RFC: PEP 608: Coordinated Python release

EDIT: I added Miro Hrončok as co-author.


While I was reading the great summary of PEP 602 and 605 on LWN, and after my first failed attempt to handle incompatible changes (PEP 606: Python Compatibility Version), I decided to write down an idea that I was thinking about for almost one year. I wrote the PEP that you can find below.

Red Hat has a team working on Fedora to ensure that all Python packages are working well with the next Python (we’re almost done with 3.8, the next will be Python 3.9). We do bug triage for each broken package and attempt to report issues to upstream (broken projects and Python upstream) as much as possible. We are also actively working on fixing these issues (with our limited resources). My colleague Miro Hrončok is doing an amazing job in this area!

The idea here is to do something similar, but directly on Python upstream with a smaller set of Python projects and make it part of the Python release process (PEP 101).

Inexact summary: I propose to include 27 projects to the Python CI :slight_smile:

HTML version:

Full text bellow to reply inline.


PEP: 608
Title: Coordinated Python release
Author: Miro Hrončok miro@hroncok.cz, Victor Stinner vstinner@python.org
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 25-Oct-2019
Python-Version: 3.9


Block a Python release until a compatible version of selected projects
is available.

The Python release manager can decide to release Python even if a
project is not compatible, if they decide that the project is going to
be fixed soon enough, or if the issue severity is low enough.


The PEP involves maintainers of the selected projects in the Python
release cycle. There are multiple benefit:

  • Detect more bugs before a Python final release
  • Discuss and maybe revert incompatible changes before a Python final
  • Increase the number of compatible projects when the new Python final
    version is released

Too few projects are involved in the Python beta phase

Currently, Python beta versions are available four months before the
final 3.x.0 release.

Bugs reported during the beta phase can be easily fixed and can block a
release if they are serious enough.

Incompatible changes are discussed during the beta phase: enhance
documentation explaining how to update code, or consider to revert these

Even if more and more projects are tested on the master branch of Python
in their CI, too many projects of the top 50 PyPI projects are only
compatible with the new Python a few weeks, or even months, after the
final Python release.

DeprecatedWarning is being ignored

Python has well defined process to deprecate features. A
DeprecatedWarning must be emitted during at least one Python release,
before a feature can be removed.

In practice, DeprecatedWarning warnings are ignored for years in major
Python projects. Usually, maintainers explain that there are too many
warnings and so they simply ignore warnings. Moreover, DeprecatedWarning
is silent by default (except in the __main__ module: PEP 565 <https://www.python.org/dev/peps/pep-0565/>_).

Even if more and more projects are running their test suite with
warnings treated as errors (-Werror), Python core developers still
have no idea how many projects are broken when a feature is removed.

Need to coordinate

When issues and incompatible changes are discovered and discussed after
the final Python release, it becomes way more complicated and expensive
to fix Python. Once an API is part of an official final release, Python
should provide backward compatibility for the whole 3.x release
lifetime. Some operating systems can be shipped with the buggy final
release and can take several months before being updated.

Too many projects are only updated to the new Python after the final
Python release, which makes this new Python version barely usable to run
large applications when Python is released.

It is proposed to block a Python release until a compatible version of
all selected projects is available.

Shorter Python release schedule

The PEP 602: Annual Release Cycle for Python <https://www.python.org/dev/peps/pep-0602/>_ and the PEP 605: A rolling feature release stream for CPython <https://www.python.org/dev/peps/pep-0605/>_ would like to release
Python more often to ship new features more quickly.

The problem is that each Python 3.x release breaks many projects.

Coordinated Python releases reduces the number of broken projects and
makes new Python release more usable.


By default, a Python release is blocked until a compatible version of
all selected projects is available.

Before releasing the final Python version, the Python release manager is
responsible to send a report of the compatibility status of each project
of the selected projects. It is recommended to send such report at
each beta release to see the evolution and detects issues as soon as

The Python release manager can decide to release Python even if a
project is not compatible, if they decide that the project is going to
be fixed soon enough, or if the issue severity is low enough.

