Deprecate Python 3.9 support (#314)

In accordance with the Python version support policy:
https://devcenter.heroku.com/articles/python-support#python-version-support-policy

See also:
https://devcenter.heroku.com/changelog-items/3095
heroku/heroku-buildpack-python#1732

GUS-W-14846951.

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Deprecated support for Python 3.9. ([#314](https://github.com/heroku/buildpacks-python/pull/314))
 - Buildpack detection now recognises more Python-related file and directory names. ([#312](https://github.com/heroku/buildpacks-python/pull/312))
 - Improved the error messages shown for EOL or unrecognised major Python versions. ([#313](https://github.com/heroku/buildpacks-python/pull/313))
 

src/main.rs

@@ -20,14 +20,15 @@ use crate::layers::python::PythonLayerError;
 use crate::layers::{pip, pip_cache, pip_dependencies, poetry, poetry_dependencies, python};
 use crate::package_manager::{DeterminePackageManagerError, PackageManager};
 use crate::python_version::{
-    PythonVersionOrigin, RequestedPythonVersionError, ResolvePythonVersionError,
+    PythonVersionOrigin, RequestedPythonVersion, RequestedPythonVersionError,
+    ResolvePythonVersionError,
 };
 use indoc::formatdoc;
 use libcnb::build::{BuildContext, BuildResult, BuildResultBuilder};
 use libcnb::detect::{DetectContext, DetectResult, DetectResultBuilder};
 use libcnb::generic::{GenericMetadata, GenericPlatform};
 use libcnb::{buildpack_main, Buildpack, Env};
-use libherokubuildpack::log::{log_header, log_info};
+use libherokubuildpack::log::{log_header, log_info, log_warning};
 use std::io;
 
 struct PythonBuildpack;
@@ -91,6 +92,32 @@ impl Buildpack for PythonBuildpack {
             )),
         }
 
+        if let RequestedPythonVersion {
+            major: 3,
+            minor: 9,
+            origin,
+            ..
+        } = requested_python_version
+        {
+            log_warning(
+                "Support for Python 3.9 is deprecated",
+                formatdoc! {"
+                    Python 3.9 will reach its upstream end-of-life in October 2025,
+                    at which point it will no longer receive security updates:
+                    https://devguide.python.org/versions/#supported-versions
+
+                    As such, support for Python 3.9 will be removed from this
+                    buildpack on 7th January 2026.
+
+                    Upgrade to a newer Python version as soon as possible, by
+                    changing the version in your {origin} file.
+
+                    For more information, see:
+                    https://devcenter.heroku.com/articles/python-support#supported-python-versions
+                "},
+            );
+        }
+
         log_header("Installing Python");
         let python_layer_path = python::install_python(&context, &mut env, &python_version)?;
 

tests/django_test.rs

@@ -34,7 +34,7 @@ fn django_staticfiles_legacy_django() {
     TestRunner::default().build(
         default_build_config("tests/fixtures/django_staticfiles_legacy_django"),
         |context| {
-            assert_empty!(context.pack_stderr);
+            // We can't `assert_empty!(context.pack_stderr)` here, due to the Python 3.9 deprecation warning.
             assert_contains!(
                 context.pack_stdout,
                 indoc! {"

tests/python_version_test.rs

@@ -60,14 +60,38 @@ fn python_3_13() {
 }
 
 fn builds_with_python_version(fixture_path: &str, python_version: &PythonVersion) {
-    let PythonVersion {
+    let &PythonVersion {
         major,
         minor,
         patch,
     } = python_version;
 
     TestRunner::default().build(default_build_config(fixture_path), |context| {
-        assert_empty!(context.pack_stderr);
+        if major == 3 && minor == 9 {
+            assert_eq!(
+                context.pack_stderr,
+                indoc! {"
+                    
+                    [Warning: Support for Python 3.9 is deprecated]
+                    Python 3.9 will reach its upstream end-of-life in October 2025,
+                    at which point it will no longer receive security updates:
+                    https://devguide.python.org/versions/#supported-versions
+                    
+                    As such, support for Python 3.9 will be removed from this
+                    buildpack on 7th January 2026.
+                    
+                    Upgrade to a newer Python version as soon as possible, by
+                    changing the version in your .python-version file.
+                    
+                    For more information, see:
+                    https://devcenter.heroku.com/articles/python-support#supported-python-versions
+                    
+                "}
+            );
+        } else {
+            assert_empty!(context.pack_stderr);
+        }
+
         assert_contains!(
             context.pack_stdout,
             &formatdoc! {"
@@ -78,6 +102,7 @@ fn builds_with_python_version(fixture_path: &str, python_version: &PythonVersion
                 Installing Python {major}.{minor}.{patch}
             "}
         );
+
         // There's no sensible default process type we can set for Python apps.
         assert_contains!(context.pack_stdout, "no default process type");
 
@@ -294,13 +319,36 @@ fn python_version_non_existent_minor() {
     });
 }
 
+// This tests that:
+// - The Python version can be specified using runtime.txt
+// - pip works with the oldest currently supported Python version (3.9.0).
+// - The Python 3.9 deprecation warning correctly lists the origin as runtime.txt.
 #[test]
 #[ignore = "integration test"]
 fn runtime_txt() {
     let config = default_build_config("tests/fixtures/runtime_txt_and_python_version_file");
 
     TestRunner::default().build(config, |context| {
-        assert_empty!(context.pack_stderr);
+        assert_eq!(
+            context.pack_stderr,
+            indoc! {"
+                
+                [Warning: Support for Python 3.9 is deprecated]
+                Python 3.9 will reach its upstream end-of-life in October 2025,
+                at which point it will no longer receive security updates:
+                https://devguide.python.org/versions/#supported-versions
+                
+                As such, support for Python 3.9 will be removed from this
+                buildpack on 7th January 2026.
+                
+                Upgrade to a newer Python version as soon as possible, by
+                changing the version in your runtime.txt file.
+                
+                For more information, see:
+                https://devcenter.heroku.com/articles/python-support#supported-python-versions
+                
+            "}
+        );
         assert_contains!(
             context.pack_stdout,
             indoc! {"