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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def login(request, user):
 """ Persist a user id and a backend in the request. This way a user doesn't have to reauthenticate on every request. Note that data set during the anonymous session is retained when the user logs in. """

    session_auth_hash = ''
    if user is None:
        user = request.user
    if hasattr(user, 'get_session_auth_hash'):
        session_auth_hash = user.get_session_auth_hash()

    if SESSION_KEY in request.session:
        session_key = request.session[SESSION_KEY]
        if session_key != user.pk or (
                session_auth_hash and
                request.session.get(HASH_SESSION_KEY) != session_auth_hash):
            # To avoid reusing another user's session, create a new, empty
            # session if the existing session corresponds to a different
            # authenticated user.
            request.session.flush()
    else:
        request.session.cycle_key()
    request.session[SESSION_KEY] = user.pk
    request.session[BACKEND_SESSION_KEY] = user.backend
    request.session[USER_DATA_SESSION_KEY] = user.user_data
    request.session[HASH_SESSION_KEY] = session_auth_hash

    update_token_in_session(request.session, user.token)

    if hasattr(request, 'user'):
        request.user = user
    rotate_token(request)
    user_logged_in.send(sender=user.__class__, request=request, user=user)