应该写成这样的形式（可以使用函数对象）
另外，需要注意安装对应版本的opencv-contrib-python
同时需要注意某些API已经不再支持
retval = cv.TrackerKCF_create( [, parameters] )
# initialize a dictionary that maps strings to their corresponding OpenCV object tracker implementations
OPENCV_OBJECT_TRACKERS = {
"csrt": cv2.TrackerCSRT_create,
"kcf": cv2.TrackerKCF_create,
"mil": cv2.TrackerMIL_create,
"goturn": cv2.TrackerGOTURN_create
}
# grab the appropriate object tracker using our dictionary of OpenCV object tracker objects
tracker = OPENCV_OBJECT_TRACKERS[args["tracker"]]()