After each Python release, the project list can be updated to remove
projects and add new ones. For example, to remove old unused
dependencies and add new ones. The list can grow if the whole process
doesn’t block Python releases for too long.

Limit the delay

When a build or test issue with the next Python version is reported to a
project, maintainers have one month to answer. With no answer, the
project can be excluded from the list of projects blocking the Python

Multiple projects are already tested on the master branch of Python in a
CI. Problems can be detected very early in a Python release which should
provide enough time to handle them. More CI can be added for projects
which are not tested on the next Python yet.

Once selected projects issues are known, exceptions can be discussed
between the Python release manager and involved project maintainers on a
case by case basis. Not all issues deserve to block a Python release.

Selected projects

List of projects blocking a Python release (total: 27).

Projects (13):

  • aiohttp
  • cryptography
  • Cython
  • Django
  • numpy
  • pandas
  • pip
  • requests
  • scipy
  • Sphinx (needed to build Python)
  • sqlalchemy
  • pytest
  • tox

Direct and indirect dependencies (14):

  • certifi (needed by urllib3)
  • cffi (needed by cryptography)
  • chardet (needed by Sphinx)
  • colorama (needed by pip)
  • docutils (needed by Sphinx)
  • idna (needed by Sphinx and requests)
  • jinja2 (needed by Sphinx)
  • MarkupSafe (needed by Sphinx)
  • psycopg2 (needed by Django)
  • pycparser (needed by cffi)
  • setuptools (needed by pip and tons of Python projects)
  • six (needed by tons of Python projects)
  • urllib3 (needed by requests)
  • wheel (needed by pip)

How projects are selected

Projects used by to build Python should be in the list, like Sphinx.

Most popular projects are picked from the most downloaded PyPI projects.

Most of project dependencies are included in the list as well, since a
single incompatible dependency can block a whole project. Some
dependencies are excluded to reduce the list length.

Test dependencies as pytest and tox should be included as well. If a
project cannot be tested, a new version cannot be shipped neither.

The list should be long enough to have a good idea of the cost of
porting a project to the next Python, but small enough to not block a
Python release for too long.

Obviously, projects which are not part of the list also are encouraged
to report issues with the next Python and to have a CI running on the
next Python version.

Incompatible changes

The definition here is large: any Python change which cause an issue
when building or testing a project.

See also the PEP 606: Python Compatibility Version <https://www.python.org/dev/peps/pep-0606/>_ for more examples of
incompatible changes.


There are different kinds of incompatible changes:

  • Change in the Python build. For example, Python 3.8 removed 'm' (which stands for pymalloc) from sys.abiflags which impacts Python vendors like Linux distributions.
  • Change in the C extensions build. For example, Python 3.8 no longer links C extensions to libpython, and Python 3.7 removed os.errno alias to the errno module.
  • Removed function. For example, collections aliases to ABC classes have been removed in Python 3.9.
  • Changed function signature. Reject a type which was previously accepted (ex: only accept int, reject float). Add a new mandatory parameter. Convert a positional-or-keyword parameter to positional-only.
  • Behavior change. For example, Python 3.8 now serializes XML attributes in their insertion order, rather than sorting them by name.
  • New warning. Since more and more projects are tested with all warnings treated as errors, any new warning can cause a project test to fail.
  • Function removed from the C API.
  • Structure made opaque in the C API. For example, PyInterpreterState became opaque in Python 3.8 which broke projects accessing interp->modules (PyImport_GetModuleDict() should be used instead).

Cleaning up Python and DeprecationWarning

One of the Zen of Python (PEP 20) <https://www.python.org/dev/peps/pep-0020/>_ motto is:

There should be one-- and preferably only one --obvious way to do it.

When Python evolves, new ways emerge inevitably. DeprecationWarning
are emitted to suggest to use the new way, but many developers ignore
these warnings which are silent by default.

Sometimes, supporting both ways has a minor maintenance cost, but Python
core developers prefer to drop the old way to clean up the Python code
base and standard library. Such kind of change is backward incompatible.

More incompatible changes than usual should be expected with the end of
the Python 2 support which is a good opportunity to cleaning up old
Python code.

