input: Write a function to check whether a string contains only digits. If the string contains only digits, return True; otherwise, return False. The function should handle strings of any length, including an empty string.
  assert is_digit_only("0123")==True
  assert is_digit_only("abc123")==False
  assert is_digit_only("12.34")==False
  assert is_digit_only("")==True
output: def is_digit_only(s):
      return s.isdigit()