compress.py

from collections import defaultdict


def encode(text):
    result = []
    freq = defaultdict(int)
    for prev, current in zip(text[:-1], text[1:]):
        freq[prev] += 1
        if current != prev:
            result.append((prev, freq[prev]))
            freq[prev] = 0
    result.append((prev, freq[prev] + 1))
    return result


def decode(data):
    return ''.join(freq * symbol for symbol, freq in data)