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Python Enhancement Proposals

PEP 2026 – Calendar versioning for Python

Author:
Hugo van Kemenade
Discussions-To:
Discourse thread
Status:
Draft
Type:
Process
Created:
11-Jun-2024
Python-Version:
3.26
Post-History:
14-Jun-2024

Table of Contents

Abstract

This PEP proposes updating the versioning scheme for Python to include the calendar year.

Calendar Versioning (CalVer) makes everything easier to translate into calendar time rather than counting versions and looking up when they will be (or were) released:

  • The support lifecycle is clear, making it easy to see when a version was first released.
  • Deprecations are easier to manage for maintainers and users.
  • It’s easier to work out when a version will reach end of life (EOL).
  • It helps people, especially new learners, understand how old their installation is.
  • It’s easier to reason about which versions of Python to support for libraries and applications.

Starting with what would have been Python 3.15, the version is 3.YY.micro where YY is the year of initial release:

  • Python 3.26 will be released in 2026 instead of Python 3.15. EOL is five years after initial release, therefore Python 3.26 will reach EOL in 2031.
  • Python 3.27 will be released in 2027, and so on.

Motivation and rationale

In 2019, we adopted an annual release cycle with PEP 602, which opened the door for calendar versioning:

Adopting an annual release calendar allows for natural switching to calendar versioning, for example by calling Python 3.9 “Python 3.20” since it’s released in October ‘20 and so on (“Python 3.23” would be the one released in October ‘23).

While the ease of switching to calendar versioning can be treated as an advantage of an annual release cycle, this PEP does not advocate for or against a change in how Python is versioned. Should the annual release cycle be adopted, the versioning question will be dealt with in a separate PEP.

This is that PEP.

Current scheme

From the General Python FAQ: How does the Python version numbering scheme work?

Python versions are numbered “A.B.C” or “A.B”:
  • A is the major version number – it is only incremented for really major changes in the language.
  • B is the minor version number – it is incremented for less earth-shattering changes.
  • C is the micro version number – it is incremented for each bugfix release.

Python predates SemVer

Semantic Versioning (SemVer) is a popular scheme which aims to communicate the intent of a release (though it doesn’t always succeed).

Given a version number MAJOR.MINOR.PATCH, increment the:
  1. MAJOR version when you make incompatible API changes
  2. MINOR version when you add functionality in a backward compatible manner
  3. PATCH version when you make backward compatible bug fixes

People often assume Python follows SemVer and complain about breaking changes in feature releases. But Python predates SemVer by at least 15 years: the SemVer spec was introduced in 2009 and the bespoke Python scheme was added to source control in 1994 for the 1.0 release.

If Python adopted SemVer, that would imply a new major bump every year when we remove deprecations.

Instead of SemVer, however, some projects have adopted another versioning scheme based on the calendar.

Calendar versioning

With Calendar Versioning (CalVer), you include some element of the date in the version number. For example, Ubuntu and Black use the year and month – Ubuntu 24.04 came out in April 2024; pip and PyCharm use only the year.

Ubuntu Black pip PyCharm
YY.0M.micro YY.MM.micro YY.minor.micro YYYY.minor.micro
23.04
23.10
24.04
24.10
23.12.1
24.1.0
24.1.1
24.2.0
23.3
23.3.1
23.3.2
24.0
2023.3.5
2024.1
2024.1.1
2024.1.2

And here are some programming language standards, all using some form of the year:

Ada Algol C C++ Fortran
ECMAScript
aka JavaScript
YY / YYYY YY YY YY YY / YYYY YYYY
83
95
2012
2022
58
60
68
89
99
11
23
98
03
11
23
66
90
2003
2023
2020
2021
2022
2023

Annual release cadence

Since 2019, we’ve made a release each year:

  • 3.15.0 will be released in 2026
  • 3.16.0 will be released in 2027
  • 3.17.0 will be released in 2028
  • 3.18.0 will be released in 2029
  • 3.19.0 will be released in 2030

This is sort of calendar-based, it’s just that it’s offset by 11 years.

CalVer for Python

The simplest CalVer option would be to stick with major version 3, and encode the year in the minor version:

  • 3.26.0 will be released in 2026
  • 3.27.0 will be released in 2027
  • 3.28.0 will be released in 2028
  • 3.29.0 will be released in 2029
  • 3.30.0 will be released in 2030

For example, 3.26 will be released in 2026. It makes it obvious when a release first came out.