Distributed CI

Checking if selected projects are compatible with the master branch
of Python can be automated using a distributed CI.

Existing CIs can be reused.

New CIs can be added for projects which are not tested on the next
Python yet.

It is recommended to treat DeprecationWarning warnings as errors when
testing on the next Python.

A job testing a project on the next Python doesn’t have to be
“mandatory” (block the whole CI). It is fine to have failures during the
beta phase of a Python release. The job only has to pass for the final
Python release.


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

Night gathers, and now my watch begins. It shall not end until my death.

1 Like

[replying here because not everybody uses the mailing list]


We are not responsible for third-party projects, and allowing them to block our progress is counter-productive.

1 Like

In general, I agree with @stoneleaf that we shouldn’t be allowing non-core projects to block a Python release. The RM already has the ability to delay a release if we hear that a 3rd party project has issues - I’d rather leave this to the RM’s judgement, instead of over-formalising the situation.

As a technical note, the list of “indirect dependencies” you quote is incomplete, because pip vendors a lot of dependencies (most notably requests) that don’t show up in the project metadata, but which would need to be included (we almost certainly wouldn’t alter our policy that we don’t patch vendored dependencies, but rather wait for the dependency to be fixed and re-vendor).

The PEP is also under-specified. What counts as a “compatible release” of a project? Pip already has a release compatible with Python 2.10, for instance, because we ship pure Python “py2.py3” wheels. Conversely, we are only just adding Python 3.8 to our CI matrix, because our CI providers are only just adding support for it. With the PEP as it stands I don’t even know how I’d confirm on behalf of pip that we are OK for Python 3.8 to be released…

1 Like

I can see how this could be problematic, and I’d assume it would only be workable as it is entirely up to the release manager - say Django decides not to support compatibility with 3.9?
Surely the engagement with project managers in speeding up compatibility work with upcoming releases as much as anything?

My other thought is…if Django, why not matplotlib, Flask…and that’s just naming the two big ones in the project I’m currently working on, matplotlib not currently working for me on 3.8.

However, along with many others, I’m sure, I would be pretty excited if on the day 3.9.0 is released I could just:
py -3.9 -m pip install -r my_project/requirements.txt
py -3.9 -m myproject/myscript
…and it just works, which isn’t the case for 3.8.0 so far.

1 Like

We also don’t know from this list if these projects are even staffed well enough to respond quickly to required fixes when they do come up. Have these projects been asked if they are prepared to take on the burden of holding up a Python release and thus are on the hook to fix their projects fast enough to not make the whole world have to wait for a new Python version to come out?

Yeah, this also brings up the question as to whether all of these projects have agreed to this as they will have to tell us what they are expecting to be compatible with and work with us to maintain that.


I think the key motivator here is to help projects be on top of upcoming changes and be prepared on release day for the latest Python version. That involves two things. One is getting projects to use -X dev or -W error when testing so they know when we have deprecated something. And two is actually testing the betas and RCs. For the latter there are two options.

One is to ask projects what they need from us to set up their CI to test against the betas and RCs (e.g. is it container images or something?). That way they can provide feedback for the betas and be prepped to release based on the RCs.

The other option is we set up our own CI just to test other projects as a way to gather our own feedback on the ramifications of a change. That would probably require projects to opt-in so we know who to reach out to in order to find out whether something break legitimately or not. This also does not help projects be prepared for a quick release based on the RCs.

So obviously suggestion 1 of helping projects get set up on the betas and RCs is my preference.

In terms of the release process, I don’t see any fundamental problem with the idea of a new “Check the status of [project compatibility testing report]” checklist item for release managers, as long as the RM would have broad discretion to decide what counts as a release blocker (“can’t build”, “can’t install”, and “can’t import” would probably qualify, “test case for obscure function x.y.z fails” probably wouldn’t).

However, the principle has never been the real problem with this idea, the problem has been figuring out how to manage it sustainably.

One possible way to tackle that would be to approach it as a status reporting problem, rather than a test execution problem.

