The Polygon package consists of two modules. One is an extension written in C
wrapping the gpc library (http://www.cs.man.ac.uk/aig/staff/alan/software/)
and its polygon clipping operations in an object oriented fashion. There are
lots of additional methods. The other is written in pure python and provides
some additional functions.
The wrapping and extension code is free software, but the core gpc library is
free for non-commercial usage only. The author says:
"This library is provided free of charge for non-commercial users. Reaching
its current state took up almost two years of personal development ...