diff --git a/repo2docker/app.py b/repo2docker/app.py
index 4625ac3f..e11f36b6 100755
--- a/repo2docker/app.py
+++ b/repo2docker/app.py
@@ -611,7 +611,7 @@ class Repo2Docker(Application):
             else:
                 # no port specified, pick a random one
                 container_port = host_port = str(self._get_free_port())
-                self.ports = {"%s/tcp" % container_port: host_port}
+                self.ports = {f"{container_port}/tcp": host_port}
             self.port = host_port
             # To use the option --NotebookApp.custom_display_url
             # make sure the base-notebook image is updated:
diff --git a/tests/unit/test_ports.py b/tests/unit/test_ports.py
index 445e6c4c..09944e87 100644
--- a/tests/unit/test_ports.py
+++ b/tests/unit/test_ports.py
@@ -96,7 +96,7 @@ def read_port_mapping_response(
         else:
             break
     else:
-        pytest.fail("Never succeeded in talking to %s" % url)
+        pytest.fail(f"Never succeeded in talking to {url}")
     assert "Directory listing" in r.text