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.

This will simplify the reasoning about what a program does since no hidden side
effects ever can take place to these data structures. You can rest assured that
the object you hold a reference to will remain the same throughout its lifetime
and need not worry that somewhere five stack levels below you in the darkest
corner of your application someone has decided to remove that element that you
expected to be there.

Pyrsistent is influenced by persistent data structures such as those found in
the standard library of Clojure. The data structures are designed to share
common elements through path copying. It aims at taking these concepts and make
them as pythonic as possible so that they can be easily integrated into any
python program without hassle.

Homepage:
https://github.com/tobgu/pyrsistent
