fix: python-uv cached build copies dependencies to the build dir

`sam build --cached` (incremental build) invokes the python-uv workflow
with a `dependencies_dir` set. Two bugs made that path fail with
`CopyDependencies - [Errno 2] No such file or directory`:

1. `PythonUvWorkflow._setup_build_actions` constructed `CopyDependenciesAction`
   with `artifact_dir` and `destination_dir` swapped. Dependencies are
   installed into `dependencies_dir`, so that is the source of the copy and
   `artifacts_dir` the destination; as written the action listed the
   not-yet-created artifacts directory and never populated it.

2. `UvRunner.install_requirements` passed `--target` through verbatim. UV runs
   with `cwd` set to the project directory, so the relative
   `.aws-sam/deps/<uuid>` dependencies dir was created under the source
   directory instead of the build root. Resolve `--target` to an absolute
   path before invoking UV.

Adds regression coverage: workflow unit test asserts the
`CopyDependenciesAction` source/artifact/dest dirs; packager unit test
asserts a relative `--target` is made absolute; the `dependencies_dir`
integration test now asserts dependencies are present in the artifacts dir.

Fixes aws#864

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tomaDev

aws_lambda_builders/workflows/python_uv/packager.py

@@ -136,8 +136,10 @@ def install_requirements(
         # Add requirements file
         args.extend(["-r", requirements_path])
 
-        # Add target directory
-        args.extend(["--target", target_dir])
+        # Resolve --target to an absolute path: UV runs with cwd set to the project directory, so a
+        # relative target (e.g. the incremental-build dependencies dir) would otherwise be created
+        # under the source directory instead of the build root.
+        args.extend(["--target", os.path.abspath(target_dir)])
 
         # Add configuration arguments
         args.extend(config.to_uv_args())

aws_lambda_builders/workflows/python_uv/workflow.py

@@ -139,13 +139,15 @@ def _setup_build_actions(self, source_dir, artifacts_dir, scratch_dir, manifest_
                 )
             )
 
-        # Advanced case: Copy dependencies from dependencies_dir to artifacts_dir if configured
+        # Advanced case: copy dependencies from dependencies_dir to artifacts_dir if configured.
+        # Dependencies were installed into dependencies_dir above, so that is the copy source
+        # (artifact_dir) and artifacts_dir is the destination.
         if self.dependencies_dir and self.combine_dependencies:
             self.actions.append(
                 CopyDependenciesAction(
                     source_dir=source_dir,
-                    artifact_dir=artifacts_dir,
-                    destination_dir=self.dependencies_dir,
+                    artifact_dir=self.dependencies_dir,
+                    destination_dir=artifacts_dir,
                     maintain_symlinks=False,
                 )
             )

tests/integration/workflows/python_uv/test_python_uv.py

@@ -121,6 +121,9 @@ def test_workflow_with_dependencies_dir(self):
         dependencies_files = set(os.listdir(self.dependencies_dir))
         self.assertEqual(expected_dependencies.intersection(dependencies_files), expected_dependencies)
 
+        # combine_dependencies defaults to True, so dependencies must also be copied into the artifacts dir.
+        self.assertEqual(expected_dependencies.intersection(output_files), expected_dependencies)
+
     @skipIf(which("uv") is None, "uv not available")
     def test_workflow_builds_numpy_successfully(self):
         """Test that UV can build numpy (same as PIP workflow test)"""

tests/unit/workflows/python_uv/test_packager.py

@@ -125,6 +125,22 @@ def test_install_requirements_success(self):
         self.assertIn("-r", args_called)
         self.assertIn("/path/to/requirements.txt", args_called)
 
+    def test_install_requirements_resolves_relative_target_to_absolute(self):
+        # UV runs with cwd=project_dir, so a relative --target must be resolved to an absolute path
+        # first, otherwise dependencies land under the source dir instead of the build root.
+        self.mock_subprocess_uv.run_uv_command.return_value = (0, "success", "")
+
+        self.uv_runner.install_requirements(
+            requirements_path="/path/to/requirements.txt",
+            target_dir=os.path.join(".aws-sam", "deps", "abc-123"),
+            scratch_dir="/scratch",
+        )
+
+        args_called = self.mock_subprocess_uv.run_uv_command.call_args[0][0]
+        target_value = args_called[args_called.index("--target") + 1]
+        self.assertTrue(os.path.isabs(target_value), f"--target should be absolute, got: {target_value}")
+        self.assertEqual(target_value, os.path.abspath(os.path.join(".aws-sam", "deps", "abc-123")))
+
     def test_install_requirements_failure(self):
         self.mock_subprocess_uv.run_uv_command.return_value = (1, "", "error message")
 

tests/unit/workflows/python_uv/test_workflow.py

@@ -74,6 +74,13 @@ def test_workflow_sets_up_actions_with_dependencies_dir(self):
         self.assertIsInstance(self.workflow.actions[2], CopyDependenciesAction)
         self.assertIsInstance(self.workflow.actions[3], CopySourceAction)
 
+        # Dependencies are installed into dependencies_dir, then copied into artifacts_dir;
+        # artifact_dir is the copy source and dest_dir the destination (must not be swapped).
+        copy_deps_action = self.workflow.actions[2]
+        self.assertEqual(copy_deps_action.source_dir, "source")
+        self.assertEqual(copy_deps_action.artifact_dir, "deps")
+        self.assertEqual(copy_deps_action.dest_dir, "artifacts")
+
     def test_workflow_sets_up_actions_without_download_dependencies_and_dependencies_dir(self):
         self.workflow = PythonUvWorkflow(
             "source",