b2
  • … beta (or alpha) releases every other month
  • 2021-04: 3.9.0b9
  • 2021-06: 3.9.0rc1 (feature freeze, ABI freeze, pyc format freeze)
  • 2021-07: 3.9.0rc2
  • 2021-08: 3.9.0
  • 2021-09: 3.9.1, 3.8.x (final 3.8.x binary maintenance release)
  • 2021-10: 3.10.0a1
  • 2021-11: 3.9.2
  • 2021-12: 3.10.0b2
  • … beta (or alpha) and maintenance releases continue in alternate months
  • 2023-04: 3.10.0b10
  • 2023-05: 3.9.11
  • 2023-06: 3.10.0rc1 (feature freeze, ABI freeze, pyc format freeze)
  • 2023-07: 3.10.0rc2, 3.9.12
  • 2023-08: 3.10.0
  • 2023-09: 3.10.1, 3.9.13 (final 3.9.x binary maintenance release)
  • 2023-10: 3.11.0a1
  • 2023-12: 3.11.0b2
  • … etc

If we assume two additional pre-releases were made that introduced breaking
changes to the full CPython ABI in the 3.9.0a5 and 3.9.0a7 releases, then the
overall calendar would look like:

Figure 1. Impact of the pre-release process changes on the calendar.

There are always two or three active maintenance branches in this model,
which preserves the status quo in that respect. The major difference is that
we would start encouraging publishers to provide pre-built binaries for the
pre-freeze rolling releases in addition to providing them for the stable
maintenance branches.

Figure 2. Testing matrix in the 18-month cadence vs. the 24-month

Package publishers targeting the full CPython ABI that choose to provide
pre-built binaries for the rolling pre-freeze releases would at least need
to build new wheel archives following the 3.9.0a1 release. Whether they needed
to publish updated binaries after subsequent alpha releases (e.g. 3.9.0a5 or
3.9.0a7 releases in the example timeline) would depend on whether or not they
were actually affected by the ABI breaks in those later releases.

As with the status quo, all package publishers wishing to provide pre-built
binaries for the final release will need to build new wheel archives following
the ABI freeze date. Unlike the status quo, this date will be clearly marked
by the publication of the first release candidate, and it will occur early
enough to give publishers a couple of months to get ready for the final release.


The current CPython pre-release and release management processes were developed
in an era where automated continuous integration and operational monitoring
systems were still relatively immature. Since that time, many organisations
have adopted deployment models that allow them to incorporate new CPython
feature releases without adding substantially more risk than they incur for any
other code change. Newer deployment models, such as lightweight task specific
application containers, also make it easier to combine an application with a
language runtime in a CI pipeline, and then keep them together until the entire
container image is later replaced by an updated one.

In light of those changes in the wider environment, PEP 602 has proposed
reducing the feature delivery latency for the Python standard library and
CPython reference interpreter by increasing the frequency of CPython feature
releases from every 18-24 months to instead occur every 12 months.

Unfortunately, for many organisations, the cost of adopting a new Python release
doesn’t automatically scale down with a reduced number of changes in the release,
as the primary costs aren’t associated with resolving any discovered issues;
the primary costs are associated with the search for issues. This search may
involve manual testing of software systems, human review of written materials,
and other activities where the time required scales with the size of the
existing system, rather than with the number of changes between the versions of

For third party library developers, costs are primarily associated with the
number of distinct Python versions in widespread usage. This currently tends
to be influenced by a combination of which releases are still actively
maintained by python-dev, and which releases are the latest versions offered
by particular redistributors (with the Debian, Ubuntu LTS, and RHEL/CentOS
system Python versions being particularly popular development targets). In
addition to the basic CI cost of testing against more Python versions, having
more variants in widespread use can make it more difficult to determine when a
fault report is an actual error in the project, or an issue in the reporting
user’s environment.

PEP 602 proposes that affected organisations and projects simply switch to
adopting every second or third CPython release, rather than attempting to adopt
every release, but that creates its own set of new problems to be resolved, both
practical (e.g. deprecations would need to cover more than one release if we’re
expecting users to routinely skip releases) and cultural (e.g. with a larger
number of versions in active use, there is a much higher chance that open source
library maintainers will receive bug reports that only occur on Python versions
that they’re not using themselves).

PEP 598 was an initial attempt by one of the authors of this PEP to propose
an alternative scheme to reduce feature delivery latency by adopting a
semantic versioning style policy that allowed for the incremental delivery of
backwards compatible features within a release series, until that series
reached feature complete status. That variant still had the undesirable
consequence of imposing visible changes on end users that are happy enough
with the current release management model.

This PEP takes the view that both PEP 598 and PEP 602 share a common flaw: they
are attempting to satisfy the needs of two quite distinct audiences within the
constraints of a single release model, which results in conflicting design
requirements, and the need for awkward trade-offs between those conflicting
requirements. The proposal in this PEP aims to avoid that flaw by proposing the
creation of two distinct production-ready release streams, with the existing
release stream being largely left alone, while the new release stream is
tailored towards the audience that would most benefit from a reduction in
feature delivery latency.

Aims of this Proposal

The core of the proposal in this PEP is changing the CPython pre-release process
to produce a rolling stream of incremental feature releases at a regular
cadence, and to ensure that most of those builds offer a sufficient level of
stability as to be suitable for use in appropriately managed production systems.

By adopting this approach, the proposal aims to provide an improved outcome
for almost all Python users and contributors:

  • for users of the new incremental feature release stream, targeting the
    pre-release phase allows for even lower feature delivery latency than the
    annual cadence proposed in PEP 602;
  • for core developers working on new features, increased frequency and adoption
    of pre-releases should improve pre-release feedback cycles;
  • for users of the established release stream, the increased adoption and
    improved feedback cycles during the pre-release period should result in
    increased feature maturity at the time of its first X.Y.0 release, as well
    as higher levels of ecosystem readiness;
  • for Python library maintainers, the rolling stream of pre-releases will
    hopefully provide more opportunities to identify and resolve design issues
    before they make it into a full stable release than is offered by the current
    pre-release management process; and
  • for developers of alternative Python implementations, the rolling stream of
    pre-releases may provide an additional incentive for extension module authors
    to migrate from the full CPython ABI to the Python stable ABI, which would
    also serve to make more of the ecosystem compatible with implementations that
    don’t emulate the full CPython C API.

