error(-215) ! Empty () In function detectMultiScale solution
Errors like this can occur when building a cascading classifier using OpenCV Python

error: (-215:Assertion failed) !empty() in function 'detectMultiScale'

Any discussion about this question on Stack Overflow at https://stackoverflow.com/questions/30508922/error-215-empty-in-function-detectmultiscale
The common perception is that XML or other files are missing or incorrectly routed.
The correct path would look like this:

cascade_fn = args.get('--cascade', "../../data/haarcascades/haarcascade_frontalface_alt.xml")
nested_fn  = args.get('--nested-cascade', "../../data/haarcascades/haarcascade_eye.xml")

cam = create_capture(video_src, fallback='synth:bg=../data/lena.jpg:noise=0.05')

There are also some references to this answer:
http://answers.opencv.org/question/52340/opencv-error-assertion-failed-empty-in-detectmultiscale/