example.py

# Example of the visitor pattern

# --- The following classes represent nodes in an expression tree
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

# --- The visitor base class

class NodeVisitor:
    def visit(self, node):
        methname = 'visit_' + type(node).__name__
        meth = getattr(self, methname, None)
        if meth is None:
            meth = self.generic_visit
        return meth(node)
    
    def generic_visit(self, node):
        raise RuntimeError('No {} method'.format('visit_' + type(node).__name__))

# --- Example 1:  An expression evaluator

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

# --- Example 2: Generate stack instructions

class StackCode(NodeVisitor):
    def generate_code(self, node):
        self.instructions = []
        self.visit(node)
        return self.instructions

    def visit_Number(self, node):
        self.instructions.append(('PUSH', node.value))

    def binop(self, node, instruction):
        self.visit(node.left)
        self.visit(node.right)
        self.instructions.append((instruction,))

    def visit_Add(self, node):
        self.binop(node, 'ADD')

    def visit_Sub(self, node):
        self.binop(node, 'SUB')
 
    def visit_Mul(self, node):
        self.binop(node, 'MUL')
       
    def visit_Div(self, node):
        self.binop(node, 'DIV')

    def unaryop(self, node, instruction):
        self.visit(node.operand)
        self.instructions.append((instruction,))
  
    def visit_Negate(self, node):
        self.unaryop(node, 'NEG')

# --- Example of the above classes in action

# Representation of 1 + 2 * (3 - 4) / 5
t1 = Sub(Number(3), Number(4))
t2 = Mul(Number(2), t1)
t3 = Div(t2, Number(5))
t4 = Add(Number(1), t3)

e = Evaluator()
print('Should get 0.6 :', e.visit(t4))

s = StackCode()
code = s.generate_code(t4)
for c in code:
    print(c)