That said, it is acknowledged that not all the outcomes of this proposal will be
beneficial for all members of the wider Python ecosystem:

  • for Python library maintainers, both this PEP and PEP 602 would likely
    result in user pressure to support the faster release cadence. While this PEP
    attempts to mitigate that by clearly marking which pre-releases include
    potentially breaking changes to the full CPython C ABI, and PEP 602 attempts
    to mitigate it by keeping the minimum time between full releases at
    12 months, it isn’t possible to eliminate this downside completely;
  • for third party extension module maintainers, both this PEP and PEP 602 would
    likely result in user pressure to start supporting the stable ABI in order to
    provide wheel archives that work on the new version as soon as it is
    available. Whether that’s a net negative or not will depend on how the request
    is presented to them (it could be a positive if the request comes in the form
    of a courteous contribution to their project from a developer interested in
    supporting the rolling pre-freeze releases);
  • for some users of the established release stream that rely on the
    availability of pre-built wheel archives, switching to adopting a new release
    every 12 months may be an acceptable rate increase, while moving consistently
    to the 24 month end of the historical 18-24 month cadence would be an
    undesirable rate reduction relative to the 18 month cycle used for recent
    releases. Whether this proposal would be a net negative for these users will
    depend on whether or not we’re able to persuade library maintainers that
    it’s worth their while to support the upcoming stable release throughout its
    pre-freeze period, rather than waiting until its API and ABI have been


The majority of the proposed changes in this PEP only affect the handling of
pre-release versions. The one change affecting full release versions is a
suggested change to their cadence.

Two year cadence for stable releases

With the rolling pre-freeze releases available to users that are looking to
use leading edge versions of the reference interpreter and standard library,
this PEP proposes that the frequency of X.Y.0 releases be adjusted to publish
a new stable release in August every two years (starting in 2021,
around two years after the release of Python 3.8.0).

This change is arguably orthogonal to the proposed changes to the handling of
the pre-freeze release period, but the connection is that without those
pre-release management changes, the downsides of a two year full release cadence
would probably outweigh the upsides, whereas the opposite is true for a 12
month release cadence (i.e. with the pre-release management changes proposed
in this PEP in place, the downsides of a 12 month full release cadence would
outweigh the upsides).

Merging of the alpha and beta phases into a “pre-freeze” phase

Rather than continuing the status quo where the pre-release alpha and beta
phases are distinct and sequential, this PEP proposes that they instead be
combined into a single “pre-freeze” phase with a monotonically increasing serial
number on the releases.

Rather than denoting distinct phases, the “alpha” and “beta” names would
instead indicate whether or not the release contains breaking changes to the
full CPython C ABI:

  • “alpha” releases would be “ABI breaking” releases where extension modules
    built against the full CPython ABI in the preceding pre-release are not
    necessarily going to load correctly

  • “beta” releases would be “binary compatible” releases, where extension modules
    built against the full CPython ABI in the preceding pre-release are expected
    to load correctly, as long as those modules abide by the following additional

    • the module must not be using any provisional or private C APIs (either from
      the previous stable release series, or the in development pre-release series)
      that were removed in this beta release, or were changed in an ABI incompatible
    • the module must not be using any C APIs that were deprecated in the previous
      stable release series, and removed in this beta release

Pre-freeze phase duration and cadence

Rather than being released monthly for a period of a few months while preparing
a new X.Y.0 release, pre-freeze releases would instead be consistently published
every two months.

The only time this would not be the case is during the two month release
candidate period for an upcoming X.Y.0 release (see the release candidate
section below for more details). This means two otherwise scheduled releases
would be skipped (one corresponding with the first release candidate date, one
with the final release date).

The pre-freeze phase would typically be expected to start 2 months after the
preceding stable X.Y.0 release.

The first pre-freeze release for any new release series will always be X.Y.0a1
(as there is no preceding release with the same ABI version markers to judge
binary compatibility against).

Pre-freeze releases would gain an additional flag in their C ABI compatibility
markers to avoid binary compatibility issues with the eventual stable release.

Release policy for beta releases

This PEP proposes that the policy for beta releases be set as follows:

  • as with current beta releases, the stable BuildBot fleet is expected to be
    green prior to preparation and publication of the beta release
  • as with current beta releases, the release manager is expected to review
    open release blocker issues prior to preparation and publication of the beta
  • as with current beta releases, any additions to the abi3 stable C ABI would
    be expected to become a permanent part of that ABI unless and until that
    stable ABI version is retired completely (Note: there are no current plans
    to increment the stable ABI version)
  • unlike current beta releases, beta releases under this PEP would not be
    considered feature complete for the next X.Y.0 release
  • unlike current beta releases, all APIs added since the last CPython feature
    release (other than additions to the stable C ABI) would be considered
  • unlike current beta releases, beta releases under this PEP would be prepared
    and published from the master development branch
  • unlike current alpha or beta releases, beta releases under this PEP would be
    required to be fully ABI compatible with the immediately preceding pre-release
    in the series (excluding any changes to provisional APIs, or the removal of
    APIs that were deprecated in the previous release series)

Release policy for alpha releases

This PEP proposes that the policy for alpha releases be set as follows:

  • as with current alpha releases, the stable BuildBot fleet is expected to be
    green prior to preparation and publication of the alpha release
  • as with current alpha releases, the release manager is expected to review
    open release blocker issues prior to preparation and publication of the beta
  • unlike current alpha release, the release manager would be expected to
    target a similar level of stability to the current beta releases, even
    for the alpha releases

Under this PEP, an alpha release would be published whenever it isn’t possible
to publish a release that satisfies the criteria for a beta release, and
allowing some additional time before making the release won’t resolve the issue.

It is expected that the full CPython API changing in a way that breaks ABI
compatibility (for example, a field may have been added to or removed from a
public struct definition) will be the most likely reason for publishing
additional alpha releases beyond the initial compatibility tag defining
X.Y.0a1 release, but the decision for any particular release rests with the
release manager.

