Repeated Sum of Digits after Exponentiation

def expo(a, b):
    res = a**b
    str_res = f"{a}^{b} = {res} => "
    L1 = list(str(res))

    while len(L1) != 1:
        sumd = 0
        for i in range(len(L1)):
            if i == 0:
                str_res += f"{L1[i]}"
            else:
                str_res += f"+{L1[i]}"

            sumd += int(L1[i])
        else:
            str_res += " = "
        L1 = list(str(sumd))

        if len(L1) == 1:
            str_res += f"{sumd}"
            break
        else:
            str_res += f"{sumd} => "

    print(str_res)

# Call the Function with 2 arguments
expo(8,8)