Support major Python version syntax in runtime.txt

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Added `runtime.txt` support for the `python-3.X` major Python version form. ([#322](https://github.com/heroku/buildpacks-python/pull/322))
 - Enabled `libcnb`'s `trace` feature. ([#320](https://github.com/heroku/buildpacks-python/pull/320))
 
 ## [0.23.0] - 2025-01-13

src/errors.rs

@@ -8,7 +8,7 @@ use crate::layers::python::PythonLayerError;
 use crate::package_manager::DeterminePackageManagerError;
 use crate::python_version::{
     RequestedPythonVersion, RequestedPythonVersionError, ResolvePythonVersionError,
-    DEFAULT_PYTHON_FULL_VERSION, DEFAULT_PYTHON_VERSION, NEWEST_SUPPORTED_PYTHON_3_MINOR_VERSION,
+    DEFAULT_PYTHON_VERSION, NEWEST_SUPPORTED_PYTHON_3_MINOR_VERSION,
     OLDEST_SUPPORTED_PYTHON_3_MINOR_VERSION,
 };
 use crate::python_version_file::ParsePythonVersionFileError;
@@ -205,17 +205,22 @@ fn on_requested_python_version_error(error: RequestedPythonVersionError) {
             log_error(
                 "Invalid Python version in runtime.txt",
                 formatdoc! {"
-                    The Python version specified in 'runtime.txt' is not in the correct format.
+                    The Python version specified in 'runtime.txt' isn't in
+                    the correct format.
 
                     The following file contents were found:
                     {cleaned_contents}
 
-                    However, the file contents must begin with a 'python-' prefix, followed by the
-                    version specified as '<major>.<minor>.<patch>'. Comments are not supported.
+                    However, the version must be specified as either:
+                    1. 'python-<major>.<minor>' (recommended, for automatic updates)
+                    2. 'python-<major>.<minor>.<patch>' (to pin to an exact version)
+
+                    Remember to include the 'python-' prefix. Comments aren't
+                    supported.
 
-                    For example, to request Python {DEFAULT_PYTHON_FULL_VERSION}, update the 'runtime.txt' file so it
-                    contains exactly:
-                    python-{DEFAULT_PYTHON_FULL_VERSION}
+                    For example, to request the latest version of Python {DEFAULT_PYTHON_VERSION},
+                    update the 'runtime.txt' file so it contains:
+                    python-{DEFAULT_PYTHON_VERSION}
                 "},
             );
         }

src/python_version.rs

@@ -13,6 +13,8 @@ pub(crate) const DEFAULT_PYTHON_VERSION: RequestedPythonVersion = RequestedPytho
     patch: None,
     origin: PythonVersionOrigin::BuildpackDefault,
 };
+
+#[cfg(test)]
 pub(crate) const DEFAULT_PYTHON_FULL_VERSION: PythonVersion = LATEST_PYTHON_3_13;
 
 pub(crate) const OLDEST_SUPPORTED_PYTHON_3_MINOR_VERSION: u16 = 9;

src/runtime_txt.rs

@@ -2,7 +2,7 @@ use crate::python_version::{PythonVersionOrigin, RequestedPythonVersion};
 
 /// Parse the contents of a `runtime.txt` file into a [`RequestedPythonVersion`].
 ///
-/// The file is expected to contain a string of form `python-X.Y.Z`.
+/// The file is expected to contain a string of form `python-X.Y.Z` or `python-X.Y`.
 /// Any leading or trailing whitespace will be removed.
 pub(crate) fn parse(contents: &str) -> Result<RequestedPythonVersion, ParseRuntimeTxtError> {
     // All leading/trailing whitespace is trimmed, since that's what the classic buildpack
@@ -30,6 +30,12 @@ pub(crate) fn parse(contents: &str) -> Result<RequestedPythonVersion, ParseRunti
             patch: Some(patch),
             origin: PythonVersionOrigin::RuntimeTxt,
         }),
+        [major, minor] => Ok(RequestedPythonVersion {
+            major,
+            minor,
+            patch: None,
+            origin: PythonVersionOrigin::RuntimeTxt,
+        }),
         _ => Err(ParseRuntimeTxtError {
             cleaned_contents: cleaned_contents.clone(),
         }),
@@ -48,6 +54,15 @@ mod tests {
 
     #[test]
     fn parse_valid() {
+        assert_eq!(
+            parse("python-1.2"),
+            Ok(RequestedPythonVersion {
+                major: 1,
+                minor: 2,
+                patch: None,
+                origin: PythonVersionOrigin::RuntimeTxt
+            })
+        );
         assert_eq!(
             parse("python-1.2.3"),
             Ok(RequestedPythonVersion {
@@ -139,12 +154,6 @@ mod tests {
                 cleaned_contents: "python-1".to_string(),
             })
         );
-        assert_eq!(
-            parse("python-1.2"),
-            Err(ParseRuntimeTxtError {
-                cleaned_contents: "python-1.2".to_string(),
-            })
-        );
         assert_eq!(
             parse("python-1.2.3.4"),
             Err(ParseRuntimeTxtError {

tests/python_version_test.rs

@@ -392,17 +392,22 @@ fn runtime_txt_invalid_version() {
             context.pack_stderr,
             &formatdoc! {"
                 [Error: Invalid Python version in runtime.txt]
-                The Python version specified in 'runtime.txt' is not in the correct format.
+                The Python version specified in 'runtime.txt' isn't in
+                the correct format.
 
                 The following file contents were found:
                 python-an.invalid.version
 
-                However, the file contents must begin with a 'python-' prefix, followed by the
-                version specified as '<major>.<minor>.<patch>'. Comments are not supported.
+                However, the version must be specified as either:
+                1. 'python-<major>.<minor>' (recommended, for automatic updates)
+                2. 'python-<major>.<minor>.<patch>' (to pin to an exact version)
+
+                Remember to include the 'python-' prefix. Comments aren't
+                supported.
 
-                For example, to request Python {DEFAULT_PYTHON_FULL_VERSION}, update the 'runtime.txt' file so it
-                contains exactly:
-                python-{DEFAULT_PYTHON_FULL_VERSION}
+                For example, to request the latest version of Python {DEFAULT_PYTHON_VERSION},
+                update the 'runtime.txt' file so it contains:
+                python-{DEFAULT_PYTHON_VERSION}
             "}
         );
     });