In CFD-post there are some macro calculation and native function that deal with these kind or demand. Mass flow rate,  Averages on areas or volumes are some of tools  available to do those calculations.

Once in this tab, select the function ( area, area average, mass flow rate,etc) , the region ( inlet, outlet, entire domain…) and if necessary, a variable to  perform a average, for example.

There is no coding, is a native feature in the post-processing tool.