pack.py

from itertools import groupby


def pack(arr):
    uniqe_items = []
    count_items = []

    for i in groupby(arr):
        if i[0] not in uniqe_items:
            count_items.append(len(list(i[1])))
            uniqe_items.append(i[0])

    return ["{}:{}".format(num, count) for num, count in zip(uniqe_items, count_items)]

print pack([5, 5, 5, 7, 7, 3, 4, 7])