Volunteer workshift scheduler

Code written in Python finds the best possible schedule for volunteers while incorporating constrains and special requirements using CP-SAT Solver (Constraint Programming - boolean SATisfiability problem Solver).

Java version is in development.
The problem

Arranging volunteers for a helpline service for the period of one month.

There are three types of work that could be done on the same day:

Phone shift: Every day.
Chat shift: Mondays and Wednesdays.
Observer shift: Any day.

Constraints:

Properties of each volunteer:

Available days.
Function: doing phone, chat, both or observation for training purposes.
Maximum amount of weekend shifts.
Whether their shifts have to be on separate weeks.
Whether they welcome observers.
Whether they want to work alone.
Whether they cannot yet work alone.
List of people they do not want to work with.