Suppose that you're writing a streaming media player that plays music and videos while downloading them from the Internet. The media is hosted by the company you work for, and you only want subscribers to that company's service to be able to access it. As you build the application, you decide that you need the following function to authenticate users before allowing them to access the media. def authenticate(connection, username: str, password: str) -> str: ''' Given a previously-established connection to the server, a username, and a password, sends the username and password to the server and obtains an access token in response. ''' You can think of an access token as being a specially-constructed, single-use string that the server will subsequently (but temporarily) accept as proof that the user is permitted to access the media. Precisely what those tokens look like, how the authentication process works, and so on, are not relevant to the question. What should this function do if the username and password are invalid (i. e., the server does not recognize them).