input: Write a function to check 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
  
  assert is_digit_only("0123") == True
  assert is_digit_only("0123a") == False
  assert is_digit_only("9876543210") == True
  assert is_digit_only("") == True
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True