git-repo/docs/python-support.md

Supported Python Versions

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 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.

repo hooks

Projects that use repo hooks run on independent schedules. 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.

If the user's Python is too new for the repo hooks, then it is up to the hooks maintainer to update.

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 we do not guarantee old repo launchers 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 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.
2. Update the "soft" versions in the codebase. This will start warning users that the older version is deprecated.
   • Update repo if the launcher needs updating. This only helps with people who download newer launchers.
   • Update main.py for the main codebase. This warns for everyone regardless of repo launcher version.
   • Update 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.
3. 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:

$ 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:

$ repo-2.32 init --repo-rev=repo-1 ...