Display a deprecation warning for runtime.txt
#1642
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) Heroku now supports the `.python-version` file as an alternative to the Heroku-specific (and now deprecated) `runtime.txt` file, and recommends apps switch to using the former since it's more widely supported by other tooling. The `.python-version` file is supported by pyenv, uv, the GitHub Actions `setup-python` action and many others. In addition, the `.python-version` file supports the version being specified as just the major version (eg `3.12` vs `3.12.N`) which means will pick up Python patch updates automatically (which is the recommended usage on Heroku). See: https://devcenter.heroku.com/changelog-items/3005 heroku/heroku-buildpack-python#1642 https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#using-the-python-version-file-input
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Support for
.python-versionfiles has been added to the Python CNB (our next-generation buildpack, currently in preview) in heroku/buildpacks-python#272, and is about to be added to this buildpack (the classic Python buildpack; the one currently used on Heroku) in #932 / #1664.The
.python-versionfile is superior toruntime.txtin several ways:runtime.txt, which is Heroku proprietary)3.Xrather than3.X.Y), so supports automatic security updates without needing to manually bump the patch version each time.As such, at some time after #932 has been implemented, we should add a deprecation warning when
runtime.txtis used, to gently encourage apps to transition to.python-versionfiles.We'll continue to support
runtime.txtin the classic buildpack, but having it show a deprecation warning will ease the future transition to the Python CNB (which itself will drop support forruntime.txtbefore GA: heroku/buildpacks-python#275).GUS-W-16878260.
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