pool.execute params should be none by default

Author: ValCapri

tornado_mysql/pools.py

@@ -80,7 +80,7 @@ def _close_conn(self, conn):
         self._opened_conns -= 1
 
     @coroutine
-    def execute(self, query, params):
+    def execute(self, query, params=None):
         """Execute query in pool.
 
         Returns future yielding closed cursor.