diff --git a/google/api_core/rest_helpers.py b/google/api_core/rest_helpers.py
index 9b3c9dc0..391351fd 100644
--- a/google/api_core/rest_helpers.py
+++ b/google/api_core/rest_helpers.py
@@ -81,7 +81,10 @@ def _flatten_value(obj, key_path, strict=False):
 
 
 def _flatten_dict(obj, key_path, strict=False):
-    items = (_flatten(value, key_path=key_path + [key], strict=strict) for key, value in obj.items())
+    items = (
+        _flatten(value, key_path=key_path + [key], strict=strict)
+        for key, value in obj.items()
+    )
     return functools.reduce(operator.concat, items, [])