Update sqlite3/db.py

sqlite3/db.py

@@ -50,3 +50,12 @@ class DB(sqlite3.Connection):
         return [row[0] for row in result]
 
     def __repr__(self): return repr(self.keys())
+
+    def exists(self, key):
+        with self:
+            cur = self.execute('''
+                SELECT 1 FROM kv_store
+                WHERE key = ?
+                LIMIT 1
+            ''', (key,))
+            return bool(cur.fetchone())