8.21小节完成
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XiongNeng
134
cookbook/c08/p21_vistor_pattern.py
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134
cookbook/c08/p21_vistor_pattern.py
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#!/usr/bin/env python
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# -*- encoding: utf-8 -*-
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"""
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Topic: 观察者模式
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Desc :
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"""
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class Node:
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pass
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class UnaryOperator(Node):
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def __init__(self, operand):
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self.operand = operand
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class BinaryOperator(Node):
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def __init__(self, left, right):
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self.left = left
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self.right = right
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class Add(BinaryOperator):
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pass
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class Sub(BinaryOperator):
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pass
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class Mul(BinaryOperator):
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pass
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class Div(BinaryOperator):
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pass
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class Negate(UnaryOperator):
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pass
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class Number(Node):
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def __init__(self, value):
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self.value = value
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# Representation of 1 + 2 * (3 - 4) / 5
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t1 = Sub(Number(3), Number(4))
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t2 = Mul(Number(2), t1)
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t3 = Div(t2, Number(5))
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t4 = Add(Number(1), t3)
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class NodeVisitor:
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def visit(self, node):
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methname = 'visit_' + type(node).__name__
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meth = getattr(self, methname, None)
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if meth is None:
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meth = self.generic_visit
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return meth(node)
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def generic_visit(self, node):
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raise RuntimeError('No {} method'.format('visit_' + type(node).__name__))
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class Evaluator(NodeVisitor):
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def visit_Number(self, node):
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return node.value
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def visit_Add(self, node):
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return self.visit(node.left) + self.visit(node.right)
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def visit_Sub(self, node):
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return self.visit(node.left) - self.visit(node.right)
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def visit_Mul(self, node):
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return self.visit(node.left) * self.visit(node.right)
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def visit_Div(self, node):
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return self.visit(node.left) / self.visit(node.right)
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def visit_Negate(self, node):
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return -node.operand
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e = Evaluator()
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print(e.visit(t4))
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class StackCode(NodeVisitor):
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def generate_code(self, node):
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self.instructions = []
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self.visit(node)
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return self.instructions
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def visit_Number(self, node):
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self.instructions.append(('PUSH', node.value))
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def binop(self, node, instruction):
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self.visit(node.left)
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self.visit(node.right)
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self.instructions.append((instruction,))
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def visit_Add(self, node):
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self.binop(node, 'ADD')
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def visit_Sub(self, node):
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self.binop(node, 'SUB')
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def visit_Mul(self, node):
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self.binop(node, 'MUL')
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def visit_Div(self, node):
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self.binop(node, 'DIV')
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def unaryop(self, node, instruction):
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self.visit(node.operand)
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self.instructions.append((instruction,))
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def visit_Negate(self, node):
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self.unaryop(node, 'NEG')
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class HTTPHandler:
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def handle(self, request):
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methname = 'do_' + request.request_method
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getattr(self, methname)(request)
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def do_GET(self, request):
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pass
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def do_POST(self, request):
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pass
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def do_HEAD(self, request):
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pass
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@@ -5,14 +5,191 @@
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----------
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问题
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----------
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todo...
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你要处理由大量不同类型的对象组成的复杂数据结构，每一个对象都需要需要进行不同的处理。
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比如，遍历一个树形结构，然后根据每个节点的相应状态执行不同的操作。
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----------
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解决方案
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----------
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todo...
