input: Write a function to check whether a given string contains only digits. If the string contains only digits return True else return False. If the string is empty return False.
  assert is_digit_only("0123")==True
  assert is_digit_only("012a3")==False
  assert is_digit_only("")==False
  assert is_digit_only("9876543210")==True
  assert is_digit_only("123 456")==False
output: def is_digit_only(s):
      return s.isdigit()