Clarity of deprecation removal

Warnings for deprecations often mention the version they will be removed in. For example:

DeprecationWarning: ‘ctypes.SetPointerType’ is deprecated and slated for removal in Python 3.15

However, once aware of CalVer, it is immediately obvious from the warning how long you have to to take action:

DeprecationWarning: ‘ctypes.SetPointerType’ is deprecated and slated for removal in Python 3.26

Clarity of support lifecycle

Right now, it’s a little tricky to work out when a release is end-of-life. First you have to look up when it was initially released, then add 5 years:

“When will Python 3.11 be EOL?”

“Well, let’s see… PEP 664 is the 3.11 release schedule, it says 3.11 was released in 2022, EOL after 5 years, so 2022 + 5 = 2027.”

But if the initial release year is right there in the version, it’s much easier:

“When will Python 3.26 be EOL?”

“26 + 5 = [20]31”

Clarity of installation age

With the year in the version, it’s easier to work out how old your installation is. For example, with the current scheme, if you’re using Python 3.15 in 2035, it’s not immediately clear that it was first released in 2026 (and has been EOL since 2031).

With knowledge of CalVer, if you’re using Python 3.26 in 2035, it’s clear it was first released nine years ago and it’s probably time to upgrade.

This can help prompt people to switch to supported releases still under security support, and help in teaching new users who may have older installations.

Clarity of version support

CalVer makes it easier to reason about which versions of Python to support.

For example, without CalVer, setting your minimum compatible Python version to 3.19 in 2031 sets an aggressive assumption regarding version adoption and support.

However, with CalVer, this is more obvious if setting the minimum to 3.30 in 2031. For wider support, perhaps you prefer setting it to 3.26.

Similarly, library maintainers supporting all CPython upstream versions need to test against five versions (or six including the pre-release).

For example, in 2030, the supported versions without CalVer would be:

  • 3.15, 3.16, 3.17, 3.18, 3.19

With CalVer they would be:

  • 3.26, 3.27, 3.28, 3.29, 3.30

A maintainer can see at a glance which versions are current and need testing.

Non-goals

Like the current scheme, only the micro version will be incremented for bug fix and security releases, with no change to the major and minor. For example:

Current scheme Proposed 3.YY.micro
Initial release (Oct ’26) 3.15.0 3.26.0
1st bugfix release (Dec ’26) 3.15.1 3.26.1
2nd bugfix release (Feb ’27) 3.15.2 3.26.2
Final security release (Oct ’31) 3.15.17 3.26.17

No change to PEP 602 (Annual Release Cycle for Python):

  • No change to the 17 months to develop a feature version: alphas, betas and release candidates.
  • No change to the support duration: two years of full support and three years of security fixes.
  • No change to the annual October release cadence.

Specification

Python versions are numbered 3.YY.micro where:

  • 3 is the major version number – it is always 3.
  • YY is the minor version number - it is the short year number: {year} - 2000.
  • micro is the micro version number - it is incremented for each bugfix or security release.

We’ll keep major version 3. Python 3 is the brand; there will be no Python 4.

In the year 2100, the minor will be 2100-2000 = 100, therefore the version will be 3.100.0.

Python 3.14 will be the last version before this change, released in 2025. Python 3.26 will be the first version after this change, released in 2026. There will be no Python 3.15 to 3.25 inclusive.

Security implications

None known. No change to durations or timing of bug fix and security phases.

How to teach this

We will announce this on blogs, in the 3.14 release notes, documentation, and through outreach to the community.

This change targets the version following 3.14: instead of 3.15 it will be 3.26. This PEP was proposed in June 2024. Development for the 3.15/3.26 release will begin in May 2025, with the first alpha in October 2025 and initial release in October 2026. We can already update documentation during the 3.14 cycle. This gives plenty of notice.

We can make preview builds which only change the version for early testing.

We could ship a python3.15 command as part of Python 3.26 that immediately errors out and tells the user to use python3.26 instead.

Rejected ideas

YY.0

For example, Python 26.0 would be released in 2026.

There’s not much appetite for Python version 4. We don’t want to repeat 2-to-3, and 4 has a lot of expectations by now. We don’t want “earth-shattering changes”.

