Internal Program Version: pyrsistent  0.19.3

Pyrsistent is a number of persistent collections
(by some referred to as functional data structures).
Persistent in  the sense that they are immutable.

All methods on a data structure that would normally
mutate it instead return a new copy of the structure
containing the requested updates. The original structure
is left untouched.

http://github.com/tobgu/pyrsistent