Support the `.python-version` file

[edmorley]

We should support the .python-version file as a means for specifying what Python version an app is using.

This file format (used by pyenv and others) now also supports specifying Python versions in the X.Y format (and not just X.Y.Z), making it a strong contender over the Heroku-specific runtime.txt file.

Officially supported syntax:

• 3.12.6
• 3.12
• comments (lines starting with #)
• blank lines
• pyenv supports specifying multiple versions, however, this buildpack can't, so we'll either want to only use the first version, or error if there are multiple versions specified

Unofficially supported syntax:

• python-3.12.6
• python-3.12 (though this currently has a bug: The auto-resolution of latest versions feature doesn't work with python- prefixes (eg python-3.12) pyenv/pyenv#3054)

...though the python- forms are undocumented and likely going to be deprecated soon (pyenv/pyenv#3054 (comment)), so I'm leaning towards not supporting them.

For more background, see:

• Python Version via .python-version heroku-buildpack-python#932
• Support flexible patch version syntax to allow for automatic Python updates heroku-buildpack-python#913 (comment)
• Internal tracking epic

[edmorley]

I found a bug in pyenv's handling of some of the more obscure syntax variants, which I filed as:
pyenv/pyenv#3054

And have opened pyenv/pyenv#3056 to fix.

[megahall]

I created a one line .python-version containing 3.12 and tested this on Heroku but it did not work for me. Is there some special procedure or ETA before this buildpack update kicks in?

remote: -----> Python app detected
remote: -----> No Python version was specified. Using the same version as the last build: python-3.12.6
remote:        To use a different version, see: https://devcenter.heroku.com/articles/python-runtimes

[edmorley]

@megahall Hi! This repository is for the Heroku Python Cloud Native Buildpack (the next generation of buildpacks, currently in preview), rather than the existing (classic) Heroku Python buildpack, which is at:
https://github.com/heroku/heroku-buildpack-python

Support for Poetry and .python-version will be backported to the classic buildpack soon, but if you wanted to try out the Python CNB in the meantime, there is more information here:
https://github.com/heroku/buildpacks

[edmorley]

Support for Poetry and .python-version will be backported to the classic buildpack soon

To track that, subscribe to:

• Python Version via .python-version heroku-buildpack-python#932
• Support for Poetry heroku-buildpack-python#796

[merwok]

if you wanted to try out the Python CNB in the meantime, there is more information here

I was curious about this, as I’ve been following recent activity, but all the docs I found talk about running tools locally, not using the CNB on heroku.