Add support for the .python-version file

Following on from the recent Python version refactoring, this now adds
support for configuring the app's Python version using a
`.python-version` file. This file is used by several tools in the Python
ecosystem (such as pyenv, `actions/setup-python`, uv), whereas the
existing `runtime.txt` file is proprietary to Heroku.

If both a `runtime.txt` file and a `.python-version` file are present,
then the `runtime.txt` file will take precedence. However, use of the
`.python-version` file is now recommended, since `runtime.txt` will
be deprecated in the future. Both the `runtime.txt` file and
`.python-version` file take precedence over any Python version specified
in a `Pipfile.lock` for Pipenv users.

We support the following `.python-version` syntax:
- Major Python version (e.g. `3.13`, which will then be resolved to the
  latest Python 3.13). (This form is recommended, since it allows for
  Python security updates to be pulled in without having to manually
  bump the version.)
- Exact Python version (e.g. `3.13.0`)
- Comments (lines starting with `#`)
- Blank lines

We don't support the following `.python-version` features:
- Specifying multiple Python versions
- Prefixing versions with `python-` (since this form is undocumented
  and will likely be deprecated by pyenv in the future)

This change is the classic Python buildpack equivalent of the Python
CNB change here:
heroku/buildpacks-python#272

In addition, the existing `runtime.txt` support has been updated to
allow specifying just the major Python version, in order to increase
feature parity between the files, and avoid confusion if users try to
use the major version only syntax from `.python-version` in their
`runtime.txt`.

Refs #913.
Refs #932.
GUS-W-7671453.
GUS-W-16821357.

Author: edmorley

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## [Unreleased]
 
+- Added support for configuring the Python version using a `.python-version` file. Both the `3.N` and `3.N.N` version forms are supported (the former is recommended). The existing `runtime.txt` file will take precedence if both files are found, however, we recommend switching to `.python-version` since it is more commonly supported in the Python ecosystem. ([#1664](https://github.com/heroku/heroku-buildpack-python/pull/1664))
+- Added support for specifying only the Python major version in `runtime.txt` instead of requiring the full Python version (for example `python-3.N` instead of `python-3.N.N`). ([#1664](https://github.com/heroku/heroku-buildpack-python/pull/1664))
 
 ## [v260] - 2024-10-10
 

README.md

@@ -8,61 +8,52 @@ This is the official [Heroku buildpack](https://devcenter.heroku.com/articles/bu
 
 Recommended web frameworks include **Django** and **Flask**, among others. The recommended webserver is **Gunicorn**. There are no restrictions around what software can be used (as long as it's pip-installable). Web processes must bind to `$PORT`, and only the HTTP protocol is permitted for incoming connections.
 