Release candidate policy, phase duration, and cadence

Given the proposed changes to the alpha and beta release phases, the release
candidate phase would see the following related adjustments:

  • Feature freeze, ABI freeze, pyc file format freeze, and maintenance branch
    creation would all correspond with the creation of X.Y.0rc1 (currently these
    occur across a mixture of X.Y.0b1, the last beta release, and X.Y.0rc1)
  • The X.Y.0 release candidate period would be extended from 3 weeks to 2 months
  • There would normally be two release candidates issued a month apart, but
    additional candidates may be published at the release manager’s discretion
  • The final X.Y.0 release would occur between 1 and 4 weeks after the final
    release candidate (depending if additional release candidates were needed
    after the second)
  • If the final X.Y.0 release is delayed beyond the August target date, the
    subsequent release series is not affected, and will still be scheduled for
    August (now slightly less than two years later).

In addition to allowing more time for end user feedback on the release
candidate, this adjusted policy also provides additional time for maintainers
of Python projects to build and publish pre-built wheel archives for the new
stable release series, significantly improving the initial user experience of
the X.Y.0 release.

Changes to management of the CPython stable C ABI

The CPython stable ABI [5_] makes the commitment that binary extension modules
built against any particular CPython release will continue to work on future
CPython releases that support the same stable ABI version (this version is
currently abi3).

Under the proposed rolling pre-freeze release model, this commitment would be
extended to also apply to the beta releases: once an intentional addition to the
abi3 stable ABI for the upcoming Python version has been shipped in a beta
release, then it will not be removed from future releases for as long as the
abi3 stable ABI remains supported.

Two main mechanisms will be available for obtaining community feedback on
additions to the stable ABI:

  • the preferred mechanism will be to add new APIs to the full CPython API first,
    and only promote them to the stable ABI after they have been included in at
    least one published beta release and received relevant user feedback
  • for APIs where that approach is unavailable for some reason (e.g. some API
    additions may serve no useful purpose when the full CPython API is available),
    then developers may request that the release manager mark the next release
    as an alpha release (even in the absence of an ABI break in the full CPython
    API), and attempt to obtain further feedback that way

As a slight readability and usability improvement, this PEP also proposes the
introduction of aliases for each major stable ABI version::

#define Py_LIMITED_API_3_3 0x03030000
#define Py_LIMITED_API_3_4 0x03040000
#define Py_LIMITED_API_3_5 0x03050000
#define Py_LIMITED_API_3_6 0x03060000
#define Py_LIMITED_API_3_7 0x03070000
#define Py_LIMITED_API_3_8 0x03080000
#define Py_LIMITED_API_3_9 0x03090000
// etc...

These would be used both in extension module code to set the target ABI


And also in the CPython interpreter implementation to check which symbols should
be made available::

#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= Py_LIMITED_API_3_9
// A Python 3.9+ addition to the stable ABI would appear here

The documentation for the rolling pre-freeze releases and the stable C ABI would
make it clear that extension modules built against the stable ABI in a later
pre-freeze release may not load correctly on an earlier pre-freeze release.

The documentation for alpha releases and the stable C ABI would make it clear
that even extension modules built against the stable ABI in an alpha release
release may not load correctly on the next release if two alpha releases are
published in a row (this situation would ideally be rare).

Changes to management of the full CPython ABI

This PEP proposes two changes to the management of the full CPython ABI.

An explicit NEWS file convention to mark ABI breaking changes

The proposal in this PEP requires that release managers be able to appropriately
mark a pre-freeze release as either an alpha or a beta release based on whether
or not it includes an ABI breaking change.

To assist in that process, core developers would be requested to include a
“(CPython ABI break)” marker at the beginning of all NEWS file snippets for
changes that introduce a breaking change in the full CPython C ABI.

The “CPython” marker is included to make it clear that these annotations relate
to the full CPython ABI, not the stable ABI.

In addition to being useful for release managers, these markers should also be
useful for developers investigating unexpected segfaults when testing against
the affected release.

Explicitly marking builds against the pre-freeze ABI

The full CPython ABI has long operated under a policy where binary
compatibility only applies within a release series after the ABI has been
declared frozen, and only source compatibility applies between different
release series.

This policy means that extension modules built against CPython pre-releases
prior to the ABI freeze for that release series may not actually load correctly
on the final release.

This is due to the fact that the extension module may be relying on provisional
or previously deprecated interfaces that were changed or removed in a later
alpha or beta release, or it may be due to public structures used by the
extension module changing size due to the addition of new fields.

Historically, adoption of alpha and beta releases has been low enough that this
hasn’t really been a problem in practice. However, this PEP proposes to actively
encourage widespread operational use of beta releases, which makes it desirable
to ensure that users of those releases won’t inadvertently publish binary
extension modules that cause segfaults for users running the release candidates
and final releases.

To that end, this PEP proposes amending the extension module SOABI marker
on non-Windows systems to include a new “p” flag for CPython pre-releases, and
only switch back to omitting that flag once the ABI for that particular X.Y.0
version has been frozen on entry to the release candidate stage.

With this change, alpha and beta releases of 3.9.0 would get an SOABI tag of
cpython-39p, while all release candidates and final builds (for both 3.9.0
and later 3.9.x releases) would get an unqualified SOABI tag of cpython-39

Debug builds would still add the “d” to the end of the tag, giving
cpython-39pd for debug builds of pre-releases.

On Windows systems, the suffix for tagged pyd files in pre-release builds
would include “p” as a pre-release marker immediately after the version number,
giving markers like “cp39p-win_amd64”.

A proposed reference implementation for this change is available at [4_] (Note:
at time of writing, that implementation had not yet been tested on Windows).

Updating Python-Requires for projects affected by full C ABI changes

When a project first opts in to providing pre-built binary wheels for the
rolling pre-freeze release series, they don’t need to do anything special: they
would add the rolling release series to their build and test matrices and
publish binary archives that are flagged as being compatible with that release
series, just as they would if providing pre-built binary wheels after the
full CPython ABI freeze for that release series.

However, if the project is affected by a CPython ABI compatibility break in the
rolling release stream, then they will need to issue a version update that
includes both the new binary build, and a new environment constrained
Python-Requires marker.

For example, if a project supporting the rolling release stream was affected by
a CPython ABI compatibility break in the 3.9.0a5 release, then they would add
the following metadata entry on the version that published the updated binary

Python-Requires: >= "3.9.0a5"; python_version == "3.9"

What this does is add an additional compatibility constraint as part of the
published packages, so Python 3.9.x versions older than 3.9.0a5 won’t consider
the updated package as a candidate for installation.

