How I can extract a face features as vector

Hello! I use OpenCV with Python to detect faces in images (with Haar cascade detector). Everything work fine, but I have a question. How I can extract a face features and put them into vector or array? For example - I detect a face, get coordinates of the face in the image, extract the face, normalize it and then I want to extract the features that differs from face to face. For me doesn't matter what is exactly meaning of the integers from feature vector, I only want to be different between two faces. How I can done this? Any suggestions? BR, Alex