diff --git a/colcon_core/task/python/test/__init__.py b/colcon_core/task/python/test/__init__.py
index 8ccb6964..a01ec478 100644
--- a/colcon_core/task/python/test/__init__.py
+++ b/colcon_core/task/python/test/__init__.py
@@ -137,9 +137,7 @@ async def step(self):
         raise NotImplementedError()
 
 
-def get_python_testing_step_extensions(
-    *, group_name='colcon_core.python_testing',
-):
+def get_python_testing_step_extensions(*, group_name=None):
     """
     Get the available Python testing step extensions.
 
@@ -147,6 +145,8 @@ def get_python_testing_step_extensions(
 
     :rtype: OrderedDict
     """
+    if group_name is None:
+        group_name = 'colcon_core.python_testing'
     extensions = instantiate_extensions(group_name, unique_instance=False)
     for name in list(extensions.keys()):
         extension = extensions[name]