PeekIter (via https://mail.python.org/pipermail/python-ideas/2014-November/029924.html)

gistfile1.py

class PeekIter:
     """ Create an iterator which allows you to peek() at the
         next item to be yielded. It also stores the exception
         and re-raises it on the next next() call.
     """
     def __init__(self, iterable):
         self._exn = None
         self._it = iter(iterable)
         self._peek = next(self._it)

     def peek(self):
         return self._peek

     def ok(self):
         #True if no exception occured when setting self._peek
         return self._exn == None

     def __iter__(self):
         return iter(self)

     def __next__(self):
         if self._exn != None:
             raise self._exn
         t = self._peek
         try:
             self._peek = next(self._it)
         except Exception as exn:
             self._exn = exn
             self._peek = None
         return t


def group_by(iterable):
     """ Yield (key, (group))'s of like values
         from an iterator.
     """
     def group(it):
         # Yield only like values.
         k = it.peek()
         while it.peek() == k:
             yield next(it)

     itr = PeekIter(iterable)
     while itr.ok():
         yield itr.peek(), list(group(itr))


print(' '.join(''.join(k) for k, v in group_by('AAAABBBCCDAABBB')))
# 'A B C D A B'

print(' '.join(''.join(g) for k, g in group_by('AAAABBBCCDAABBB')))
# 'AAAA BBB CC D AA BBB'