Note that context manager should be in separate classes if they implement non-obvious behaviors.

pep-0008.txt

@@ -837,6 +837,20 @@ Programming Recommendations
           # Will also catch KeyError raised by handle_value()
           return key_not_found(key)
 
+- Context managers should be in separate classes whenever they do
+  something other than acquire and release resources.  For example:
+
+      Yes:     with auto_commit_or_rollback(conn):
+                   do_transaction(conn)
+
+      No:      with conn:
+                   do_transaction(conn)
+
+   The latter example doesn't provide any information to indicate that
+   the __enter__ and __exit__ methods are doing something other than
+   closing the connection after a transaction.  Being explicit is
+   important in this case.
+
 - Use string methods instead of the string module.
 
   String methods are always much faster and share the same API with