Stop including pip in the final app image

After #254, pip is now installed into its own layer rather than into the
system site-packages directory inside the Python layer.

This means its now possible to exclude pip from the final app image, by
making the pip layer be a build-only layer.

Excluding pip from the final app image:
- Prevents several classes of user error/confusion/bad app design
  patterns seen in support tickets (see #255 for more details).
- Reduces app image supply chain surface area.
- Reduces app image size by 13 MB and layer count by 1, meaning less
  to have to push to the remote registry.
- Matches the approach used for Poetry, where we don't make Poetry
  available at run-time either.

Users that need pip at run-time for a temporary debugging task can run
`python -m ensurepip --default-pip` in the container at run-time to make
it available again (this command doesn't even have to download anything
- it uses the pip bundled with Python).

Or if pip is an actual run-time dependency of the app, then the app can
add `pip` to its `requirements.txt` (which much more clearly conveys the
requirements of the app, and also allows the app to pick what pip
version it needs at run-time).

Should we find that pip's absence causes confusion in the future, we
could always add a wrapper/shim `pip` script in the app image which does
something like:

```
echo "pip isn't installed at run-time, if you need it temporarily run 'python -m ensurepip --default-pip' to install it"
exit 1
```

...to improve discoverability.

We'll also document pip (and Poetry) being available at build-time only
in the docs that will be added by #11.

Closes #255.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Removed
+
+- pip is now only available at build time, and not also in the final app image. ([#264](https://github.com/heroku/buildpacks-python/pull/264))
+
 ## [0.17.0] - 2024-09-04
 
 ### Added

src/layers/pip.rs

@@ -31,7 +31,7 @@ pub(crate) fn install_pip(
         layer_name!("pip"),
         CachedLayerDefinition {
             build: true,
-            launch: true,
+            launch: false,
             invalid_metadata_action: &|_| InvalidMetadataAction::DeleteLayer,
             restored_layer_action: &|cached_metadata: &PipLayerMetadata, _| {
                 let cached_pip_version = cached_metadata.pip_version.clone();
@@ -49,15 +49,15 @@ pub(crate) fn install_pip(
         // reduce build log spam and prevent users from thinking they need to manually upgrade.
         // https://pip.pypa.io/en/stable/cli/pip/#cmdoption-disable-pip-version-check
         .chainable_insert(
-            Scope::All,
+            Scope::Build,
             ModificationBehavior::Override,
             "PIP_DISABLE_PIP_VERSION_CHECK",
             "1",
         )
         // Move the Python user base directory to this layer instead of under HOME:
         // https://docs.python.org/3/using/cmdline.html#envvar-PYTHONUSERBASE
         .chainable_insert(
-            Scope::All,
+            Scope::Build,
             ModificationBehavior::Override,
             "PYTHONUSERBASE",
             layer.path(),

tests/pip_test.rs

@@ -5,7 +5,6 @@ use libcnb_test::{assert_contains, assert_empty, BuildpackReference, PackResult,
 
 #[test]
 #[ignore = "integration test"]
-#[allow(clippy::too_many_lines)]
 fn pip_basic_install_and_cache_reuse() {
     let mut config = default_build_config("tests/fixtures/pip_basic");
     config.buildpacks(vec![
@@ -69,14 +68,13 @@ fn pip_basic_install_and_cache_reuse() {
 
         // Check that at run-time:
         // - The correct env vars are set.
-        // - pip is available (rather than just during the build).
-        // - Both pip and Python can find the typing-extensions package.
+        // - pip isn't available.
+        // - Python can find the typing-extensions package.
         let command_output = context.run_shell_command(
             indoc! {"
                 set -euo pipefail
                 printenv | sort | grep -vE '^(_|HOME|HOSTNAME|OLDPWD|PWD|SHLVL)='
-                echo
-                pip list
+                ! command -v pip > /dev/null || { echo 'pip unexpectedly found!' && exit 1; }
                 python -c 'import typing_extensions'
             "}
         );
@@ -85,18 +83,12 @@ fn pip_basic_install_and_cache_reuse() {
             command_output.stdout,
             formatdoc! {"
                 LANG=C.UTF-8
-                LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib:/layers/heroku_python/pip/lib
-                PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/layers/heroku_python/pip/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
-                PIP_DISABLE_PIP_VERSION_CHECK=1
+                LD_LIBRARY_PATH=/layers/heroku_python/venv/lib:/layers/heroku_python/python/lib
+                PATH=/layers/heroku_python/venv/bin:/layers/heroku_python/python/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
                 PIP_PYTHON=/layers/heroku_python/venv
                 PYTHONHOME=/layers/heroku_python/python
                 PYTHONUNBUFFERED=1
-                PYTHONUSERBASE=/layers/heroku_python/pip
                 VIRTUAL_ENV=/layers/heroku_python/venv
-                
-                Package           Version
-                ----------------- -------
-                typing_extensions 4.12.2
             "}
         );