OpenCV: Couldn't read video stream from file

Python
,
9 
import cv2
import numpy as np

cam = cv2.VideoCapture(0) #get the first webcam

####defining the codec and creating the VideoWriter Obj
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out=cv2.VideoWriter('./assets/output.avi',fourcc,20.0,(1280,960))

while(cam.isOpened()):
    ret, frame = cam.read()
    if ret==True:
        out.write(frame)
        cv2.imshow('frame',frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    else:
        break

cam.release()
out.release()
cv2.destroyAllWindows()

this is the code (the one i posted by mistake) i can change XVID with MP42 AVC1, but what would be the file extension in the name? is there a way to check which is the right codec? i tried to check the list of codec with ffmpeg -encoders