Pytube is a lightweight, Pythonic, dependency-free, library (and command-line utility) for downloading YouTube Videos.
Python
from pytube import YouTube
YouTube('https://youtu.be/9bZkp7q19f0').streams.first().download()
yt = YouTube('http://youtube.com/watch?v=9bZkp7q19f0')
yt.streams.filter(progressive=True, file_extension='mp4').order_by('resolution').desc().first().download()