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python-support: update with current status & guidelines

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This doc was written back in 2019 when we were planning on the Python 3
migration.  It isn't relevant anymore, and people are reading it thinking
we still support Python 2.  Rewrite it to match current requirements and
to make it clear there is no support for older versions.

Bug: 302871152
Change-Id: I2acf3aee1816a03ee0a70774db8bf4a23713a03f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389455
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
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Mike Frysinger 2023-10-14 02:10:39 +05:45 committed by LUCI
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# Supported Python Versions
With Python 2.7 officially going EOL on [01 Jan 2020](https://pythonclock.org/),
we need a support plan for the repo project itself.
Inevitably, there will be a long tail of users who still want to use Python 2 on
their old LTS/corp systems and have little power to change the system.
This documents the current supported Python versions, and tries to provide
guidance for when we decide to drop support for older versions.
## Summary
* Python 3.6 (released Dec 2016) is required by default starting with repo-2.x.
* Older versions of Python (e.g. v2.7) may use the legacy feature-frozen branch
based on repo-1.x.
* Python 3.6 (released Dec 2016) is required starting with repo-2.0.
* Older versions of Python (e.g. v2.7) may use old releases via the repo-1.x
branch, but no support is provided.
## Overview
We provide a branch for Python 2 users that is feature-frozen.
Bugfixes may be added on a best-effort basis or from the community, but largely
no new features will be added, nor is support guaranteed.
Users can select this during `repo init` time via the [repo launcher].
Otherwise the default branches (e.g. stable & main) will be used which will
require Python 3.
This means the [repo launcher] needs to support both Python 2 & Python 3, but
since it doesn't import any other repo code, this shouldn't be too problematic.
The main branch will require Python 3.6 at a minimum.
If the system has an older version of Python 3, then users will have to select
the legacy Python 2 branch instead.
### repo hooks
## repo hooks
Projects that use [repo hooks] run on independent schedules.
They might migrate to Python 3 earlier or later than us.
To support them, we'll probe the shebang of the hook script and if we find an
interpreter in there that indicates a different version than repo is currently
running under, we'll attempt to reexec ourselves under that.
Since it's not possible to detect what version of Python the hooks were written
or tested against, we always import & exec them with the active Python version.
For example, a hook with a header like `#!/usr/bin/python2` will have repo
execute `/usr/bin/python2` to execute the hook code specifically if repo is
currently running Python 3.
If the user's Python is too new for the [repo hooks], then it is up to the hooks
maintainer to update.
For more details, consult the [repo hooks] documentation.
## Repo launcher
The [repo launcher] is an independent script that can support older versions of
Python without holding back the rest of the codebase.
If it detects the current version of Python is too old, it will try to reexec
via a newer version of Python via standard `pythonX.Y` interpreter names.
However, this is provided as a nicety when it is not onerous, and there is no
official support for older versions of Python than the rest of the codebase.
If your default python interpreters are too old to run the launcher even though
you have newer versions installed, your choices are:
* Modify the [repo launcher]'s shebang to suite your environment.
* Download an older version of the [repo launcher] and don't upgrade it.
Be aware that there is no guarantee old repo launchers are WILL work with
current versions of repo. Bug reports using old launchers will not be
accepted.
## When to drop support
So far, Python 3.6 has provided most of the interesting features that we want
(e.g. typing & f-strings), and there haven't been features in newer versions
that are critical to us.
That said, let's assume we need functionality that only exists in Python 3.7.
How do we decide when it's acceptable to drop Python 3.6?
1. Review the [Project References](./release-process.md#project-references) to
see what major distros are using the previous version of Python, and when
they go EOL. Generally we care about Ubuntu LTS & current/previous Debian
stable versions.
* If they're all EOL already, then go for it, drop support.
* If they aren't EOL, start a thread on [repo-discuss] to see how the user
base feels about the proposal.
1. Update the "soft" versions in the codebase. This will start warning users
that the older version is deprecated.
* Update [repo](/repo) if the launcher needs updating.
This only helps with people who download newer launchers.
* Update [main.py](/main.py) for the main codebase.
This warns for everyone regardless of [repo launcher] version.
* Update [requirements.json](/requirements.json).
This allows [repo launcher] to display warnings/errors without having
to execute the new codebase. This helps in case of syntax or module
changes where older versions won't even be able to import the new code.
1. After some grace period (ideally at least 2 quarters after the first release
with the updated soft requirements), update the "hard" versions, and then
start using the new functionality.
## Python 2.7 & 3.0-3.5
> **There is no support for these versions.**
> **Do not file bugs if you are using old Python versions.**
> **Any such reports will be marked invalid and ignored.**
> **Upgrade your distro and/or runtime instead.**
Fetch an old version of the [repo launcher]:
```sh
$ curl https://storage.googleapis.com/git-repo-downloads/repo-2.32 > ~/.bin/repo-2.32
$ chmod a+rx ~/.bin/repo-2.32
```
Then initialize an old version of repo:
```sh
$ repo-2.32 init --repo-rev=repo-1 ...
```
[repo-discuss]: https://groups.google.com/forum/#!forum/repo-discuss
[repo hooks]: ./repo-hooks.md
[repo launcher]: ../repo