serial: SerialBase with is idempotent
diff --git a/serial/serialutil.py b/serial/serialutil.py
index e4df90f..7d51752 100644
--- a/serial/serialutil.py
+++ b/serial/serialutil.py
@@ -557,6 +557,8 @@
     # context manager
 
     def __enter__(self):
+        if not self.is_open:
+            self.open()
         return self
 
     def __exit__(self, *args, **kwargs):