'Get selector for a video vertical with display name specified by `video_display_name`.
Arguments:
video_display_name (str or None): Display name of a Video. Default vertical selector if None.
Returns:
str: Vertical Selector for video.'

def get_video_vertical_selector(self, video_display_name=None):

    if video_display_name:
        video_display_names = self.q(css=CSS_CLASS_NAMES['video_display_name']).text
        if (video_display_name not in video_display_names):
            raise ValueError("Incorrect    Video    Display    Name:    '{0}'".format(video_display_name))
        return '.vert.vert-{}'.format(video_display_names.index(video_display_name))
    else:
        return '.vert.vert-0'
