diff --git a/dataset/database.py b/dataset/database.py
index b1ba25e..748ed15 100644
--- a/dataset/database.py
+++ b/dataset/database.py
@@ -79,7 +79,7 @@ class Database(object):
         """Connection against which statements will be executed."""
         with self.lock:
             tid = threading.get_ident()
-            if not tid in self.connections:
+            if tid not in self.connections:
                 self.connections[tid] = self.engine.connect()
             return self.connections[tid]