If it’s approached that way, then you can start to define concrete requirements for what you would like the reporting service to do:

  1. Accept information on a project’s existing CI management system, both via a web interface and by inspection of known testing config files in a repo
  2. Track what Python versions are being tested in that CI
  3. Report via a web API and UI which Python versions the tracked projects are testing against
  4. Report via a web API and UI which Python versions have successful builds, successful installs, and passing tests on those projects
  5. Allow service admins (or even users) to specify a set of projects of interest and publish summary reports for a particular Python version showing which of those projects are testing against that version, and which of those tests are passing

Similar to our “stable” and “unstable” BuildBot fleets, a service structured that way could potentially distinguish between “Critical” projects and “Desirable” projects, and perhaps even offer more fine-grained distinctions based on use case (e.g. a Flask app wouldn’t need Django, and a client app or data analysis script wouldn’t need either, but might need requests or numpy)

There would likely need to be some back & forth discussion between the tracking service and the tracked projects around making the CI results readily consumable for this purpose (especially when it comes to distinguish between build failures, install failures, and test failures), but a plausible starting point would be being able to report on:

  • GitHub projects using the Travis CI Python test matrix
  • GitHub projects running a tox test matrix in Travis CI

Assuming such a compatibility testing tracking service existed, it shouldn’t be a problem to get either the PSF or a redistributor to host a generally available instance of it.

Howeve, the initial contribution of developer time to bring such service into existence would likely need to come from one or more of Python’s commercial redistributors (and it sounds like RH may be willing to consider that possibility?)


Let me try to reply to everybody.

First of all, this PEP doesn’t propose drastic changes: most items of the PEP were already done before Python 3.8.0 final release, but it seems like most people didn’t notice :slight_smile:

  • Most if not all selected projects were compatible with Python 3.8 when Python 3.8.0 has been released. This happened because different people worked hard to make this happening.
  • Python core developers and maintainers of their projects are testing various projects on the next Python, especially during the beta phase
  • Bugs and regressions are reported to Python upstream.
  • Incompatible changes are discussed on python-dev, the bug tracker, Twitter, on projects bug tracker, etc. Some incompatible changes have been reverted during Python 3.8 beta phase.
  • Some bugs were considered as serious enough to get the “release blocker” priority which blocks a release according to the PEP 101. As far as I know, all these release blocker issues have been fixed.
  • There are already multiple CIs running different projects on the next Python. For example, at Red Hat, we rebuilt Fedora on Python 3.8 beta: it’s not just a “few projects”, but the whole operating system: more than 200+ Python packages (I don’t know the exact number, maybe it’s 500+, Miro knows that better than me, maybe 200 was the number of packages broken by Python 3.8)
  • The release manager is already free to change a release blocker issue to deferred release blocker, or even “reject” the release blocker priority if they consider that it’s not severe enough.

The Python 3.8 beta phase discovered the PyCode_New() C API change: it was decide to revert this change, and add a new function instead.

This PEP is not about doing new things, but more about better communicating around this work which is already done, and better coordinate.

IMO the counter-productive part is more that each Python release breaks tons of Python projects :slight_smile: This PEP proposes a concrete solution to make Python releases less painful for users.

Many users remind the pain of upgrading from Python 2 to Python 3. We are close to have Python 2 behind us. It’s time to look how to prevent such situation to happen again.

It seems like the PEP doesn’t clearly explain when the compatibility is tested and who is going to pay attention to this.

I don’t expect that a single person, the release manager, will handle all issues. It’s the opposite: I would like to involve all Python core developers and all maintainers of all selected projects in the process. It’s more a human issue than a technical issue. That’s why the PEP title is “Coordinated Python release”: we need more interactions and coordination than what we have currently.

I don’t expect that selected projects will only be checked the day before the expected final release. No. My intent is to check these projects every day using a CI: see the “Distributed CI” section, where CI stands for Continuous Integration :wink: The formal part is that the release manager is intented to send a report at each beta release. I expect that the release manager will point to projects which need most help to be updated.

One practical issue is that project dependencies can evolve more quickly than the PEP will be updated. So I chose to only select dependencies which are the most likely to be broken, but also select dependencies which are the most commonly used. For example, urllib3 is the top #1 the most downloaded library on PyPI. If urllib3 is broken, you should expect a huge amount of projects to be broken on the next Python. On the other side, I chose to ignore distlib which is very specific to pip and packaging.

