group_by.py

def group_by(f_key):
    def gen(it):
        iterator = iter(it)

        try:
            first = next(iterator)
        except StopIteration:
            return

        key = f_key(first)
        acc = [first]
        for head in iterator:
            head_key = f_key(head)
            if head_key != key:
                yield acc
                acc = [head]
                key = head_key
                continue

            acc += [head]
        yield acc

    return gen


source = range(100)
key_10 = lambda x : x // 10

group_by_mod_10 = group_by(key_10)

for group in group_by_mod_10(source):
    print(group)