test: show output if subprocess fails

src/darker/tests/test_main.py

@@ -11,7 +11,7 @@
 from argparse import ArgumentError
 from io import BytesIO
 from pathlib import Path
-from subprocess import PIPE, run  # nosec
+from subprocess import PIPE, CalledProcessError, run  # nosec
 from textwrap import dedent
 from types import SimpleNamespace
 from unittest.mock import ANY, Mock, call, patch
@@ -969,14 +969,19 @@ def test_stdout_newlines_subprocess(newline):
 
     """
     code = f"import collections{newline}import sys{newline}".encode()
+    try:
 
-    darker_subprocess = run(  # nosec
-        ["darker", "--stdout", "--isort", "--stdin-filename=new-file.py", "-"],
-        input=code,
-        stdout=PIPE,
-        check=True,
-    )
+        darker_subprocess = run(  # nosec
+            ["darker", "--stdout", "--isort", "--stdin-filename=new-file.py", "-"],
+            input=code,
+            stdout=PIPE,
+            check=True,
+        )
 
+    except CalledProcessError as e:
+        print(e.stdout.decode())
+        print(e.stderr.decode())
+        raise
     assert darker_subprocess.stdout == code