Perhaps Python 4 could be reserved for something big like removing the GIL (PEP 703), but the Steering Council made it clear the free-threading rollout must be gradual. Will we stick with version 3 forever?

Another option would be to put the year in the major version and jump to 26.0. This could mean we could leapfrog all that 4.0 baggage.

Platform compatibility tags

Changing the major version would complicate packaging, however.

The Platform compatibility tags specification says the Python version tag used in wheel filenames is given by sysconfig.get_config_var("py_version_nodot"), where the major and minor versions are joined together without a dot. For example, 3.9 is 39.

During the 3.10 alpha, there was ambiguity because 310 can be interpreted as 3.10, 31.0, or 310.

The specification says an underscore can be used if needed, and PEP 641 (“Using an underscore in the version portion of Python 3.10 compatibility tags”) proposed this:

Version → tag → version PEP 641 proposed version
Pre-3.10 3.9 → 39
Ambiguity after 3.10 3.10 → 310 → 3.10 or 31.0 or 310? 3_10
Ambiguity with YY.xx 26.0 → 260 → 2.60 or 26.0 or 260? 26_0

However, PEP 641 was rejected because it was unknown what side effects there would be on code we’re not aware of.

We would need something like this for YY.0 versioning, which would be a significant amount of complex work.

Ecosystem changes

Would changing the major version to double digits break code?

Yes, any novel change to the version inevitably does because people make assumptions, such as the major always being 3, or that the version parts are always single digits. For example:

Version change Example Expected Actual
2.7.9 → 2.7.10
'this is Python {}'.format(sys.version[:5])
2.7.10 2.7.1
3.9 → 3.10
".%s-%s" % (get_platform(), sys.version[0:3])
3.10 3.1
3 → 4
if sys.version_info[1] >= 9:
4.0 0
3 → 26
if sys.version[0] == '3':
26 2

The last one here is most relevant for YY.0 versioning. Therefore the 3.YY scheme is the safest and requires fewest changes, because the shape of the version doesn’t change: it’s still a 3 followed by two digits.

Tip

Use Ruff’s YTT rules or Flake8’s flake8-2020 plugin to help find the problems like these.

python3 command

PEP 394 (The “python” Command on Unix-Like Systems) outlines recommendations for the python, python2 and python3 commands. python can map to either python2 or python3. These would need revisiting if the major version changed, and started changing annually.

Four years after Python 2.7’s end-of-life, we could recommend python only maps to the the latest Python 3+ version. But what would python3 map to when Python 26.0 is out? This would introduce additional complexity and cost.

CPython changes

In addition to python3 command changes, there are at least four places in CPython that assume the major version is 3 and would need updating:

YY.0 rejection

The benefits of calendar versioning are not so big compared to the combined costs for YY.0 versioning. Therefore, YY.0 versioning is rejected.

YY.MM

For example, Python 26.10 would be released in October 2026.

Building upon YY.0 versioning, we could also include the release month as the minor version, like Ubuntu and Black. This would make it clear when in the year it was released, and also when in the year it will reach end-of-life.

However, YY.MM versioning is rejected for many of the same reasons as YY.0 versioning.

3.YYYY

For example, Python 3.2026 would be released in 2026.

It’s clearer that the minor version is a year when using a four digits, and avoids confusion with Ubuntu versions which use YY.MM. However, this is rejected as changing from two to four digits would break more code than 3.YY versioning.

Editions

For example, Python 3.15 (2026 Edition) would be released in 2026.

The Rust language uses “Editions” to introduce breaking changes. Applying this to Python would require big changes to PEP 387 (Backwards Compatibility Policy) and is out of scope for this PEP.

We could apply a year label to releases, such as “Python 3.15 (2026 Edition)”, but this is rejected because we’d have to keep track of two numbers.

Adopt SemVer and skip 4

For example, Python 5.0 would be released in 2026, 6.0 in 2027, and so on.

We could skip the problematic 4.0 entirely and adopt SemVer. Because deprecations are removed in every feature release, we would get a new major bump every year.

This is rejected because we wouldn’t get the benefit of calendar versioning, and moving away from 3.x would also break code.

Change during 3.14 cycle

The Python 3.14 release must go ahead because: π.

