8.21.py

from __future__ import print_function, division

class Node:
    pass


class UnaryOperator(Node):
    def __init__(self, operand):
        self.operand = operand


class BinaryOperator(Node):
    def __init__(self, left, right):
        self.left = left
        self.right = right


class Add(BinaryOperator):
    pass


class Sub(BinaryOperator):
    pass


class Mul(BinaryOperator):
    pass


class Div(BinaryOperator):
    pass


class Negate(UnaryOperator):
    pass


class Number(Node):
    def __init__(self, value):
        self.value = value


t1 = Sub(Number(3), Number(4))
t2 = Mul(Number(2), t1)
t3 = Div(t2, Number(5))
t4 = Add(Number(1), t3)


class NodeVisitor:
    def visit(self, node):
        print(node)
        method_name = 'visit_' + node.__class__.__name__
        method = getattr(self, method_name, None)
        if method is None:
            mathod = self.generic_visit
        return method(node)


class Evaluator(NodeVisitor):
    def visit_Number(self, node):
        return node.value

    def visit_Add(self, node):
        return self.visit(node.left) + self.visit(node.right)

    def visit_Sub(self, node):
        return self.visit(node.left) - self.visit(node.right)

    def visit_Mul(self, node):
        return self.visit(node.left) * self.visit(node.right)

    def visit_Div(self, node):
        return self.visit(node.left) / self.visit(node.right)

    def visit_Negate(self, node):
        return - node.operand


e = Evaluator()
print(e.visit(t4))