Salesforce problem

Generate the following sequence:
[1, 11, 21, 1211, 111221, 312211, 13112221 ]

Starting with "1", generate a sequence of numbers that describe the previous member of the sequence.
In this example, "1" is followed by "11" because there's "one 1". Then, to describe "11", you use "21" because
there are "two 1's". The next, 1211, describes "21" because there's "1 2, and 1 1". The next is "1 1, 1 2, 2 2s, and 2 1s"
hence "111221", Which leads to "3 1, 2 2, 1 1".

def make_sequence(num_to_produce):
    initial = "1"
    for sequence_num in range(0,num_to_produce):
        this_string = ""
        current_num = None
        current_count = 0
        for index, number in enumerate(initial):
            if number == current_num:
                current_count += 1
            else:
                if current_count >= 1:
                    this_string += "{}{}".format(current_count, current_num)
                current_num = number
                current_count = 1
            if len(initial) == index + 1:
                this_string += "{}{}".format(current_count, current_num)
        initial = this_string
        yield this_string
    
if __name__ == "__main__":
    print list(make_sequence(50))