Caveats and Limitations

Actual release dates may be scheduled up to a month earlier or later at
the discretion of the release manager, based on release team availability, and
the timing of other events (e.g. PyCon US, or the annual core developer
sprints). However, as one goal of the proposal is to provide a consistent
release cadence, adjustments should ideally be rare.

Within a release series, the exact frequency of maintenance releases would
still be up to the release manager and the binary release team; this PEP
only proposes an expected cadence for pre-releases and X.Y.0 releases.

However, for the sake of the example timelines, the PEP assumes maintenance
releases every other month, allowing them to alternate months with the rolling
pre-freeze releases.

Design Discussion

Why rolling pre-freeze releases over simply doing more frequent X.Y.0 releases?

For large parts of Python’s user base, availability of new CPython feature
releases isn’t the limiting factor on their adoption of those new releases
(this effect is visible in such metrics as PyPI download metadata).

As such, any proposal based on speeding up full feature releases needs to strike
a balance between meeting the needs of users who would be adopting each release
as it became available, and those that would now be in a position of adopting
every 2nd, 3rd, or 4th release, rather than being able to migrate to almost
every release at some point within its lifecycle.

This proposal aims to approach the problem from a different angle by defining a
new production-ready release stream that is more specifically tailored to the
interests of operating environments that are able to consume new releases as
fast as the CPython core team is prepared to produce them.

Is it necessary to keep the “alpha” and “beta” naming scheme?

Using the “a” and “b” initials for the proposed rolling releases is a design
constraint imposed by some of the pragmatic aspects of the way CPython version
numbers are published.

Specifically, alpha releases, beta releases, and release candidates are reported
in some places using the strings “a”, “b”, and “c” respectively, while in others
they’re reported using the hex digits 0xA, 0xB, and 0xC. We want to
preserve that, while also ensuring that any Python-Requires constraints
are expressed against the beta releases rather than the alpha releases (since
the latter may not enforce the abi3 stability requirements if two alpha
releases occur in succession).

However, there isn’t anything forcing us to say that the “a” stands for “alpha”
or the “b” stands for “beta”.

That means that if we wanted to increase adoption amongst folks that were
only being put off by the “beta” label, then it may make sense to emphasise
the “*A*BI breaking” and “*B*inary compatible” names over the “alpha”
and “beta” names, giving:

  • 3.9.0a1: ABI breaking pre-freeze release
  • 3.9.0b2: binary compatible pre-freeze release
  • 3.9.0rc1: release candidate
  • 3.9.0: final release

This iteration of the PEP doesn’t go that far, as limiting initial adoption
of the rolling pre-freeze releases to folks that are comfortable with the
“beta” label is likely to be a good thing, as it is the early adopters of these
releases that are going to encounter any unexpected consequences that occur
at the level of the wider Python ecosystem, and we’re going to need them to
be willing to take an active part in getting those issues resolved.

Moving away from the “beta” naming would then become an option to keep in mind
for the future, assuming the resulting experience is sufficiently positive that
we decide the approach is worth continuing.

Why rolling pre-freeze releases rather than alternating between stable and unstable release series?

Rather than using the beta period for rolling releases, another option would be
to alternate between traditional stable releases (for 3.8.x, 3.10.x, etc), and
release series that used the new rolling release cadence (for 3.9.x, 3.11.x,

This idea suffers from the same core problem as PEP 598 and PEP 602: it imposes
changes on end users that are happy with the status quo without offering them
any clear compensating benefit.

It’s also affected by one of the main concerns raised against PEP 598: at least
some core developers and end users strongly prefer that no particular semantics
be assigned to the value of any of the numbers in a release version. These
community members instead prefer that all the semantic significance be
associated with the position within the release number that is changing.

By contrast, the rolling pre-freeze release proposal aims to address that concern by
ensuring that the proposed changes in policy all revolve around whether a
particular release is an alpha release, beta release, release candidate, or
final release.

Why not use Calendar Versioning for the rolling release stream?

Steve Dower’s initial write-up of this proposal [1_] suggested the use of
calendar versioning for the rolling release stream (so the first rolling
pre-release after Python 3.8.0 would have been Python 2019.12 rather than

Paul Moore pointed out [2_] two major practical problems with that proposal:

  • it isn’t going to be clear to users of the calendar-based versions where they
    stand in relation to the traditionally numbered versions
  • it breaks Python-Requires metadata processing in packaging tools with
    no clear way of fixing it reliably (since all calendar versions would appear
    as newer than any standard version)

This PEP aims to address both of those problems by using the established beta
version numbers for the rolling releases.

As an example, consider the following question: “Does Python 2021.12 include
all the new features released in Python 3.9.0?”. With calendar versioning on
the rolling releases, that’s impossible to answer without consulting a release
calendar to see when 3.9.0rc1 was branched off from the rolling release series.

By contrast, the equivalent question for rolling pre-freeze releases is
straightforward to answer: “Does Python 3.10.0b2 include all the new features
released in Python 3.9.0?”. Just from formulating the question, the answer is
clearly “Yes, unless they were provisional features that got removed”.

The beta numbering approach also avoids other questions raised by the calendar
versioning concept, such as how sys.version_info, PY_VERSION_HEX,
site-packages directory naming, and installed Python binary and extension
module naming would work.

How would users of the rolling pre-freeze releases detect API changes?

When adding new features, core developers would be strongly encouraged to
support feature detection and graceful fallback to alternative approaches via
mechanisms that don’t rely on either sys.version_info or runtime code object

In most cases, a simple hasattr check on the affected module will serve this
purpose, but when it doesn’t, alternative approaches would be considered as part
of the feature addition. Prior art in this area includes the
pickle.HIGHEST_PROTOCOL attribute, the hashlib.algorithms_available set,
and the various os.supports_* sets that the os module already offers for
platform dependent capability detection.

It would also be possible to add features that need to be explicitly enabled
via a __future__ import when first included in the rolling pre-freeze releases,
even if that feature flag was subsequently enabled by default before its first
appearance in an X.Y.0 release candidate.

