Issue #9512: Made comment more informative.

Lib/logging/handlers.py

@@ -103,8 +103,13 @@ class RotatingFileHandler(BaseRotatingHandler):
 
         If maxBytes is zero, rollover never occurs.
         """
+        # If rotation/rollover is wanted, it doesn't make sense to use another
+        # mode. If for example 'w' were specified, then if there were multiple
+        # runs of the calling application, the logs from previous runs would be
+        # lost if the 'w' is respected, because the log file would be truncated
+        # on each run.
         if maxBytes > 0:
-            mode = 'a' # doesn't make sense otherwise!
+            mode = 'a'
         BaseRotatingHandler.__init__(self, filename, mode, encoding, delay)
         self.maxBytes = maxBytes
         self.backupCount = backupCount