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<h1 id="python设计模式之备忘录模式"><a href="http://dongweiming.github.io/python-memento.html"Permalink to python设计模式之备忘录模式"" target="_blank">python设计模式之备忘录模式</a></h1><h2 id="memento模式">Memento模式</h2><p>备忘录模式一个最好想象的例子：undo! 它对对象的一个状态进行了'快照'，
在你需要的时候恢复原貌。做前端会有一个场景：你设计一个表单，当点击提交会对表单内容
验证，这个时候你就要对用户填写的数据复制下来，当用户填写的不正确或者格式不对等问题， 就可以使用快照数据恢复用户已经填好的，而不是让用户重新来一遍，不是嘛？</p><h2 id="python的例子">python的例子</h2><p>这里实现了一个事务提交的例子</p><pre><code>import copy
def Memento(obj, deep=False):
# 对你要做快照的对象做快照
state = (copy.copy if deep else copy.deepcopy)(obj.__dict__)
def Restore():
obj.__dict__ = state
return Restore
class Transaction:
deep = False
def __init__(self, *targets):
self.targets = targets
self.Commit()
# 模拟事务提交，其实就是初始化给每个对象往self.targets赋值
def Commit(self):
self.states = [Memento(target, self.deep) for target in self.targets]
# 回滚其实就是调用Memento函数，执行其中的闭包，将__dict__恢复
def Rollback(self):
for state in self.states:
state()
# 装饰器的方式给方法添加这个事务的功能
def transactional(method):
# 这里的self其实就是要保存的那个对象，和类的实例无关
def wrappedMethod(self, *args, **kwargs):
state = Memento(self)
try:
return method(self, *args, **kwargs)
except:
# 和上面的回滚一样，异常就恢复
state()
raise
return wrappedMethod
class NumObj(object):
def __init__(self, value):
self.value = value
def __repr__(self):
return '<%s: %r>' % (self.__class__.__name__, self.value)
def Increment(self):
self.value += 1
@transactional
def DoStuff(self):
# 赋值成字符串，再自增长肯定会报错的
self.value = '1111'
self.Increment()
if __name__ == '__main__':
n = NumObj(-1)
print n
t = Transaction(n)
try:
for i in range(3):
n.Increment()
print n
# 这里事务提交会保存状态从第一次的-1到2
t.Commit()
print '-- commited'
for i in range(3):
n.Increment()
print n
n.value += 'x' # will fail
print n
except:
# 回滚只会回顾到上一次comit成功的2 而不是-1
t.Rollback()
print '-- rolled back'
print n
print '-- now doing stuff ...'
try:
n.DoStuff()
except:
print '-> doing stuff failed!'
import traceback
traceback.print_exc(0)
pass
# 第二次的异常回滚n还是2, 整个过程都是修改NumObj的实例对象
print n</code></pre>
<h2 id="注意">注意</h2><p>当你要保存的状态很大，可能会浪费大量内存</p>
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