Example:

>>> import pprint
>>> class S(set): pass
... 
>>> S('abc')
S({'a', 'b', 'c'})
>>> pprint.pprint(S('abc'))
S({'a', 'b', 'c'})
>>> pprint.pprint(S('abc'), width=1)
{'a',
 'b',
 'c'}

And same for frozenset.

Here is a patch which fixes this issue. With a patch:

>>> pprint.pprint(S('abc'), width=1)
S({'a',
   'b',
   'c'})