Shapely provides classes that implement, more or less, the interfaces in the
OGC's simple feature access 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. |