如何使用Opencv python在视频流中逐帧处理视频图像

I am a beginner in OpenCV. I want to do some image processing on the frames of a video which is being uploaded to my server. I just want to read the available frames and write them in a directory. And wait for other part of the video to be uploaded and write the frames to the directory. And I should wait for each frame to be completely uploaded then write it to a file.I am a beginner in OpenCV. I want to do some im