This change stops sleekxmpp from spending huge amounts of time unnecessarily computing logging data that may never be used. This is a HUGE performance improvement; in some of my test runs, unnecessary string creation was accounting for > 60% of all CPU time.

Note that using % in a string will _always_ perform the sting substitutions, because the strings are constructed before the function is called. So log.debug('%s' % expensiveoperation()) will take about the same CPU time whether or not the logging level is DEBUG or INFO. if you use , no substitutions are performed unless the string is actually logged

sleekxmpp/xmlstream/matcher/xmlmask.py

@@ -102,8 +102,7 @@ class MatchXMLMask(MatcherBase):
             try:
                 mask = ET.fromstring(mask)
             except ExpatError:
-                log.warning("Expat error: %s\nIn parsing: %s" % ('', mask))
-
+                log.warning("Expat error: %s\nIn parsing: %s" , '', mask)
         if not use_ns:
             # Compare the element without using namespaces.
             source_tag = source.tag.split('}', 1)[-1]