input: Write a function to check if a string contains only digits. Return True if the string contains only digits, otherwise return False. If the string is empty, return False.
  def is_digit_only(string):
      if not string:
          return False
      for char in string:
          if not char.isdigit():
              return False
      return True
  
  assert is_digit_only("0123")==True
  assert is_digit_only("12a3")==False
  assert is_digit_only("")==False
  assert is_digit_only("0000")==True
  assert is_digit_only("123.45")==False
output: def is_digit_only(string):
      if not string:
          return False
      for char in string:
          if not char.isdigit():
              return False
      return True