The rationale behind these approaches is that explicit detection/enabling like
this would make it straightforward for users of the rolling pre-freeze release
stream to notice when we remove or change provisional features
(e.g. from __future__ imports break on compile if the feature flag no
longer exists), or to safely fall back on previous functionality.

The interpreter’s rich attribute lookup machinery means we can also choose to
add warnings for provisional or deprecated imports and attributes that we don’t
have any practical way to add for checks against the value of

Why add a new pre-freeze ABI flag to force rebuilds after X.Y.0rc1?

The core development team currently actively discourage the creation of
public pre-built binaries for an X.Y series prior to the ABI freeze date.

The reason we do that is to avoid the risk of painful debugging sessions
on the stable X.Y.0 release that get traced back to “Oh, our dependency
‘superfast-binary-operation’ was affected by a CPython ABI break in
X.Y.0a3, but the project hasn’t published a new build since then”.

With the proposed pre-freeze ABI flag in place, this aspect of the
release adoption process continues on essentially unchanged from the
status quo: a new CPython X.Y release series hits ABI freeze -> package
maintainers publish new binary extension modules for that release
series -> end users only get segfaults due to actual bugs, not just
builds against an incompatible ABI.

The primary goal of the new pre-freeze ABI flag is then to improve
the user experience of the rolling pre-freeze releases themselves, by
allowing pre-built binary archives to be published for those releases
without risking the problems that currently cause us to actively
discourage the publication of binary artifacts prior to ABI freeze.

In the ideal case, package maintainers will only need to publish
one pre-freeze binary build at X.Y.0a1, and then a post-freeze
build after X.Y.0rc1. The only situations that should require
a rebuild in the meantime are those where the project was
actually affected by a CPython ABI break in an intervening alpha

As a concrete example, consider the scenario where we end up having three
releases that include ABI breaks: X.Y.0a1, X.Y.0a5, X.Y.0a7. The X.Y.0a7 ABI is
then the ABI that carries through all the subsequent beta releases and into
X.Y.0rc1. (This is the scenario illustrated in figure 1)

Forcing everyone to rebuild the world every time there’s an alpha release in
the rolling release stream would almost certainly lead to publishers deciding
supporting the rolling releases was more trouble than it was worth, so we want
to allow modules built against X.Y.0a1 to be loaded against X.Y.0a7, as they’re
probably going to be compatible (there are very few projects that use every
C API that CPython publishes, and most ABI breaks affect a single specific API).

Once we publish X.Y.0rc1 though, we want to ensure that any binaries that were
built against X.Y.0a1 and X.Y.0a4 are completely removed from the end user
experience. It would be nice to be able to keep the builds against X.Y.0a7 and
any subsequent beta releases (since it turned out those actually were built
against the post-freeze ABI, even if we didn’t know that at the time), but
losing them isn’t any worse than the status quo.

This means that the pre-freeze flag is “the simplest thing that could possibly
work” to solve this problem - it’s just a new ABI flag, and we already have
the tools available to deal with ABI flags (both in the interpreter and in
package publication and installation tools).

Since the ABI flags have changed relative to the pre-releases, projects don’t
even need to publish a new release: they can upload new wheel archives to their
existing releases, just as they can today.

A cleverer scheme that was able to retroactively accept everything built
against the last alpha or subsequent beta releases would likely be possible,
but it isn’t considered necessary for adoption of this PEP, as even if we
initially start out with a simple pre-release ABI flag, it would still be
possible to devise a more sophisticated approach in the future.

Implications for CPython core development

The major change for CPython core development is the need to keep the master
branch more consistently release ready.

While the main requirement for that would be to keep the stable BuildBot fleet
green, there would also be encouragement to keep the development version of
the documentation up to date for the benefit of users of the rolling pre-freeze
releases. This will include providing draft What’s New entries for changes as
they are implemented, although the initial versions may be relatively sparse,
and then expanded based on feedback from beta release users.

For core developers working on the CPython C API, there would also be a new
requirement to consistently mark ABI breaking changes in their NEWS file

On the specific topic of the stable ABI, most API designs will be able to go
through a process where they’re first introduced as a provisional part of the
full CPython API (allowing changes between pre-freeze releases), and only
promoted to the stable ABI once developers are confident that the interface
is genuinely stable.

It’s only in rare cases where an API serves no useful purpose outside the
stable ABI that it may make sense to publish an alpha release containing a
provisional stable ABI addition rather than iterating on the design in the
provisional CPython API instead.

Implications for Python library development

If this PEP is successful in its aims, then supporting the rolling pre-freeze
release stream shouldn’t be subtantially more painful for library authors than
supporting the stable releases.

For publishers of pure Python packages, this would be a matter of publishing
“py3” tagged wheel archives, and potentially adding the rolling pre-freeze
release stream to their test matrix if that option is available to them.

For publishers of binary extension modules, the preferred option would be to
target the stable C ABI (if feasible), and thus enjoy an experience similar to
that of pure Python packages, where a single pre-built wheel archive is able to
cover multiple versions of Python, including the rolling pre-freeze release

This option isn’t going to be viable for all libraries, and the desired outcome
for those authors is that they be able to support the rolling releases by
building and publishing one additional wheel archive, built against the initial
X.Y.0a1 release. The subsequent build against X.Y.0rc1 or later is then the same
build that would have been needed if only supporting the final stable release.

Additional wheel builds beyond those two should then only be needed if that
particular library is directly affected by an ABI break in any other alpha
release that occurs between those two points.

Having a rolling pre-freeze release stream available may also make it more feasible
for more CI providers to offer a “CPython beta release” testing option. At the
moment, this feature is only available from CI providers that are willing and
able to put the necessary time and effort into creating, testing, and publishing
their own builds from the CPython master branch (e.g. [6_]).

Implications for the proposed Scientific Python ecosystem support period

Based on discussions at SciPy 2019, NEP (NumPy Enhancement Proposal) 29 has
been drafted [3_] to propose a common convention across the Scientific Python
ecosystem for dropping support for older Python versions.

While the exact formulation of that policy is still being discussed, the initial
proposal is very simple: support any Python feature release published within
the last 42 months.

For an 18 month feature release cadence, that works out to always supporting at
least the two most recent feature releases, and then dropping support for all
X.Y.Z releases around 6 months after X.(Y+2).0 is released. This means there is
a 6 month period roughly every other year where the three most recent feature
releases are supported.