Obviously, if distutils is broken by Python 3.9, pip tests will fail, and so the Python 3.9 will be indirectly blocked by distutils, even if it’s not explicitly listed in selected projects.

I tried to explain that, but very shortly, in the “How projects are selected” section: “Some dependencies are excluded to reduce the list length.”

About “(most notably requests)”: requests is explicitly listed as a “project” in the PEP list. I’m aware of pip vendored dependencies (src/pip/_vendor/).

Obviously, I’m open to discuss the exact list of selected projects :wink: It’s a Request For Comments and a draft PEP :slight_smile:

With my Red Hat :wink: , the “compatibility” check basically means that building a Fedora package does not fail, knowing that tests are run as part of the build process. But this definition may be too specific to Fedora, since Fedora uses specific versions, which can be different than versions chosen by the upstream project for their CI.

If a project has a CI running on the next Python, the CI is expected to pass: it must be possible to install dependencies, to build the package (ex: build C extension), and its test suite must pass.

The CI can be run by the project directly, or it can be a CI run by Python core developers, or both. That’s why I use the “Distributed CI” term, instead of requiring an unique CI.

For example, when I discussed with numpy developers, they told me that they like to control how dependencies are installed: which version, which OS, etc. For example, the OpenBLAS version.

Obviously, having multiple CIs testing different configuration are not counter-productive: they can detect more bugs. But we will have to decide at some point which CI is the one used to validate a Python version :wink: Maybe this choice should be delegated to each selected project? I guess that the natural choice will be upstream CI run by the project.

If Django decides to not support Python 3.9, maybe it’s a strong signal to Python core developers that something gone wrong and that we have to discuss to understand why Django doesn’t want to upgrade. Maybe we are putting too many incompatible changes and it’s time to slow down this trend?

Maybe Python core developers and other volunteers can offer their help to actually port Django. This happens very commonly. It’s common that core developers who introduce incompatible changes propose directly pull requests to update projects broken by their change.

It happened for the new incompatible types.CodeType constructor: Pablo (and others) proposed different pull requests. It also decided me to introduce the new method CodeType.replace(): projects using it will not longer be broken if CodeType constructor changes again (gets a new mandatory parameter). I proposed pull requests to different projects to use it.

If Django doesn’t want to support Python 3.9, doesn’t want to accept pull requests or pull requests cannot be written, well, the Python release manager should be able to exclude Django from the selected projects. I expect that such decision will be a group decision.

Maybe ignoring Django is fine. But what about pip or urllib3? What if Python 3.9 is released even if we know that pip or urllib3 don’t want or cannot support Python 3.9? Is Python 3.9 without pip/urllib3 still relevant? That’s also the question asked indirectly by the PEP.

For the specific case of Django, maybe Django code base is too big and Django release cycle is too slow, to include Django in selected projects. I’m fine with dropping it from the PEP if you prefer. But it would be nice to have clear rules to include or not a project.

If you consider that the selected projects list is too long, we can make it way shorter. Maybe Python 3.9 should start only with pip and nothing else?

Ok, it’s now time for me to introduce you a very experimental project that I started a few weeks ago: https://github.com/vstinner/pythonci

This project is supposed to be a way to test the selected projects on a custom Python binary with custom Python options. For example, using -X dev or -Werror (passed as command line arguments or as environment variables).

I consider the project as experimental because I have different issues:

  • First, I chose to hardcode commands used to install dependencies and to run the test suite of a project. I’m not sure that this approach is going to scale. I was scared when I saw the complexity of the tox.ini project of the coverage project.
  • I wrote a task for coverage which uses tox, but I failed to run coverage with the specific custom Python binary (the task ignores the custom Python and uses “python3.7” instead).
  • Python 3.9 already introduced incompatible changes which cause the job to fail early, while installing dependencies. In short, pip is currently somehow broken in Python 3.9. In fact, pip has been fixed (bundled html5lib no longer uses collections ABC aliases but collections.abc), but pythonci runs an old pip version which isn’t compatible with Python 3.9… I’m not sure why, I should investigate.
  • All jobs fail very early using -Werror because even pip emits many warnings (not only DeprecationWarning). My plan is to experimental to only treat DeprecationWarning as error… But pip fails with -W error::DeprecationWarning: again, because pythonci picks an outdated pip version.
  • I wrote pythonci for different usage: test the master branch of Python with a patch, test a project with -X dev, test a project with -Werror.

