input: Write a function to check whether a string contains only digits. Name the function is_digit_only and it should take a single parameter string. Return True if the string contains only digits, otherwise return False. If the string is empty, return True.
  
  assert is_digit_only("0123")==True
  assert is_digit_only("abc123")==False
  assert is_digit_only("")==True
  assert is_digit_only("123.45")==False
  assert is_digit_only("9876543210")==True
output: def is_digit_only(string):
      return string.isdigit()