For a 12 month release cadence, it would work out to always supporting at
least the three most recent feature releases, and then dropping support for all
X.Y.Z releases around 6 months after X.(Y+3).0 is released. This means that
for half of each year, the four most recent feature releases would be supported.

For a 24 month release cadence, a 42 month support cycle works out to always
supporting at least the most recent feature release, and then dropping support
for all X.Y.Z releases around 18 months after X.(Y+1).0 is released.
This means there is a 6 month period every other year where only one feature
release is supported. Under the proposal in this PEP, that period would
correspond to the final few rolling pre-freeze releases and the release candidate
phase for the upcoming stable feature release.

Release cycle alignment for core development sprints

With the proposal in this PEP, it is expected that the focus of core
development sprints would shift slightly based on the current location
in the two year cycle.

In release years, the timing of PyCon US is suitable for new contributors to
work on bug fixes and smaller features before the first release candidate goes
out, while the Language Summit and core developer discussions can focus on
plans for the next release series.

The pre-alpha core development sprint in release years will provide an
opportunity to incorporate feedback received on the previous release, either
as part of the next maintenance release (for bug fixes and feedback on
provisional APIs), or as part of the first alpha release of the next release
series (for feedback received on stable APIs).

Those initial alpha releases would also be the preferred target for ABI breaking
changes to the full CPython ABI (while changes later in the release cycle
would still be permitted as described in this PEP, landing them in the X.Y.0a1
release means that they won’t trigger any additional work for publishers of
pre-built binary packages).

The Steering Council elections for the next release cycle are also likely to
occur around the same time as the pre-alpha development sprints.

In non-release years, the focus for both events would just be on the upcoming
maintenance and pre-freeze releases. These less intense years would hopefully
provide an opportunity to tackle various process changes and infrastructure
upgrades without impacting the release candidate preparation process.

Release cycle alignment for prominent Linux distributions

Some rolling release Linux distributions (e.g. Arch, Gentoo) may be in a
position to consume the new rolling pre-freeze releases proposed in this PEP,
but it is expected that most distributions would continue to use the established

The specific dates for final releases proposed in this PEP are chosen to align
with the feature freeze schedules for the annual October releases of the Ubuntu
and Fedora Linux distributions.

For both Fedora and Ubuntu, it means that the release candidate phase aligns
with the development period for a distro release, which is the ideal time for
them to test a new version and provide feedback on potential regressions and
compatibility concerns.

For Ubuntu, this also means that their April LTS releases will have benefited
from a full short-term release cycle using the new system Python version, while
still having that CPython release be open to upstream bug fixes for most of the
time until the next Ubuntu LTS release.

The one Linux release cycle alignment that is likely to be consistently poor
with the specific proposal in this PEP is with Debian, as that has been released
in the first half of odd-numbered years since 2005 (roughly 12 months offset
from Ubuntu LTS releases).

With the annual release proposal in PEP 602, both Debian and Ubuntu LTS would
consistently get a system Python version that is around 6 months old, but
would also consistently select different Python versions from each other.

With a two year cadence, and CPython releases in the latter half of the year,
they’re likely to select the same version as each other, but one of them will
be choosing a CPython release that is more than 18 months behind the latest beta
releases by the time the Linux distribution ships.

If that situation does occur, and is deemed undesirable (but not sufficiently
undesirable for Debian to choose to adjust their release timing), then that’s
where the additional complexity of the “incremental feature release” proposal
in PEP 598 may prove worthwhile.

(Moving CPython releases to the same half of the year as the Debian and Ubuntu
LTS releases would potentially help mitigate the problem, but also creates
new problems where a slip in the CPython release schedule could directly affect
the release schedule for a Linux distribution, or else result in a distribution
shipping a Python version that is more than 18 months old)

Implications for simple deployment environments

For the purposes of this PEP, a “simple” deployment environment is any use case
where it is straightforward to ensure that all target environments are updated
to a new Python release at the same time (or at least in advance of the rollout
of new higher level application versions), and any pre-release testing that
occurs need only target a single Python micro version.

The simplest such case would be scripting for personal use, where the testing
and target environments are the exact same environment.

Similarly simple environments would be containerised web services, where the
same Python container is used in the CI pipeline as is used on deployment, and
any application that bundles its own Python runtime, rather than relying on a
pre-existing Python deployment on the target system.

For these use cases, there is a straightforward mechanism to minimise the
impact of this PEP: continue using the stable releases, and ignore the rolling
pre-freeze releases.

To actually adopt the rolling pre-freeze releases in these environments, the
main challenge will be handling the potential for extension module segfaults
when the next pre-freeze release is an alpha release rather than a beta
release, indicating that the CPython ABI may have changed in an incompatible

If all extension modules in use target the stable ABI, then there’s no problem,
and everything will work just as smoothly as it does on the stable releases.

Alternatively, “rebuild and recache all extension modules” could become a
standard activity undertaken as part of updating to an alpha release.

Finally, it would also be reasonable to just not worry about it until something
actually breaks, and then handle it like any other library compatibility issue
found in a new alpha or beta release.

Aside from extension module ABI compatibilty, the other main point of additional
complexity when using the rolling pre-freeze releases would be “roll-back”
compatibility for independently versioned features, such as pickle and SQLite,
where use of new or provisional features in the beta stream may create files
that are not readable by the stable release. Applications that use these
kinds of features and also require the ability to reliably roll-back to a
previous stable CPython release would, as today, be advised to avoid adopting
pre-release versions.

Implications for complex deployment environments

For the purposes of this PEP, “complex” deployment environments are use cases
which don’t meet the “simple deployment” criteria above. They may involve
multiple distinct versions of Python, use of a personalised build of Python,
or “gatekeepers” who are required to approve use of a new version prior to

For example, organisations that install Python on their users’ machines as part
of a standard operating environment fall into this category, as do those that
provide a standard build environment. Distributions such as conda-forge or
WinPython that provide collections of consistently built and verified packages
are impacted in similar ways.

These organisations tend to either prefer high stability (for example, all of
those who are happily using the system Python in a stable Linux distribution
like Debian, RHEL/CentOS, or Ubuntu LTS as their preferred Python environment)
or fast turnaround (for example, those who regularly contribute toward the
latest CPython pre-releases).

