scratch.py

# string input
# function, short version
#aaaa: a4
# aaaabbbcccaaaa
# a4b3c3a4


def count_chars(str_: str):
    if str_ == "":
        return ""

    response = []
    count = 0
    previous = ''
    for s in str_:
        if previous == '':
            previous = s
            count += 1
        else:
            if previous != s:
                if count == 1:
                    response.append(f"{previous}")
                else:
                    response.append(f"{previous}{count}")
                count = 1
                previous = s
            else:
                count += 1
    if count == 1:
        response.append(f"{previous}")
    else:
        response.append(f"{previous}{count}")
    return "".join(response)


assert count_chars("aaaabbbcccaaaa") == "a4b3c3a4"
assert count_chars("a") == "a"
assert count_chars("aabc") == "a2bc"
assert count_chars("") == ""