input: Write a function to determine if a string contains only digits. If the string contains only digits, return True; otherwise, return False.
  
  def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True
  
  print(is_digit_only("0123"))
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True