In some cases, both usage models may exist within the same organisation for
different purposes, such as:

  • using a stable Python environment for mission critical systems, but allowing
    data scientists to use the latest available version for ad hoc data anaylsis
  • a hardware manufacturer deploying a stable Python version as part of their
    production firmware, but using the latest available version in the development
    and execution of their automated integration tests

Under any release model, each new release of Python generates work for these
organisations. This work may involve legal, security or technical reviews of
Python itself, assessment and verification of impactful changes, reapplication
of patches, recompilation and testing of third-party dependencies, and
only then deployment.

Organisations that can take updates quickly should be able to make use of the
more frequent beta releases. While each update will still require similar
investigative work to what they require today, the volume of work required per
release should be reduced as each release will be more similar to the previous
than it is under the present model. One advantage of the proposed
release-every-2-months model is that organisations can choose their own adoption
cadence from adopting every beta release, to adopting one per quarter, or one
every 6 months, or one every year. Beyond that, it would likely make more sense
to continue using the stable releases instead.

For organisations with stricter evaluations or a preference for stability, the
longer release cycle for stable releases will reduce the annual effort required
to update, the longer release candidate period will allow more time to do
internal testing before the X.Y.0 release, and the greater use by others
during the beta period will provide more confidence in the initial releases.
Meanwhile, the organisation can confidently upgrade through maintenance
releases for a longer time without fear of breaking changes.


Thanks to Łukasz Langa for creating PEP 602 and prompting this discussion of
possible improvements to the CPython release cadence, and to Kyle Stanley
and h-vetinari for constructive feedback on the initial draft of this PEP.


… [1] Steve Dower’s initial “Fast and Stable releases” proposal
(PEP 602: Annual Release Cycle for Python)

… [2] Paul Moore’s initial comments on Steve’s proposal
(PEP 602: Annual Release Cycle for Python)

… [3] NEP 29 proposes a common policy for dropping support of old Python versions

… [4] Example implementation for a pre-release SOABI flag

… [5] CPython stable ABI documentation

… [6] Travis CI nightly CPython builds


This document is placed in the public domain or under the CC0-1.0-Universal
license, whichever is more permissive.

-1 from me. The reasons are very simple. The stated expectation here is that people using the beta release stream will be more likely to need to build their own copies of 3rd party libraries from source, and in particular if I understand the explanation given above, the scientific stack would only publish binaries for release versions.

So for me as a user who can’t build binaries for all the dependencies I use, this amounts to a slower release frequency, and as a library maintainer it amounts to an increased expectation from our users that we support Python beta releases. I’m not happy with either of these consequences.

Unless you believe that 3rd party library developers will expand their matrices to support multiple active/bugfix versions of Python, and yet won’t expand it to include a single beta release, then that would mean neither current proposal is okay and you’re advocating for status quo? (Which is fine, I’m not against status quo here, just wanting to clarify whether this is a “-1 vs the other proposal” or “-1 vs any change”.)

That does seem possible as this proposal does seem to suggest that something could come in but then get removed between betas which would increase support costs (e.g. adding to an API in a beta that’s positional-or-keyword but then making it keyword-only in another beta when you realized that’s better).

That’s quite unlikely to happen for projects that try to stay compatible across several python releases, in that the codebase obviously cannot use features from python 3.(X+1).0bY (which as you say might be changed in 3.(X+1).0bZ) while being usable across several python minor versions. Usually, having the beta version in the CI matrix is only meant to catch deprecations and breakages in current APIs early on, and stay compatible with the newest (pre-)release.

One non-negligble cost I see is for the many projects that exclusively use conda to manage their dependencies in the CI (e.g. pandas). I don’t want to restart the pip-vs-conda debate here, but this is where things stand currently. For example, I’ve tried to package the 3.8.0 alpha/beta releases (1, 2, 3) for precisely this purpose, but couldn’t get things to work (3.8.0a3 was merged through my PR, but all the work was by @jjhelmus). The most recent PRs have been stalled for a while as well. And because everyone is maximally busy, pandas still has no 3.8 builds or CI.

If those packages were to get built in a timely manner after each beta-release (i.e. not taking 1-2 months out of the 2 months until the next beta), then it would be relatively easy for these projects (which are using conda for dependency management) to follow suit in their CI. This at least should be made easier by the smaller increments between betas.

Projects using a different setup for their CI might conversely benefit from an easily downloadable CPython interpreter builds for those betas, cf. this discussion thread. Certainly, few projects will be able to build CPython betas from source as part of their regular CI.

Hmm, it would be nice if you could make the PEP a bit terser. Right now it’s a wall of text, and I’ve only read parts of it.

Edit: actually, I was reading Nick’s “design discussion” post. But still :slight_smile:


That would be helpful. One way you can achieve that is by introducing pictures. Everybody likes pictures and so far PEP 602 is clearly superior in that regard :laughing:

Joking aside, I doubt we’d see much adoption of the beta stream as described. For users to be able to tap into it, libraries would need to be consistently available for it. For a library maintainer making binary wheels available for those betas is laborious enough but first they’d need to make sure their library works at all. If this is already a concern for an annual release cadence, I can only imagine it being a deal breaker in an every-other-month cadence. This is my -1 right there.

But there’s another thing that bugs me here. One of my goals with PEP 602 was to help the core developer, the volunteer. Make things more predictable, easier, smaller. PEP 605 on the other hand leaves the current status quo intact (stable releases made rarely and thus large) while adding additional complications to core developers. The expectation would be to keep the master branch suitable for production use. Last month and the many adventures we went through during that time shows the team is not quite ready for that.

Finally, despite what the PEP here suggests, I still believe the annual release cadence would get features faster to users. Both compared to the status quo and to what PEP 605 is proposing. Sure, maybe not every user would be able to use the latest version every time. However, every new version of Python would see adoption of a significant subset of those users. After all, it’s not like all users would orchestrate skipping the same versions, right? On the other hand, I feel that the beta stream wouldn’t see significant adoption. So, new features would be guaranteed to reach almost all users slower.

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Note that our available data indicates the most widely used Python 3 version is Python 3.6. There isn’t a strong reason to believe that would be any different if we were publishing 3.9 right now instead of 3.8. (It does lend weight to the argument that folks are OK with just skipping releases, though)

I do agree on the “wall of text” problem, but it isn’t clear where pictures would help in this case.

