8.22小节完成
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XiongNeng
110
cookbook/c08/p22_vistor_norecursiion.py
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110
cookbook/c08/p22_vistor_norecursiion.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: 非递归的观察者模式
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Desc :
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"""
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import types
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class Node:
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pass
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class NodeVisitor:
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def visit(self, node):
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stack = [node]
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last_result = None
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while stack:
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try:
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last = stack[-1]
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if isinstance(last, types.GeneratorType):
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stack.append(last.send(last_result))
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last_result = None
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elif isinstance(last, Node):
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stack.append(self._visit(stack.pop()))
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else:
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last_result = stack.pop()
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except StopIteration:
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stack.pop()
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return last_result
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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 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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# A sample visitor class that evaluates expressions
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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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yield (yield node.left) + (yield node.right)
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def visit_Sub(self, node):
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yield (yield node.left) - (yield node.right)
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def visit_Mul(self, node):
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yield (yield node.left) * (yield node.right)
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def visit_Div(self, node):
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yield (yield node.left) / (yield node.right)
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def visit_Negate(self, node):
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yield - (yield node.operand)
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if __name__ == '__main__':
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# 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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# Evaluate it
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e = Evaluator()
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print(e.visit(t4)) # Outputs 0.6
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@@ -5,14 +5,222 @@
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----------
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问题
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----------
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todo...
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你使用观察者模式遍历一个很深的嵌套树形数据结构，并且因为超过嵌套层级限制而失败。
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你想消除递归，并同时保持观察者编程模式。
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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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通过巧妙的使用生成器可以在树遍历或搜索算法中消除递归。
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在8.21小节中，我们给出了一个观察者类。
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下面我们利用一个栈和生成器重新实现这个类：
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.. code-block:: python
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import types
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class Node:
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pass
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class NodeVisitor:
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def visit(self, node):
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stack = [node]
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last_result = None
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while stack:
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try:
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last = stack[-1]
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if isinstance(last, types.GeneratorType):
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stack.append(last.send(last_result))
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last_result = None
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elif isinstance(last, Node):
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stack.append(self._visit(stack.pop()))
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else:
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last_result = stack.pop()
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except StopIteration:
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stack.pop()
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return last_result
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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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如果你使用这个类，也能达到相同的效果。事实上你完全可以将它作为上一节中的观察者模式的替代实现。
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考虑如下代码，遍历一个表达式的树：
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.. code-block:: python
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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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# A sample visitor class that evaluates expressions
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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 -self.visit(node.operand)
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if __name__ == '__main__':
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# 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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# Evaluate it
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e = Evaluator()
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print(e.visit(t4)) # Outputs 0.6
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如果嵌套层次太深那么上述的Evaluator就会失效：
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.. code-block:: python
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>>> a = Number(0)
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>>> for n in range(1, 100000):
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... a = Add(a, Number(n))
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...
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>>> e = Evaluator()
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>>> e.visit(a)
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Traceback (most recent call last):
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...
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File "visitor.py", line 29, in _visit
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return meth(node)
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File "visitor.py", line 67, in visit_Add
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return self.visit(node.left) + self.visit(node.right)
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RuntimeError: maximum recursion depth exceeded
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>>>
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现在我们稍微修改下上面的Evaluator：
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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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yield (yield node.left) + (yield node.right)
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def visit_Sub(self, node):
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yield (yield node.left) - (yield node.right)
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def visit_Mul(self, node):
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yield (yield node.left) * (yield node.right)
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def visit_Div(self, node):
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yield (yield node.left) / (yield node.right)
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def visit_Negate(self, node):
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yield - (yield node.operand)
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再次运行，就不会报错了：
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.. code-block:: python
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>>> a = Number(0)
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>>> for n in range(1,100000):
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... a = Add(a, Number(n))
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...
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>>> e = Evaluator()
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>>> e.visit(a)
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4999950000
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>>>
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如果你还想添加其他自定义逻辑也没问题：
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.. code-block:: python
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class Evaluator(NodeVisitor):
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...
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def visit_Add(self, node):
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print('Add:', node)
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lhs = yield node.left
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print('left=', lhs)
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rhs = yield node.right
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print('right=', rhs)
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yield lhs + rhs
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...
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下面是简单的测试：
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.. code-block:: python
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>>> e = Evaluator()
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>>> e.visit(t4)
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Add: <__main__.Add object at 0x1006a8d90>
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left= 1
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right= -0.4
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0.6
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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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这一小节我们演示了生成器和协程在程序控制流方面的强大功能。
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避免递归的一个通常方法是使用一个栈或队列的数据结构。
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例如，深度优先的遍历算法，第一次碰到一个节点时将其压入栈中，处理完后弹出栈。``visit()`` 方法的核心思路就是这样。
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另外一个需要理解的就是生成器中yield语句。当碰到yield语句时，生成器会返回一个数据并暂时挂起。
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上面的例子使用这个技术来代替了递归。例如，之前我们是这样写递归：
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.. code-block:: python
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value = self.visit(node.left)
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现在换成yield语句：
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.. code-block:: python
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value = yield node.left
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它会将 ``node.left`` 返回给 ``visti()`` 方法，然后 ``visti()`` 方法调用那个节点相应的 ``vist_Name()`` 方法。
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yield暂时将程序控制器让出给调用者，当执行完后，结果会赋值给value，
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看完这一小节，你也许想去寻找其它没有yield语句的方案。但是这么做没有必要，你必须处理很多棘手的问题。
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例如，为了消除递归，你必须要维护一个栈结构，如果不使用生成器，代码会变得很臃肿，到处都是栈操作语句、回调函数等。
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实际上，使用yield语句可以让你写出非常漂亮的代码，它消除了递归但是看上去又很像递归实现，代码很简洁。
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