-See it in Action
-----------------
-```
-$ ls
-my-application		requirements.txt	runtime.txt
-
-$ git push heroku main
-Counting objects: 4, done.
-Delta compression using up to 8 threads.
-Compressing objects: 100% (2/2), done.
-Writing objects: 100% (4/4), 276 bytes | 276.00 KiB/s, done.
-Total 4 (delta 0), reused 0 (delta 0)
-remote: Compressing source files... done.
-remote: Building source:
-remote:
-remote: -----> Python app detected
-remote: -----> Installing python
-remote: -----> Installing pip
-remote: -----> Installing SQLite3
-remote: -----> Installing requirements with pip
-remote:        Collecting flask (from -r /tmp/build_c2c067ef79ff14c9bf1aed6796f9ed1f/requirements.txt (line 1))
-remote:          Downloading ...
-remote:        Installing collected packages: Werkzeug, click, MarkupSafe, Jinja2, itsdangerous, flask
-remote:        Successfully installed Jinja2-2.10 MarkupSafe-1.1.0 Werkzeug-0.14.1 click-7.0 flask-1.0.2 itsdangerous-1.1.0
-remote:
-remote: -----> Discovering process types
-remote:        Procfile declares types -> (none)
-remote:
-```
-
-A `requirements.txt` must be present at the root of your application's repository to deploy.
-
-To specify your python version, you also need a `runtime.txt` file - unless you are using the default Python runtime version.
-
-Current default Python Runtime: Python 3.12.7
-
-Alternatively, you can provide a `setup.py` file, or a `Pipfile`.
-Using `pipenv` will generate `runtime.txt` at build time if one of the field `python_version` or `python_full_version` is specified in the `requires` section of your `Pipfile`.
-
-Specify a Buildpack Version
----------------------------
-
-You can specify the latest production release of this buildpack for upcoming builds of an existing application:
-
-    $ heroku buildpacks:set heroku/python
-
-
-Specify a Python Runtime
-------------------------
-
-Supported runtime options include:
-
-- `python-3.13.0` on all [supported stacks](https://devcenter.heroku.com/articles/stack#stack-support-details)
-- `python-3.12.7` on all [supported stacks](https://devcenter.heroku.com/articles/stack#stack-support-details)
-- `python-3.11.10` on all [supported stacks](https://devcenter.heroku.com/articles/stack#stack-support-details)
-- `python-3.10.15` on all [supported stacks](https://devcenter.heroku.com/articles/stack#stack-support-details)
-- `python-3.9.20` on all [supported stacks](https://devcenter.heroku.com/articles/stack#stack-support-details)
-- `python-3.8.20` on Heroku-20 only
+## Example Usage
+
+## Getting Started
+
+See the [Getting Started on Heroku with Python](https://devcenter.heroku.com/articles/getting-started-with-python) tutorial.
+
+## Application Requirements
+
+A `requirements.txt` or `Pipfile` file must be present in the root (top-level) directory of your app's source code.
+
+## Configuration
+
+### Python Version
+
+We recommend that apps specify a Python version rather than relying on the buildpack's default Python version.
+
+For example, to request the latest patch release of Python 3.13, create a `.python-version` file in
+the root directory of your app containing:
+`3.13`
+
+The buildpack will look for a Python version in the following places (in descending order of precedence):
+
+1. `runtime.txt` file (deprecated)
+2. `.python-version` file (recommended)
+3. The `python_full_version` field in the `Pipfile.lock` file
+4. The `python_version` field in the `Pipfile.lock` file
+
+If none of those are found, the buildpack will use a default Python version (currently Python 3.12)
+for the first build of an app, and then subsequent builds of that app will be pinned to that version
+unless the build cache is cleared or you request a different version.
+
+The supported Python versions are:
+
+- Python 3.13
+- Python 3.12
+- Python 3.11
+- Python 3.10
+
+These Python versions are deprecated on Heroku:
+
+- Python 3.9
+- Python 3.8 (only available on the [Heroku-20](https://devcenter.heroku.com/articles/heroku-20-stack) stack)
+
+Python versions older than those listed above are no longer supported, since they have reached
+end-of-life [upstream](https://devguide.python.org/versions/#supported-versions).
+
+## Documentation
+
+For more information about using Python on Heroku, see [Dev Center](https://devcenter.heroku.com/categories/python-support).

bin/compile

@@ -154,6 +154,9 @@ fi
 python_version::read_requested_python_version "${BUILD_DIR}" "${package_manager}" "${cached_python_version}" requested_python_version python_version_origin
 meta_set "python_version_reason" "${python_version_origin}"
 
+# TODO: More strongly recommend specifying a Python version (eg switch the messaging to
+# be a warning instead, after version resolution, and mention .python-version inline)
+# TODO: Add runtime.txt deprecation warning.
 case "${python_version_origin}" in
 	default)
 		puts-step "No Python version was specified. Using the buildpack default: Python ${requested_python_version}"

bin/steps/python

@@ -18,7 +18,7 @@ PYTHON_URL="${S3_BASE_URL}/python-${python_full_version}-ubuntu-${UBUNTU_VERSION
 # TODO: Update this message to be more specific once Python 3.8 support is dropped.
 if ! curl --output /dev/null --silent --head --fail --retry 3 --retry-connrefused --connect-timeout 10 "${PYTHON_URL}"; then
 	display_error <<-EOF
-		Error: Python ${python_full_version} is not available for this stack (${STACK}).
+		Error: Python ${python_full_version} isn't available for this stack (${STACK}).
 
 		For a list of the supported Python versions, see:
 		https://devcenter.heroku.com/articles/python-support#supported-runtimes
@@ -35,6 +35,8 @@ function warn_if_patch_update_available() {
 	# Extract the patch version component of the version strings (ie: the '5' in '3.10.5').
 	local requested_patch_number="${requested_full_version##*.}"
 	local latest_patch_number="${latest_patch_version##*.}"
+	# TODO: Update this message to suggest using the .python-version major version syntax to stay up to date,
+	# once runtime.txt is deprecated and sticky-versioning only pins to the major version.
 	if ((requested_patch_number < latest_patch_number)); then
 		puts-warn
 		puts-warn "A Python security update is available! Upgrade as soon as possible to: Python ${latest_patch_version}"