python creating custom syntax

Question

or-tools module in python adds custom syntax where functions can take arbitrary expressions as arguments (something like below), which is evaluated not instantaneously but solved as a constraint later

model.Add(x + 2 * y -1 >= z)

When I print the type of the argument from my function, it shows

<class 'ortools.sat.python.cp_model.BoundedLinearExpression'>

A simple way is to pass the expression as a string, but it feels better. I wish to understand how this is achieved. Is this a way to create custom syntax in python? Does it require updating the parser or something like that?

Here is the simple program

from ortools.sat.python import cp_model


def foo(expr):
    print(expr, type(expr))

def main():
    model = cp_model.CpModel()
    var_upper_bound = max(50, 45, 37)
    x = model.NewIntVar(0, var_upper_bound, 'x')
    y = model.NewIntVar(0, var_upper_bound, 'y')
    z = model.NewIntVar(0, var_upper_bound, 'z')
    a = 0
    b = 0
    c = 0

    model.Add(2*x + 7*y + 3*z == 50)

    solver = cp_model.CpSolver()
    status = solver.Solve(model)

    if status == cp_model.OPTIMAL:
        print('x value: ', solver.Value(x))
        print('y value: ', solver.Value(y))
        print('z value: ', solver.Value(z))


    foo(2*x + 7*y + 3*z == 50)
    foo(2*a + 7*b + 3*c == 50)



if __name__ == '__main__':
    main()

x, y, z are special variables (instances of some class) and the expression with x, y, z is stored as expression

a, b, c are simple integers and the expression is evaluated immediately and the result is stored as a bool

Answer 1

They override the python operators.

References:

• https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types
• https://github.com/google/or-tools/blob/b37d9c786b69128f3505f15beca09e89bf078a89/ortools/sat/python/cp_model.py#L212-L233

    def __mul__(self, arg):
        if isinstance(arg, numbers.Integral):
            if arg == 1:
                return self
            elif arg == 0:
                return 0
            cp_model_helper.AssertIsInt64(arg)
            return _ProductCst(self, arg)
        else:
            raise TypeError('Not an integer linear expression: ' + str(arg))