If you were clipping a line, you'd need a different algorithm anyway, since this would cause the line to trace around the edge of the clipping region, rather than just inserting a MOVETO.  This algorithm assumes closed polys in, closed polys out.  We have an algorithm for clipping lines to boxes in `PathClipper`, which doesn't currently have a direct way to be used from Python.  I think eventually -- and here we really in the realm of new feature and not bug fix -- we should probably generalize this to find the polygons and lines within the path and shunt off to the appropriate algorithm for each, and then recombine directly into a path (since in that case each entry could be either a polygon or a line, we'd have to do the appropriate thing for each).

In the short term, though, we should update the docstring to document the fact that this assumes polygons.