Take a string as input and count the numbers of characters in their occurrence and return the result with their number of occurrences and the letter.

CountCharacters.py

def countCharacters(string):
    single = []
    whole = []
    result = ""
    for i in string:
        if (len(single) == 0):
            single.append(i)
        elif (i == single[0]):
            single.append(i)
        else:
            whole.append(single)
            single = []
            single.append(i)
    if (len(single) != 0):
        whole.append(single)
    for i in whole:
        result = result + "" + str(len(i)) + i[0]
        
    return result

print(countCharacters('aaabcccdeefg'))