How to use the pytube.extract function in pytube

def test_extract_watch_url():
    video_id = '9bZkp7q19f0'
    watch_url = extract.watch_url(video_id)
    assert watch_url == 'https://youtube.com/watch?v=9bZkp7q19f0'

self.vid_info_url = None  # the url to vid info, parsed from watch html

		self.watch_html = None     # the html of /watch?v=
		self.embed_html = None
		self.player_config_args = None  # inline js in the html containing
		# streams
		self.age_restricted = None

		self.fmt_streams = []  # list of :class:`Stream ` instances
		self.caption_tracks = []

		# video_id part of /watch?v=
		self.video_id = extract.video_id(url)

		# https://www.youtube.com/watch?v=
		self.watch_url = extract.watch_url(self.video_id)

		self.embed_url = extract.embed_url(self.video_id)
		# A dictionary shared between all instances of :class:`Stream `
		# (Borg pattern).
		self.stream_monostate = {
			# user defined callback functions.
			'on_progress': on_progress_callback,
			'on_complete': on_complete_callback,
		}

		if proxies:
			install_proxy(proxies)

		if not defer_prefetch_init:
			self.prefetch_init()