RH pointed out that discard(element) doesn't do the transformation on

gvanrossum · gvanrossum · commit e399d08a4a64 · 2002-08-23T14:45:02.000Z
the element if necessary.  Fixed by calling self.remove(element).
diff --git a/Lib/sets.py b/Lib/sets.py
@@ -450,7 +450,7 @@ def discard(self, element):
         If the element is not a member, do nothing.
         """
         try:
-            del self._data[element]
+            self.remove(element)
         except KeyError:
             pass
 
