Run-Length Encoding

Compress a string so aaabccddd becomes a3b1c2d3, then write the decoder and prove it round-trips.

a3b1c2d3
aaabccddd
True

Walk the string tracking the current character and a count; flush when it changes. For decode, pair each letter with the digits that follow it.

def encode(s):
    out = ""
    count = 1
    for i in range(1, len(s) + 1):
        if i < len(s) and s[i] == s[i - 1]:
            count += 1
        else:
            out += s[i - 1] + str(count)
            count = 1
    return out

def decode(s):
    out = ""
    i = 0
    while i < len(s):
        ch = s[i]
        j = i + 1
        while j < len(s) and s[j].isdigit():
            j += 1
        out += ch * int(s[i + 1:j])
        i = j
    return out

text = "aaabccddd"
print(encode(text))
print(decode(encode(text)))
print(decode(encode(text)) == text)

Run your code to check it…