input: Write a function to check whether a 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("abc123")==False
  assert is_digit_only("456")==True
  assert is_digit_only("789xyz")==False
  assert is_digit_only("")==True
output: def is_digit_only(s):
      return s.isdigit()