By the way, pythonci includes patches on pip and setuptools to fix a few warnings, to be able to experiment -Werror.

In short, I would say that right now, Python 3.9 is in a bad shape: it’s barely usable, most basic functions like pip are broken… Maybe I’m wrong and it will be fine in practice.

All these issues also decided me to propose this PEP.

I don’t think that a single CI can reply to all open questions. Some jobs may only be relevant to Python core developers.

For example, I would like to drop the “U” mode of the open() function: https://bugs.python.org/issue37330 But I have no idea how many projects would be broken by this change… 4 months ago, when I tried, even building Python was broken… because of Sphinx… because docutils was ignoring the DeprecationWarning since Python 3.4. Moreover, when I reported the issue to docutils with a patch… I was told that docutils was already fixed, but there was no release yet! (A new docutils version has been released with the fix in the meanwhile.)

It would be great to have a tool (like pythonci?) to check that the selected projects still run fine while working on an incompatible change: run the tool manually before merging a PR.

This is not a theoretical issue: pip was broken by the removal of collections ABC aliases. It was an issue in html5lib which has been fixed: a new compatible pip version has been released in the meanwhile.

About DeprecationWarning, one reason why developers ignore them may be that it’s not easy to separate warnings emitted by the stdlib, and warnings emitted by third party code.

At least for Python core developers, it would help to run selected projects with DeprecationWarning treated as errors, but only for warnings emitted by the stdlib.

Would it be possible to develop a special warnings filter for that?

1 Like

(Reply to Paul Moore’s email on python-dev)

Le ven. 25 oct. 2019 à 17:37, Paul Moore p.f.moore@gmail.com a écrit :

It’s also worth noting that even with pre-release testing, there’s
still the non-trivial problem of getting any necessary fixes
implemented and co-ordinated. That’s a people-management issue,
though, and IMO needs handling flexibly and in a way that’s
sympathetic to the fact that most of the projects involved are
volunteer-based and resource starved.

It seems like you understood the deep roots of the PEP :slight_smile:

Many Python projects are understaffed, whereas Python core developers are putting incompatible changes into Python. It’s done without any kind of coordination. Once Python is released, it’s too late to revert changes.

Incompatible changes should be reduced to ensure that understaffed projects are able to handle them.

As I wrote in my previous comments, in practice, core developers are already helping to update many core Python projects for incompatible changes.

If selected projects fail to be updated to next Python, we will be in a very bad situation which smells more and more like Python 2 => Python 3 failure.

The question is more why Python is breaking projects at each release, and if it’s worth it.

One of my biggest reservations about Victor’s proposal is that it’s
basically removing flexibility and demanding extra testing, with
little or no explanation of how we’d make that sustainable.

If we stop to put incompatible changes in the next Python, the PEP 608 becomes basically free :slight_smile:

We have to find the right balance for incompatible changes.

About the “extra testing”, I replied in previous comments.


I would like to propose PyPA & PyCQA projects & celery to be considered as well :slight_smile:

I would think the PEPs are over-optimistic of what can be achieved for Redhat/Mac/Windows.

Limiting the focus on the common ultra-basic would already be great:

  • zero constraint on Python delivery, just get community focus/efforts on the bottlenecks to get the whole eco-system be ready sooner (which can vary at each version or beta cycle, depending of PEPs)
  • suspect “always a bottleneck” : C.I. Chains + Cython + Numpy + Jupyter Notebook-
  • ugly patches are ok in alpha/beta to keep the bottlenecks away for the rest of the community,

and maybe some environments shall be droped, at least at alpha/beta stage, as increasing the effort for ever shrinking effect:

  • Windows 7/8, Windows 32 bit (I see 32 bit mostly dying with Windows7),
  • Old Mac/Linux versions.

