diff --git a/fastapi/openapi/utils.py b/fastapi/openapi/utils.py
index 46b685198..da7bebab8 100644
--- a/fastapi/openapi/utils.py
+++ b/fastapi/openapi/utils.py
@@ -423,7 +423,7 @@ def get_openapi(
             if result:
                 path, security_schemes, path_definitions = result
                 if prefix:
-                    route.path_format= prefix + route.path_format
+                    route.path_format = prefix + route.path_format
                 if path:
                     paths.setdefault(route.path_format, {}).update(path)
                 if security_schemes: