欢迎使用CSDN-markdown编辑器

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 from the 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：
file = open(“/tmp/foo.txt”)
data = file.read()
file.close()

There are two annoying things here. First, you end up forgetting to close the file handler. The second is how to handle exceptions that may occur once the file handler has been obtained. One could write something like this to get around this:
这里有两个问题。一是可能忘记关闭文件句柄；二是文件读取数据发生异常，没有进行任何处理。下面是处理异常的加强版本：
file = open(“/tmp/foo.txt”)
try:
data = file.read()
finally:
file.close()

While this works well, it is unnecessarily verbose. This is where with is useful. The good thing about with apart from the better syntax is that it is very good handling exceptions. The above code would look like this, when using with:
虽然这段代码运行良好，但是太冗长了。这时候就是with一展身手的时候了。除了有更优雅的语法，with还可以很好的处理上下文环境产生的异常。下面是with版本的代码：
with open(“/tmp/foo.txt”)
as file:
data = file.read()

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。

After the 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：

#!/usr/bin/env
python
#
with_example01.py

class Sample:
def enter(self):
print “In
enter()”
return “Foo”
def exit(self, type,
value, trace):
print “In
exit()”

def get_sample():
return Sample()

with
get_sample() as sample:
print “sample:”,
sample

When executed, this will result in:
运行代码，输出如下
bash-3.2$
./with_example01.py
In
enter()
sample:
Foo
In
exit()

As you can see,
The enter() function is executed
The value returned by it - in this case “Foo” is assigned to sample
The body of the block is executed, thereby printing the value of sample ie. “Foo”
The exit() function is called.
What makes with really powerful 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。

#!/usr/bin/env
python
#
with_example02.py

class Sample:
def enter(self):
return self

def __exit__(self, type,

value, trace):
print “type:”, type
print “value:”,
value
print “trace:”,
trace

def do_something(self):    bar = 1/0    return bar + 10

with
Sample() as sample:
sample.do_something()

Notice how in this example, instead of get_sample(), with takes Sample(). It does not matter, as long as the statement that follows with evaluates to an object that has an enter() and exit() functions. In this case, Sample()’s enter() returns the 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：
bash-3.2$
./with_example02.py
type:

定义列表

Markdown　Extra　定义列表语法：

项目１

项目２

定义 A

定义 B

项目３

定义 C

定义 D

定义D内容

代码块

代码块语法遵循标准markdown代码，例如：

@requires_authorizationdef somefunc(param1='', param2=0):    '''A docstring'''    if param1 > param2: # interesting        print 'Greater'    return (param2 - param1 + 1) or Noneclass SomeClass:    pass>>> message = '''interpreter... prompt'''

脚注

生成一个脚注1.

目录

用 [TOC]来生成目录：

数学公式

使用MathJax渲染LaTex 数学公式，详见math.stackexchange.com.

• 行内公式，数学公式为：Γ(n)=(n−1)!∀n∈N 。
• 块级公式：

x=−b±b 2 −4ac − − − − − − −  √ 2a  

更多LaTex语法请参考 这儿.

UML 图:

可以渲染序列图：

或者流程图：

• 关于 序列图 语法，参考 这儿,
• 关于 流程图 语法，参考 这儿.

离线写博客

即使用户在没有网络的情况下，也可以通过本编辑器离线写博客（直接在曾经使用过的浏览器中输入write.blog.csdn.net/mdeditor即可。Markdown编辑器使用浏览器离线存储将内容保存在本地。

用户写博客的过程中，内容实时保存在浏览器缓存中，在用户关闭浏览器或者其它异常情况下，内容不会丢失。用户再次打开浏览器时，会显示上次用户正在编辑的没有发表的内容。

博客发表后，本地缓存将被删除。　

用户可以选择 把正在写的博客保存到服务器草稿箱，即使换浏览器或者清除缓存，内容也不会丢失。

注意：虽然浏览器存储大部分时候都比较可靠，但为了您的数据安全，在联网后，请务必及时发表或者保存到服务器草稿箱。

浏览器兼容

1. 目前，本编辑器对Chrome浏览器支持最为完整。建议大家使用较新版本的Chrome。
2. IE９以下不支持
3. IE９，１０，１１存在以下问题
   
   1. 不支持离线功能
   2. IE9不支持文件导入导出
   3. IE10不支持拖拽文件导入

---

---

1. 这里是 脚注 的 内容. ↩