how to access camera feed in Jetson Nano

I have tried many attempts to run the cameras connected to my jetson nano when i run this command

gst-launch-1.0 -v nvarguscamerasrc ! 'video/x-raw(memory:NVMM),format=NV12,width=1280,height=720,framerate=30/1' ! autovideosink

The camera works but when i try to perform a simple camera read in python with this code

import sys
import cv2

def read_cam():
    G_STREAM_TO_SCREEN = "videotestsrc num-buffers=50 ! videoconvert ! appsink"
    cap = cv2.VideoCapture(G_STREAM_TO_SCREEN, cv2.CAP_GSTREAMER)
    if cap.isOpened():
        cv2.namedWindow("demo", cv2.WINDOW_AUTOSIZE)
        while True:
            ret_val, img = cap.read()
            cv2.imshow('demo',img)
            cv2.waitKey(1)
    else:
     print ("Unable to use pipline")

    cv2.destroyAllWindows()


if __name__ == '__main__':
    read_cam()

the camera does not work in the code above and it returns

Unable to use pipline"

what am doing wrong and how can access the camera feed in python ?

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