diff --git a/testing/functional/__init__.py b/testing/functional/__init__.py
index 6c82d2c7..22163fa3 100644
--- a/testing/functional/__init__.py
+++ b/testing/functional/__init__.py
@@ -94,8 +94,8 @@ class FunctionalTestCase(DuplicityTestCase):
         for item in passphrase_input:
             assert isinstance(item, str), f"item {os.fsdecode(item)} in passphrase_input is not unicode"
 
-        # set python path to be dev directory
-        os.environ["PYTHONPATH"] = _top_dir
+        # prepend dev directory to python path
+        os.environ["PYTHONPATH"] = _top_dir + ":" + os.environ["PYTHONPATH"]
 
         cmd_list = []