diff --git a/lib/iris/tests/test_coding_standards.py b/lib/iris/tests/test_coding_standards.py
index 96c87b2734..4964d560c5 100644
--- a/lib/iris/tests/test_coding_standards.py
+++ b/lib/iris/tests/test_coding_standards.py
@@ -115,7 +115,7 @@ def test_python_versions():
             f"PY_VER={latest_supported[:1]}.{latest_supported[1:]}",
         ),
         (benchmark_runner_file, f'python_version = "{latest_supported}"'),
-        (tox_config_file, '[testenv:py{' + ','.join(all_supported)+ '}-'),
+        (tox_config_file, "[testenv:py{" + ",".join(all_supported) + "}-"),
     ]
 
     for ver in all_supported: