def get_default_username(check_db=True):
    """
    Try to determine the current system user's username to use as a default.

    :param check_db: If ``True``, requires that the username does not match an
        existing ``auth.User`` (otherwise returns an empty string).
    :returns: The username, or an empty string if no username can be
        determined.
    """
    # If the User model has been swapped out, we can't make any assumptions
    # about the default user name.
    if auth_app.User._meta.swapped:
        return ''

    default_username = get_system_username()
    try:
        default_username = unicodedata.normalize('NFKD', default_username)\
            .encode('ascii', 'ignore').decode('ascii').replace(' ', '').lower()
    except __HOLE__:
        return ''

    # Run the username validator
    try:
        auth_app.User._meta.get_field('username').run_validators(default_username)
    except exceptions.ValidationError:
        return ''

    # Don't return the default username if it is already taken.
    if check_db and default_username:
        try:
            auth_app.User._default_manager.get(username=default_username)
        except auth_app.User.DoesNotExist:
            pass
        else:
            return ''
    return default_username
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/contrib/auth/management/__init__.py/get_default_username