Backwards compatibility

This version change is the safest of the CalVer options considered (see rejected ideas): we keep 3 as the major version, and the minor version is still two digits. The minor will eventually change to three digits but this is predictable, a long way off and can be planned for.

We retain the python3 executable.

Version mapping

Versions 3.15 to 3.25 inclusive will be skipped. Features, deprecations and removals planned for these will be remapped to the new version numbers.

For example, a deprecation initially planned for removal in 3.16 will instead be removed in 3.27.

Old version New version Initial release
3.14 3.14 (no change) 2025
3.15 3.26 2026
3.16 3.27 2027
3.17 3.28 2028
3.18 3.29 2029
3.19 3.30 2030
3.20 3.31 2031
3.21 3.32 2032
3.22 3.33 2033
3.23 3.34 2034
3.24 3.35 2035
3.25 3.36 2036

Forwards compatibility

Future change in cadence

This PEP proposes no change to the annual release cadence as defined in PEP 602, which lays out many good reasons for annual releases (for example, smaller releases with a predictable release calendar, and syncing with external redistributors). However unlikely, should we decide to change the cadence in the future, CalVer does not preclude doing so.

Less frequent

If we went to fewer than one release per year, the proposed CalVer scheme still works; indeed, it even helps people know in which year to expect the release. For example, if we released every second year starting in 2036:

  • 3.36.0 would be released in 2036
  • 3.38.0 would be released in 2038
  • and so on

Ecosystem changes depend in part on how the the hypothetical cadence-changing PEP updates PEP 387 (Backwards Compatibility Policy). If, for example, it requires that the deprecation period must be at least one feature release and not the current two (to maintain the minimum two years), CalVer has the benefit over the status quo in requiring no changes to planned removal versions (other than adjusting any falling in non-release years).

More frequent

If we went to more than one release per year, here are some options. For example, if we released in April and October starting in 2036, the next four releases could be:

Scheme Notes 2036 a 2036 b 2037 a 2037 b
YY.MM.micro Year as major, month as minor 36.04.0 36.10.0 37.04.0 37.10.0
YY.x.micro Year as major, serial number as minor 36.1.0 36.2.0 37.1.0 37.2.0
3.YYMM.micro Combine year and month as minor 3.3604.0 3.3610.0 3.3704.0 3.3710.0
3.YYx.micro Combine year and serial number as minor 3.360.0 3.361.0 3.370.0 3.371.0
3.YY.MM.micro Add an extra month segment 3.36.04.0 3.36.10.0 3.37.04.0 3.37.10.0
3.major.micro No more CalVer: increment minor 3.36.0 3.37.0 3.38.0 3.39.0
3.50.0 3.51.0 3.52.0 3.53.0
3.100.0 3.101.0 3.102.0 3.103.0
4.major.micro No more CalVer: increment major 4.0.0 4.1.0 4.2.0 4.3.0
5.major.micro 5.0.0 5.1.0 5.2.0 5.3.0

The YY options would require addressing issues around the platform compatibility tags, the python3 command, and code assuming the version always begins with 3.

The options keeping major version 3 but changing the minor to three or four digits would also need to address code assuming the version is always two digits.

The option adding an extra month segment is the biggest change as code would need to deal with a four-part version instead of three.

The options dropping CalVer would be the most conservative allowing the major and minor to be chosen freely.

No more CalVer

Adopting CalVer now does not preclude moving away CalVer in the future, for example, back to the original scheme, to SemVer or another scheme. Some options are listed in the table above. If wanting to make it clear the minor is no longer the year, it can be bumped to a higher round number (for example, 3.50 or 3.100) or the major version can be bumped (for example, to 4.0 or 5.0). Additionally, a version epoch could be considered.

Footnotes

The author proposed calendar versioning at the Python Language Summit 2024; this PEP is a result of discussions there and during PyCon US.

Read the slides and blogpost of the summit talk.

Acknowledgements

Thanks to Seth Michael Larson for the Language Summit Q&A notes and blogpost, and to everyone who gave feedback at the summit and PyCon US.

Thank you to Łukasz Langa and Alex Waygood for reviewing a draft of this PEP.


Source: https://github.com/python/peps/blob/main/peps/pep-2026.rst

Last modified: 2024-07-11 15:35:42 GMT