Switch pattern for marking unused argument
diff --git a/src/python/grpcio/grpc/_interceptor.py b/src/python/grpcio/grpc/_interceptor.py
index fdd484c..6c4e396 100644
--- a/src/python/grpcio/grpc/_interceptor.py
+++ b/src/python/grpcio/grpc/_interceptor.py
@@ -126,8 +126,7 @@
     def traceback(self, ignored_timeout=None):
         return self._traceback
 
-    def add_callback(self, callback):
-        del callback
+    def add_callback(self, unused_callback):
         return False
 
     def add_done_callback(self, fn):