import cv2

img = cv2.imread("imagem.jpg")
face_cascade = cv2.CascadeClassifier(cv2.data.haarcascades + "haarcascade_frontalface_default.xml")
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
faces = face_cascade.detectMultiScale(gray, 1.1, 4)

for (x, y, w, h) in faces:
    face_crop = img[y:y+h, x:x+w]
    cv2.imwrite("imagem_cropada.jpg", face_crop)