Function responsibility in code

[CodeWolf]

I've heard the whole "design functions to do one thing well" mantra, but it's still a little vague to me. Recently I've been working on an input processing script which is based off of instructions from a book. Right now I'm worried if I've split up the functions too much. This is what I have so far:

def check_input(digit_input):
    if digit_input.isdigit() is not True:
        print("Error: input contains non-numbers. Exiting.")
    else:
        if len(digit_input) == 6:
            return digit_input
        else:
            print("Error: number of digits is not six. Exiting")


def input_to_list(digit_input):
    digit_list = list(digit_input)
    return digit_list


def chksum_extraction(digit_list):
    check_sum = digit_list.pop(-1)
    return check_sum
    return digit_list


digit_input = input("Enter six digit ID: ")
check_input(digit_input)

I think knowing whether or not this is organized well will give me a concrete example when working on future programs.