Display the top 10 lines in git diff for each modified file in error message.

Author: Qian-HuiChen

src/model_config_tests/config_tests/test_bit_reproducibility.py

@@ -333,7 +333,7 @@ def test_repro_determinism_restart(
 @pytest.mark.repro_payu_setup
 def test_repro_payu_setup(control_path, output_path):
     """
-    Test payu setup with --reproduce which errors if payu manifests full hashes are changed
+    Test payu setup with `git diff` which errors if any files in payu manifests are changed.
     """
     experiment = setup_exp(control_path, output_path, exp_name="repro_payu_setup")
     try:

src/model_config_tests/exp_test_helper.py

@@ -110,8 +110,23 @@ def setup_reproduce(self):
             os.chdir(owd)
 
         if result.stdout != "":
+            # Collect and display the top 10 lines of the diff for each modified file
+            files = result.stdout.strip().split("\n")
+            error_message = ""
+            for file in files:
+                error_message += f"Modifications are detected in {file}:\n"
+                diff_details = sp.run(
+                    ["git", "diff", f"{file}"],
+                    capture_output=True,
+                    text=True,
+                )
+                diff_lines = diff_details.stdout.splitlines()
+                top_lines = "\n".join(diff_lines[2:12]) 
+                if len(diff_lines) > 12:
+                    top_lines += "\n... (truncated)"
+                error_message += f"Changes are: \n {top_lines}\n"
             raise RuntimeError(
-                f"Manifests have been modified. The modified files include: {result.stdout}.\n"
+                f"{error_message}"
             )
 
     def setup_for_test_run(self):