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这里遇到的问题在编程领域中是很普遍的，有时候会构建一个由大量不同对象组成的数据结构。
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假设你要写一个表示数学表达式的程序，那么你可能需要定义如下的类：
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.. code-block:: python
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class Node:
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pass
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class UnaryOperator(Node):
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def __init__(self, operand):
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self.operand = operand
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class BinaryOperator(Node):
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def __init__(self, left, right):
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self.left = left
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self.right = right
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class Add(BinaryOperator):
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pass
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class Sub(BinaryOperator):
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pass
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class Mul(BinaryOperator):
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pass
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class Div(BinaryOperator):
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pass
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class Negate(UnaryOperator):
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pass
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class Number(Node):
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def __init__(self, value):
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self.value = value
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然后利用这些类构建嵌套数据结构，如下所示：
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.. code-block:: python
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# Representation of 1 + 2 * (3 - 4) / 5
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t1 = Sub(Number(3), Number(4))
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t2 = Mul(Number(2), t1)
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t3 = Div(t2, Number(5))
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t4 = Add(Number(1), t3)
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这样做的问题是对于每个表达式，每次都要重新定义一遍，有没有一种更通用的方式让它支持所有的数字和操作符呢。
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这里我们使用观察者模式可以达到这样的目的：
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.. code-block:: python
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class NodeVisitor:
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def visit(self, node):
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methname = 'visit_' + type(node).__name__
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meth = getattr(self, methname, None)
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if meth is None:
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meth = self.generic_visit
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return meth(node)
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def generic_visit(self, node):
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raise RuntimeError('No {} method'.format('visit_' + type(node).__name__))
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为了使用这个类，可以定义一个类继承它并且实现各种 ``visit_Name()`` 方法，其中Name是node类型。
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例如，如果你想求表达式的值，可以这样写：
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.. code-block:: python
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class Evaluator(NodeVisitor):
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def visit_Number(self, node):
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return node.value
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def visit_Add(self, node):
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return self.visit(node.left) + self.visit(node.right)
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def visit_Sub(self, node):
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return self.visit(node.left) - self.visit(node.right)
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def visit_Mul(self, node):
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return self.visit(node.left) * self.visit(node.right)
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def visit_Div(self, node):
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return self.visit(node.left) / self.visit(node.right)
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def visit_Negate(self, node):
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return -node.operand
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使用示例：
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.. code-block:: python
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>>> e = Evaluator()
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>>> e.visit(t4)
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0.6
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>>>
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作为一个不同的例子，下面定义一个类在一个栈上面将一个表达式转换成多个操作序列：
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.. code-block:: python
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class StackCode(NodeVisitor):
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def generate_code(self, node):
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self.instructions = []
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self.visit(node)
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return self.instructions
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def visit_Number(self, node):
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self.instructions.append(('PUSH', node.value))
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def binop(self, node, instruction):
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self.visit(node.left)
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self.visit(node.right)
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self.instructions.append((instruction,))
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def visit_Add(self, node):
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self.binop(node, 'ADD')
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def visit_Sub(self, node):
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self.binop(node, 'SUB')
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def visit_Mul(self, node):
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self.binop(node, 'MUL')
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def visit_Div(self, node):
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self.binop(node, 'DIV')
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def unaryop(self, node, instruction):
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self.visit(node.operand)
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self.instructions.append((instruction,))
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def visit_Negate(self, node):
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self.unaryop(node, 'NEG')
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使用示例：
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.. code-block:: python
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>>> s = StackCode()
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>>> s.generate_code(t4)
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[('PUSH', 1), ('PUSH', 2), ('PUSH', 3), ('PUSH', 4), ('SUB',),
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('MUL',), ('PUSH', 5), ('DIV',), ('ADD',)]
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>>>
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----------
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讨论
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----------
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todo...
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刚开始的时候你可能会写大量的if/else语句来实现，
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这里观察者模式的好处就是通过 ``getattr()`` 来获取相应的方法，并利用递归来遍历所有的节点：
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.. code-block:: python
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def binop(self, node, instruction):
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self.visit(node.left)
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self.visit(node.right)
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self.instructions.append((instruction,))
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还有一点需要指出的是，这种技术也是实现其他语言中switch或case语句的方式。
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比如，如果你正在写一个HTTP框架，你可能会写这样一个请求分发的控制器：
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.. code-block:: python
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class HTTPHandler:
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def handle(self, request):
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methname = 'do_' + request.request_method
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getattr(self, methname)(request)
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def do_GET(self, request):
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pass
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def do_POST(self, request):
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pass
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def do_HEAD(self, request):
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pass
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观察者模式一个缺点就是它严重依赖递归，如果数据结构嵌套层次太深可能会有问题，
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有时候会超过Python的递归深度限制(参考 ``sys.getrecursionlimit()`` )。
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可以参照8.22小节，利用生成器或迭代器来实现非递归遍历算法。
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在跟解析和编译相关的编程中使用观察者模式是非常常见的。
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Python本身的 ``ast`` 模块值的关注下，可以去看看源码。
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9.24小节演示了一个利用 ``ast`` 模块来处理Python源代码的例子。
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