diff --git a/fastapi/routing.py b/fastapi/routing.py
index eaaeee7481..512f718f8c 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -1506,7 +1506,9 @@ class APIRouter(routing.Router):
                 )
             elif isinstance(route, routing.Mount):
                 if isinstance(route.app, StaticFiles):
-                    self._mount(prefix + router.prefix + route.path, route.app, name=route.name)
+                    self._mount(
+                        prefix + router.prefix + route.path, route.app, name=route.name
+                    )
         for handler in router.on_startup:
             self.add_event_handler("startup", handler)
         for handler in router.on_shutdown:
diff --git a/tests/test_apirouter_mount.py b/tests/test_apirouter_mount.py
index 1a2de54822..e6c18b7687 100644
--- a/tests/test_apirouter_mount.py
+++ b/tests/test_apirouter_mount.py
@@ -1,4 +1,5 @@
 """Tests for mounting StaticFiles under APIRouter (issue #10180)."""
+
 from pathlib import Path
 
 import pytest