python中with as 用法

with as 呢，就是个python控制流语句，像 if ,while。
with as 语句的结构:

with expression [as variable]:      with-block  

with expresion as variable的执行过程是

• 首先执行_enter_函数，它的返回值会赋给as后面的variable，想让它返回什么就返回什么，只要你知道怎么处理就可以了，如果不写as variable，返回值会被忽略。
• 然后，开始执行with-block中的语句，不论成功失败(比如发生异常、错误，设置sys.exit())，在with-block执行完成后，会执行_exit_函数。

等价于：
try:
执行 _enter_的内容
执行 with_block.
finally:
执行 _exit_内容

    附：try finally功能演示例子 (顺序执行try内语句，如果有错跳进except报错语句执行，随后直接进入finally。不管是否有错，都要执行finally)

try:    a=1/2    print (a)      b = 1 / 0 # an abnormal number/variable      print (b)    c = 2 / 1 # a normal number/variable      print (c)  except NameError:    print("ops")except ZeroDivisionError:    print("wrong math")except:    print("error")#else:#    print("no error")finally:    print("end")

例子1(无报错):

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  

执行步骤：

• 调用get_sample()函数，返回Sample()类;

• 执行Sample类中的enter()方法，打印”In__enter_()”字符串，并将字符串“Foo”赋值给as后面的sample变量;

• 执行with-block码块，即打印”sample: %s”字符串，结果为”sample: Foo”
• 执行with-block码块结束，返回Sample()类，执行类方法exit()。因为在执行with-block码块时并没有错误返回，所以type,value,trace这三个arguments都没有值。直接打印 print “In__exit__()”

结果：

In __enter__()sample:  FooIn __exit__()

例子2（有报错）:

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()  

结果：

type: <class 'ZeroDivisionError'>value: division by zerotrace: <traceback object at 0x000000000EC6F7C8>Traceback (most recent call last):  File "<ipython-input-16-1f63b4f99f4d>", line 13, in <module>    sample.do_something()  File "<ipython-input-16-1f63b4f99f4d>", line 9, in do_something    bar = 1/0ZeroDivisionError: division by zero