python语法-[with来自动释放对象]

python语法-[with来自动释放对象]

http://www.cnblogs.com/itech/archive/2011/01/13/1934779.html

一 with

python中的with的作用是自动释放对象，即使对象在使用的过程中有异常抛出。可以使用with的类型必须实现__enter__ __exit__。我的理解是=try...finally{},在finally中调用了释放函数。

[类似与CSharp中的using(){}关键字，用来自动确保调用对象的dispose()方法，即使对象有异常抛出。C#中可以使用using{}的对象必须已经实现了IDispose接口。]

def TestWith():
  with open("myfile.txt") as f:
     for line in f:
         print (line)
  f.readline() #f is already clean up here, here will meet ValueError exception
   
TestWith()

在with语句执行完以后，f对象马上就被释放了。所以下面在调用f.readline()会出错。

二 with + try...except

既能让对象自动释放，又包含了异常捕获的功能。

class controlled_execution(object):
    def __init__(self, filename):
        self.filename = filename
        self.f = None

    def __enter__(self):
        try:
            f = open(self.filename, 'r')
            content = f.read()
            return content
        except IOError  as e:
            print (e)

    def __exit__(self, type, value, traceback):
        if self.f:
            print ('type:%s, value:%s, traceback:%s' % (str(type), str(value), str(traceback)))
            self.f.close()

def TestWithAndException():
    with controlled_execution("myfile.txt") as thing:
        if thing:
            print(thing)

#TestWithAndException()

参考：

http://jianpx.javaeye.com/blog/505469