Closes SF bug #628246.

The _update method detected mutable elements by trapping TypeErrors.
Unfortunately, this masked useful TypeErrors raised by the iterable
itself.  For cases where it is possible for an iterable to raise
a TypeError, the iterable is pre-converted to a list outside the
try/except so that any TypeErrors propagate through.

Lib/sets.py

@@ -320,6 +320,8 @@ class BaseSet(object):
             return
 
         value = True
+        if type(iterable) not in (list, tuple, dict, file, xrange, str):
+            iterable = list(iterable)
         it = iter(iterable)
         while True:
             try: