hex_to_bin.py

prompt = "Enter a hex digit: "
reject_too_long = "Only one number must be input."
reject_invalid_input = "Invalid input"

hex_candidate = input(prompt).upper()
if len(hex_candidate) != 1:
    print(reject_too_long)
elif not hex_candidate.isalnum():
    print(reject_invalid_input)
elif not "A" <= hex_candidate <= "F":
    # by this point digit is alphanumeric
    # so we only have to check if it's in range [A, F]
    print(reject_invalid_input)
else:
    numeric = int(hex_candidate, 16)
    print(bin(numeric)[2:])

isbn.py

prompt = "Enter the first 9 digits of an ISBN string: "
reject_non_nine = "Incorrect Input. It must have exact 9 digits"
reject_non_numeric = "Incorrect Input. All should be digits."
accept_msg = "The ISBN-10 number is"
nine_digits = input(prompt)

if len(nine_digits) != 9:
    print(reject_non_nine)
elif not nine_digits.isdecimal():
    # should not be needed but required for safety
    print(reject_non_numeric)
else: # numeric and length 9
    running_total = 0
    for index,digit in enumerate(nine_digits):
        running_total += (index+1) * int(digit)

    checksum_string = str(running_total % 11)
    if checksum_string == "10":
        checksum_string = "X"

    print(accept_msg, nine_digits+checksum_string)