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:])