理解Python的With语句
2015-07-06 15:28
246 查看
With语句是什么?
Python’s with statement provides a very convenient way of dealing with the situation where you have to do a setup and teardown to make something happen. A very good example for this is the situation where you want to gain a handler to a file, read data fromthe file and the close the file handler.有一些任务,可能事先需要设置,事后做清理工作。对于这种场景,Python的with语句提供了一种非常方便的处理方式。一个很好的例子是文件处理,你需要获取一个文件句柄,从文件中读取数据,然后关闭文件句柄。Without the with statement, one would write something along the lines of:如果不用with语句,代码如下:
with如何工作?
while this might look like magic, the way Python handles with is more clever than magic. The basic idea is that the statement after with has to evaluate an object that responds to an __enter__() as well as an __exit__() function.这看起来充满魔法,但不仅仅是魔法,Python对with的处理还很聪明。基本思想是with所求值的对象必须有一个__enter__()方法,一个__exit__()方法。Afterthe statement that follows with is evaluated, the __enter__() function on the resulting object is called. The value returned by this function is assigned to the variable following as. After every statement in the block is evaluated, the __exit__() function
is called.紧跟with后面的语句被求值后,返回对象的__enter__()方法被调用,这个方法的返回值将被赋值给as后面的变量。当with后面的代码块全部被执行完之后,将调用前面返回对象的__exit__()方法。This can be demonstrated with the following example:下面例子可以具体说明with如何工作:
is the fact that it can handle exceptions. You would have noticed that the __exit__() function for Sample takes three arguments - val, type and trace. These are useful in exception handling. Let’s see how this works by modifying the above example.正如你看到的,1.
__enter__()方法被执行2. __enter__()方法返回的值 - 这个例子中是"Foo",赋值给变量'sample'3. 执行代码块,打印变量"sample"的值为 "Foo"4. __exit__()方法被调用with真正强大之处是它可以处理异常。可能你已经注意到Sample类的__exit__方法有三个参数- val, type 和 trace。 这些参数在异常处理中相当有用。我们来改一下代码,看看具体如何工作的。
newly created instance of Sample and that is what gets passed to sample.这个例子中,with后面的get_sample()变成了Sample()。这没有任何关系,只要紧跟with后面的语句所返回的对象有__enter__()和__exit__()方法即可。此例中,Sample()的__enter__()方法返回新创建的Sample对象,并赋值给变量sample。When executed:代码执行后:
case, you can see that there was a ZeroDivisionError exception being thrown. People implementing libraries can write code that clean up resources, close files etc. in their __exit__() functions.实际上,在with后面的代码块抛出任何异常时,__exit__()方法被执行。正如例子所示,异常抛出时,与之关联的type,value和stack
trace传给__exit__()方法,因此抛出的ZeroDivisionError异常被打印出来了。开发库时,清理资源,关闭文件等等操作,都可以放在__exit__方法当中。Thus, Python’s with is a nifty construct that makes code a little less verbose and makes cleaning up during exceptions a bit easier.因此,Python的with语句是提供一个有效的机制,让代码更简练,同时在异常产生时,清理工作更简单。I
have put the code examples given here on
Github.示例代码可以在Github上面找到。译注:本文原文见Understanding
Python's "With" Statement
相关文章推荐
- Python 学习 --- 基础知识
- 判断Python输入是否为数字
- Python实现图像几何变换
- Python统计日志中每个IP出现次数的方法
- python BinaryTree
- python统计日志ip访问数的方法
- python检查指定文件是否存在的方法
- vim的python代码检测工具
- windows上安装numpy和scipy
- 安装python至windows
- Python单例模式的4种实现方法
- Python - 属性访问
- python代码运行显示PC参数
- Python正则表达式
- XGBoost:在Python中使用XGBoost
- Python 拷贝对象(深拷贝deepcopy与浅拷贝copy)
- python转换字符串为摩尔斯电码的方法
- python实现文件路径和url相互转换的方法
- 详解Python中的Cookie模块使用
- python计算文本文件行数的方法