How to know total number of Frame in a file with cv2 in python

With a newer OpenCV version (I use 3.1.0) it works like this:

import cv2

cap = cv2.VideoCapture("video.mp4")
length = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
print( length )

And similar for other video properties cv2.CAP_PROP_*