<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_username(details, backend, response, *args, **kwargs):
 """Sets the `username` argument. If the user exists already, use the existing username. Otherwise generate username from the `new_uuid` using the `helusers.utils.uuid_to_username` function. """


    user = details.get('user')
    if not user:
        user_uuid = kwargs.get('uid')
        if not user_uuid:
            return

        username = uuid_to_username(user_uuid)
    else:
        username = user.username

    return {
        'username': username
    }