I think that works to solve the problem that I pointed out.  In my stack overflow question (http://stackoverflow.com/a/43926058/748858) it has been pointed out that there are other opportunities for weirdness here.Specifically, if if I skip processing 2 groups and then I process a third group whose key is the same as the first:inputs = [(x > 5, x) for x in range(10)]inputs += [(False, 10), (True, 11)]g = groupby(inputs2 + [(True, 11)], key=itemgetter(0))_, a = next(g)_, b = next(g)_, c = next(g)print(list(a))print(list(b))Both `a` and `b` should probably be empty at this point, but they aren't.  What if you kept track of the last iterable group and just consumed it at whenever `next` is called?  I think then you also need to keep track of whether or not the input iterable has been completely consumed, but that's not too bad either:_sentinel = object()class groupby:    # [k for k, g in groupby('AAAABBBCCDAABBB')] --> A B C D A B    # [list(g) for k, g in groupby('AAAABBBCCD')] --> AAAA BBB CC D    def __init__(self, iterable, key=None):        if key is None:            key = lambda x: x        self.keyfunc = key        self.it = iter(iterable)        self.last_group = self.currkey = self.currvalue = _sentinel        self.empty = False    def __iter__(self):        return self    def __next__(self):        if self.last_group is not _sentinel:            for _ in self.last_group:                pass        if self.empty:            raise StopIteration        if self.currvalue is _sentinel:            try:                self.currvalue = next(self.it)            except StopIteration:                self.empty = True                raise            self.currkey = self.keyfunc(self.currvalue)        self.last_group = self._grouper(self.currkey, self.currvalue)        return (self.currkey, self.last_group)    def _grouper(self, tgtkey, currvalue):        while self.currkey == tgtkey:            yield self.currvalue            try:                self.currvalue = next(self.it)            except StopIteration:                self.empty = True                return            self.currkey = self.keyfunc(self.currvalue)I haven't tested this to make sure it passes the test suite -- I also don't know if this would have major performance implications or anything.  If it did have severe performance implications, then it probably isn't worthwhile...