I think this is an excellent idea in general, even if it will still take some time to hammer out the details. Testing a some core packages to be compatible with the new release is very good for finding allowing libraries to know what to adapt to (generally, projects that are actively maintained would surely be willing to work on removing deprecated methods, or at the very least accept pull requests to that effect), and for having a wider range of projects available at release time.

Most projects (even those as big as pandas) do not test against python master, and this is something that should conceivably be done for CPython itself.

This mode of development is also not unprecedented, for example there’s the community build for scala or crater runs for Rust.

IMO, minimal list to start with could also include Sphinx, particularly since we depend upon it for building Python and our documentation.

Also, I think it would be helpful to have more than one category of projects that the list would include, such as a “suggested release blocking” section that includes only a few stable projects and a “non-blocking” section that includes significantly more packages. This would allow us to expand upon the list further over time, without increasing the maintenance cost as much for each addition. It could potentially be only two sections or perhaps many sections with increasing compatibility priorities.

I would propose that new packages added to the list could start in a lower compatibility priority section; moving up or down depending on if they are proven to be stable and responsive to fixes over time. Regardless of the priority level of any package, we would of course maintain the discretion for issues to be a release blocker or not (as the PEP suggests).

These different sections could help give us an idea of where to focus our efforts, and give us a better understanding of compatibility issues. It’s far more meaningful if a known “stable” package suddenly breaks, in comparison to one that is new to the list or commonly has compatibility issues. In a way, it’s not entirely different from how we think of the buildbots.

My concerns about the ambiguity here remain. As a pip core developer, my question to you in that case would be, precisely what commitments are you expecting from the pip developers if this proposal were to be accepted? What would change for pip?

  • Are you expecting us to add the dev version of Python to our CI? Is “whatever Travis/Appveyor/Azure happen to have available” sufficient, or will we be expected to add a CPython build to our CI?
  • Are you expecting us to re-run our CI when a new Python release occurs (at the moment we only run CI when a pip commit happens)?
  • Are you expecting us to run CI repeatedly on our release tags and/or master? At the moment we run CI on commit but we don’t run CI again unless another commit occurs.
  • Are you expecting us to let the CPython project know if our tests fail? Do we need to triage such failures to confirm if they are caused by a CPython change or something else (environmental issues, pip relying on undocumented behaviour, etc)?
  • Who do you propose communicates these issues to our vendored dependencies? Are you OK with our policy that we generally don’t patch vendored dependencies, but wait for a downstream fix to be released?
  • Do you have any expectation that we prioritise fixing failures that are triggered by CPython changes? What if our test failures are delaying a release?

I could go on - there are many questions that would need answering here, if this were to become a formal part of any process. Of course, if it’s simply a general “good intention”, that we try to ensure that pip works when a new Python version is released, then (a) that wouldn’t involve much extra work from the pip developers, but (b) it’s what we already do… And general “good intentions” don’t need a PEP :wink:

But as it stands, I’d personally be against pip agreeing to be part of this PEP. (The other pip developers may have different opinions - we’d have to discuss this internally and come up with a project response if it came to the point of signing this PEP off).

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Hi Paul,

Thanks for these interesting questions. It seems like the PEP needs some clarification :slight_smile:


It’s better if a selected project has a job to test the project on the next Python (the job doesn’t have to be mandatory), but it’s not required by the PEP.

My plan is that Python core developers (at least me) will add a CI for projects which are not tested on next Python yet. Maybe maintainers of selected projects will be kind enough to help us on this task :wink:


For the short term, my plan is more to have a script similar to my experimental https://github.com/vstinner/pythonci project which would be run manually. Only the Python release manager is expected to do such manual check.

But for the long term, it would be better to have a CI, so run continuously.


For projects less active than CPython (ex: if the CI is not run daily), we can do manual checks and/or have a separated CI.

IHMO we will need a separated CI anyway, especially to test Python changes on selected projects before merging a change. I’m thinking at changes known to be incompatible, like my https://github.com/python/cpython/pull/16959


The Python release manager will have to check the status of selected projected at each beta, rc and final release.

