How to add a Variable

Adding a New Variable to the Shift Scheduling System

This guide explains how to create and integrate a new variable into the shift scheduling system.

Overview

Variables are the decision elements that the solver can set when creating a schedule. They represent choices like "Does employee X work shift Y on day Z?"

Why Variable Classes?

• Consistent Referencing: Variables are used in multiple constraints and objectives. Having dedicated classes makes it easier to reference them consistently instead of needing to remember the keys.
• Export Integration: Variables are part of the export functionality. Having well-defined classes ensures proper serialization and data exchange.

Step 1: Create the Variable Class

Create a new variable in cp/variables/variable.py:

# cp/variables/your_new_variable.py

# an example template, which you could follow
class YourNewVariable:
    """
    A dictionary wrapper that allows indexing with different objects (e.g., Employee, Day, and Shift).

    Usage: wrapper_dict[...]...[...] -> Variable
    """

    def __init__(self, internal_dict: dict[key, value]):
        self._data = internal_dict

    def __getitem__(self, key) -> value:
        return self._data[employee.get_key()]

    def __len__(self) -> int:
        l = ...
        # compute length
        return l

Step 2: Export the Variable

Add your variable to the __init__.py files:

# cp/variables/__init__.py
from .your_new_variable import YourNewVariable as YourNewVariable

# cp/__init__.py
from .variables import (
    # ... existing imports ...
    YourNewVariable as YourNewVariable
)

CP-SAT Variable Types

Boolean Variables (True/False decisions):

var = model.new_bool_var(name)

Integer Variables (Numeric values):

var = model.new_int_var(min_value, max_value, name)