Design Notes

Shapely provides classes that implement, more or less, the interfaces in the OGC’s simple feature acess specification [1]. The classes are defined in similarly named modules under shapely.geometry: Point is in shapely.geometry.point, MultiPolygon is in shapely.geometry.multipolygon. These classes derive from shapely.geometry.base.BaseGeometry. The simple features methods of BaseGeometry call functions registered in a class variable impl. For example, BaseGeometry.area calls BaseGeometry.impl['area'].

The default registry is in the shapely.impl module. Its items are classes that operate on single geometric objects or pairs of geometric objects. Pluggability is a goal of this design, but we’re not there yet. Some work needs to be done before anybody can use CGAL as a Shapely backend.

In sum, Shapely’s stack is 4 layers:

• Python geometry classes in shapely.geometry
• An implementation registry: an abstraction that permits alternate geometry engines, even a mix of geometry engines. The default is in shapely.impl.
• The GEOS implementations of methods for the registry in shapely.geos.
• libgeos: algorithms written in C++.

[1]	John R. Herring, Ed., “OpenGIS Implementation Specification for Geographic information - Simple feature access - Part 1: Common architecture,” Oct. 2006.