An example of decoding RLE with itertools.groupby()

run_length.py

def decode(compressed):
    '''
    Given an Run Length Encoded (RLE) string, decodes it
    into its original format, and returns this decoded string.
    '''

    groups = groupby(compressed, lambda x: x.isdigit())

    output = ''
    count = 1

    for key, group in groups:
        if key is True:
            count = int(''.join(list(group)))

        else:
            letters = list(group)
            output += (letters.pop(0) * count) + ''.join(letters)

    return output