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if you do a :

 >>> help(cv2.calcOpticalFlowFarneback)
Help on built-in function calcOpticalFlowFarneback:

calcOpticalFlowFarneback(...)
    calcOpticalFlowFarneback(prev, next, flow, pyr_scale, levels, winsize, iterations, poly_n, poly_sigma, flags) -> flow

in the python repl, you can see the signature.

so, the problem, seems the flow param, you can either use None, like in the tutorial, but, if you simply omit it, any following param must be adressed by name, like:

cv2.calcOpticalFlowFarneback(gray1, gray2, pyr_scale=0.5, levels=3, winsize=15, iterations=2, poly_n=5, poly_sigma=1.2, flags=0)

if you do a :

 >>> help(cv2.calcOpticalFlowFarneback)
Help on built-in function calcOpticalFlowFarneback:

calcOpticalFlowFarneback(...)
    calcOpticalFlowFarneback(prev, next, flow, pyr_scale, levels, winsize, iterations, poly_n, poly_sigma, flags) -> flow

in the python repl, you can see the signature.

so, the problem, seems the flow param, you can either have to use None, like in the tutorial, but, if you simply omit it, any following param must be adressed by name, like:,

flow = cv2.calcOpticalFlowFarneback(gray1, gray2, pyr_scale=0.5, levels=3, winsize=15, iterations=2, poly_n=5, poly_sigma=1.2, flags=0)
None, 0.5, 3, 15, 2, 5, 1.2, 0)