persistence using afw table is prone to causing Python circular imports in python. When folks at HSC added table persistence to afw.geom.Polygon they found that this caused a circular dependency in Python. They resolved this moving Polygon a level deeper. That worked, but has the unfortunate effect of moving Polygon to a surprising namespace and making it harder to access.
This ticket asks for changes that reduce the danger of circular dependencies and allow a more natural namespace for Polygon and future classes that are persisted using afw table.
I do not fully understand the issue, but it I saw it while moving the HSC code over, and what I observed is that "import lsst.afw.geom" imported afw table, which in turn imported afw.coord.Coord, which in turn imported afw geom. I'm not sure why adding Polygon persistence caused this, nor how we got away with the current design before Polygon adding persistence.