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"))
  assert is_digit_only("0123")==True
  assert is_digit_only("1a2b3")==False
  assert is_digit_only("9876543210")==True
  assert is_digit_only("123 456")==False
  assert is_digit_only("")==True
output: def is_digit_only(string):
      for char in string:
          if not char.isdigit():
              return False
      return True