I get a ximgproc_DisparityWLSFilter from cv2.ximgproc.createDisparityWLSFilter(left_matcher),
but I cannot get ximgproc_DisparityWLSFilter.filter() to work.
The error I get is 

OpenCV Error: Assertion failed (!disparity_map_right.empty() && (disparity_map_right.depth() == CV_16S) && (disparity_map_right.channels() == 1)) in cv::ximgproc::DisparityWLSFilterImpl::filter, file ......\opencv_contrib\modules\ximgproc\src\disparity_filters.cpp, line 262

In general, how do I figure out how to use this, when there isn't a single google result for "ximgproc_DisparityWLSFilter"?
Unlike c++, Python doesn't work well with pointers. So the arguments are 
Filtered_disp = ximgproc_DisparityWLSFilter.filter(left_disp,left, None,right_disp)
Note that it's no longer a void function in Python!
I figured this out through trial and error though.
What are the ximgproc_DisparityWLSFilter.filter() Arguments?