input: Write a function to check whether a given string contains only digits. If the string contains only digits, return True; otherwise, return False.
  assert is_digit_only("0123")==True
  assert is_digit_only("12a3")==False
  assert is_digit_only("")==True
  assert is_digit_only("12.3")==False
  assert is_digit_only("123 ")==False
output: def is_digit_only(s):
      return s.isdigit()