For wheel archive availability on beta releases, we probably need to better emphasise that for projects that also support the latest stable release:

  • compatible pure Python wheels can be published once and left alone
  • binary wheels can also be published once and left alone as long as they target the stable ABI

So pysip based projects, for example, would be usable, as long as pysip itself was able to keep up with the beta releases.

We also wouldn’t necessarily need to start with a 2-month cadence straight away. Instead, we might start out with a 6 month cadence (speeding up before a feature release)

The situation is actually even better than that: for projects that also support the latest stable release, the only cases that are likely to cause extension module breakage are those where public structs change size, and even then, extension modules are mostly accessing those through pointers provided by the runtime rather than allocating memory themselves.

The PEP currently isn’t focused on those cases - it’s focused on in-house modules that are actively using the provisional APIs, and hence need to be updated when they change.

Given the initial feedback, it sounds like we should flip that, and emphasize that extension module rebuilds are only likely to be outright required at the following points:

  • on the first alpha or beta release of the new series
  • on each subsequent alpha release (with “Do we want public extension modules rebuilt?” being one of the criteria for making an alpha release)
  • on the first release candidate (to remove the pre-release qualifier)

Some of the other text in the PEP would also need to change, as at that point we would be encouraging library authors to publish pre-built wheels for the beta series (we’d just be trying to minimise how often they needed to update them).

(Note: pyarrow is an especially complex case that’s already hard to handle reliably on stable CPython releases. We’re hoping conda-press may help on that front, by allowing conda-forge to be used as a wheel building environment)

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I have had data shared with me saying 3.7, so I don’t know if we can ever get a clear, definitive data point on this.

You focus a lot on users, and very little on the package maintainers responsible for your -1 vote.

As Nick pointed out, the churn between each beta release would likely not require changes each time for most 3rd party packages, and an actual guarantee on the stable ABI being stable even throughout beta should help move extensions onto it and ensure their stability as well.

In contrast, you’re offering breaking changes once a year, and despite users having the flexibility to skip versions, maintainers do not have the same flexibility. As far as “orchestrating skipping the same versions”, I would expect that any version skipped by numpy is going to be skipped by the entire data ecosystem out of necessity, and meanwhile users are faced with a stable release that doesn’t have what they need.

This is true, but I think more aspirational/hopeful than how you present it. The more rare stable releases will have had much more usage and be more ready than the status quo, and the “additional complications” are probably the best way to prevent the “many adventures” we have when changes aren’t getting real use. I don’t see a problem with injecting a need for more caution into our process - each Python release impacts literally millions of people, and we ought to take that pretty seriously.

Under PEP 602, we would already have shipped async streams (now being removed, last I heard) and a “stable” initialization API (now no longer guaranteed stable), which would mean those changes would have been permanent. Under PEP 605, the lead time on committing to those designs would have been longer but releases containing them would have come sooner, giving us a better chance of avoiding the issues entirely.

I suspect the biggest sticking point is going to be calling the fast-track releases “beta”, even though it is defined differently from how we’ve used it in the past.

Perhaps we should just choose a totally new name, but keep the versioning scheme as described here? That way it still works nicely on a technical level, but won’t scare people away solely on the name.

I think if binary wheels can be readily made available without placing a significant burden on publishers, and don’t need to be routinely rebuilt for every beta release, we should be able to work through those reservations.

The initial draft spends a lot of words on “How things can go wrong”, as I spent a bunch of time thinking about that.

Even in the full CPython API though, ABI breaks are relatively rare (mainly relating to public structs changing size), so it would be better to have a clear distinction between “ABI breaking” pre-releases and “binary compatible” pre-releases.

What is the actual motivation to change away from the status que. Unless I am missing something the arguments for are “because we can”, and the arguments against are “This will create too much extra work and confusion”.

Is a faster release cadence solving a real problem that people actually have? If it is, fine. I don’t like changing it because it creates more work for me, and slows down feature access because I will end up ignoring most python releases.

But if it is not solving someone’s real problems, whats the point? Just because Azure Pipelines makes it so we can, does not mean that we should.

1 Like

I see what you’re saying but please note that your choice of language is not helpful. Assume good faith. Saying that it seems the argument for the change is “because we can” is needlessly adding negative emotion to the equation.

As for reasons for the change, they are documented both in PEP 602 and in PEP 605.

This is fundamentally the problem with PEP 605, though. Nick’s “Design notes” seems to pretty clearly suggest that numpy won’t be publishing binaries for the beta stream.

IMO, it’s pretty much a showstopper for any proposal if we can’t be reasonably certain that the key binary extension providers (the scientific stack being the most obvious example) will provide binaries for all the releases that we expect to be generally usable.

Something I think both our PEPs are assuming without spelling out is that readers already understand the potential impact of faster turnaround times for all kinds of contributions, but especially commercial ones.

I believe you, Steve, and I agree on the basic shape of the problem to be addressed, so maybe this is one of those cases where we should split out a separate shared background PEP, so we can cut that aspect out of the individual proposal PEPs?

Are you sure? It’s true that in both cases the stakeholders changed their minds later but the moment they did was informed more by the release calendar than by the time it took to understand the flaws in the respective implementations. In fact, the entire point of reverting Streams was to avoid shipping a flawed design. A faster release cadence would inform this even better, pushing some of those work-in-progress to PyPI instead.

More importantly, I feel this argument is invalid in the sense that there’s always some likelihood that a decision was wrong and it will have to be reverted in the future. This is a problem, yes, but if you want to optimize against stable users ever seeing this kind of thing, we should maximize the number of years between releases. What you’d get in result is the Python 2 to Python 3 migration.

Maybe in the case of Streams and the initialization API an 18-month cycle was drawn out enough to catch those deficiencies. But we’ll see after the release if there were other decisions that should have been different. Heck, we’re still discovering deficiencies in decisions made around Python 3.5… or even 19 years ago.


There in lies some of the problem here. Both PEP rationals do a really poor job of defining the actual problem they are trying to solve, with one not even attempting to define the problem. They both really do read like Because CI tools exist, we can do it, so we should do it.

Maybe that’s the bias of someone who thinks the 18 month lead time between releases is a feature of the language rather than a flaw leaking through. I don’t see a problem in the rationals.