I hope that project maintainers of selected projects will be more proactive to report issues upstream, but it’s not a PEP requirement.

Honestly, I don’t want to go so far in term of organization detail in the PEP. I prefer to let developers organize themselves :slight_smile: Most developers are kind enough to report issues directly to the proper project.

The strict minimum required by the PEP is that the release manager detects that pip is broken. That would an enhancement compared to the current blurry status quo.

In my experience, the issue is first reported to pip, and then it is reported to the downstream project. For example, I was somehow involved in reporting the deprecated collections ABC issue to html5lib and getting a html5lib release. You may notice that Python core developers already coordinated with pip to wait until pip is being fixed before removing collections ABC aliases :wink: Such coordination is not something new.

Let me try to explain that differently. The PEP doesn’t require pip developers to do more work. The PEP gives a trigger to pip developers to force core developers to revert a Python change if something goes wrong.

The PEP makes the assumption that all open source projects are understaffed and that CPython has a larger team than other selected projects. Even if it’s not written down, my expectation is that core developers will help to fix the broken selected projects, because these projects would block the final Python release.

My expectation is that if pip developers are simply too busy to fix an issue, CPython must be fixed to not bother pip developers. Or the CPython release is blocked until pip is fixed, if it’s better to let pip developers to fix the issue themselves. pip and CPython are not exclusive: they are Python core developers working on pip :wink:

But sometimes, it’s just a matter of weeks or even of days. That’s why the PEP gives the freedom to the release manager to ignore a broken project for a release.

IMHO it’s fine if Cython is broken for the first Python beta releases, but it’s not ok for the first rc release. And it must block a final release.

Let’s say that Cython is badly broken on Python 3.9 because of a last minute bugfix between two rc releases. Cython is fixed, but the Cython release manager is traveling. The Python release manager can try coordinate the Python and the Cython releases, or take in account that Cython is going to be released soon with a fix, and release Python anyway.

The coordination can be relaxed when a project is already fixed, but I would prefer to not release Python 3.9 final knowing that pip is badly broken, that no fix is available and no one is available to fix it. The PEP is a process to reduce the risk of ending in such situation, by making sure that bugs are detected before the final release.

If the PEP is modified to not require anything, IMHO the whole PEP becomes useless.

The DeprecationWarning is being ignored section of the PEP is a concrete example of such issue. Developers are “expected” to take DeprecationWarning warnings in account, but they don’t. Result? Python break frequently random projects when removing a feature, even if it was deprecated for 10 years.

The PEP must to be strict on some specific points. I expect you to help me to clarify what should be strict and what doesn’t have to be strict :slight_smile:

Technically, Python doc can be built even if Sphinx is not compatible with the latest version of Python: you can use an older Python version. No?

But Python without pip is barely usable.

I’m thinking aloud about the strict minimum set for selected projects. Adding any project means adding indirectly many other dependencies and Sphinx has many dependencies.

The PEP gives many new tasks to the release manager. I would prefer to not add confusion with “non blocking projects”.

I would prefer to stick the PEP to the minimum requirements for a Python release. Obviously, everything helping to get a better release can be done without a PEP and is welcomed (as already written in the PEP!) :slight_smile: You are free to add tons of CIs. You don’t need the Python core developers approval nor select projects maintainers approval :wink:

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I heard that such “world rebuild” is also done in CPAN before a Perl release.

But there’s a big difference between trying to do this and requiring we do this. So it’s great what you want to accomplish with the PEP, but that’s best-effort compared to requiring we do it. And if you mean to make this a goal and not a rule then this should either be an informational or process PEP or instead be in the devguide and not be a PEP.

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What about if this were describing a project that the PSF could offer a grant for?

Resourcing it is the big challenge - everyone should agree this is a good thing to achieve, but that doesn’t magically produce the ability to do it.

But perhaps if this PEP could become a statement of work, we’d be able to pay someone to do it.

@ewa.jodlowska - any thoughts?

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What part of “it” would it pay? Even if you pay someone to test all listed projects and file issues and PRs for the detected regressions, the PRs must still be reviewed and approved by a core developer of each of those projects.