Run the code like this:
python face_detect.py abba.png
If you want to understand how the code works, the details are here:
https://realpython.com/blog/python/face-recognition-with-python/
Update: Now supports OpenCV3. This change has been made by furetosan ( https://github.com/furetosan) and tested on Linux.
To run the OpenCV3 version, run facedetect_cv3.py.