Python-深入理解with语句

使用实例：

with open('./data.txt') as file:
    file.read()

基本思想：with所求值的对象必须有__enter__方法和__exit__方法

基本流程：with后面的语句被求值后，enter方法被调用，返回一个对象赋予as后的变量，with模块中所有的语句被执行完毕后，调用前面返回对象的exit方法

In [4]: class Sample:
   ...:     def __enter__(self):
   ...:         print('enter')
   ...:         return "对象"
   ...:     def __exit__(self,type,value,trace):
   ...:         print('exit')

In [5]: def get_Sample():
   ...:     return Sample()
   ...:

In [6]: with get_Sample() as sample:
   ...:     print('sample',sample)
enter
sample 对象
exit

完整功能实例（报异常）：可以在exit()中输出异常

In [9]: 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
   ...:

In [10]: with Sample() as sample:
    ...:     sample.do_Something()
    ...:
type: <class 'ZeroDivisionError'>
value division by zero
trace <traceback object at 0x000002777858E748>
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-10-5e52b3552bb5> in <module>
      1 with Sample() as sample:
----> 2     sample.do_Something()
      3

<ipython-input-9-3ea07c9cf578> in do_Something(self)
      7         print('trace',trace)
      8     def do_Something(self):
----> 9         bar = 1/0
     10         return bar+10
     11

ZeroDivisionError: division by zero

创建新的上下文使用实例：

In [21]: import time

In [22]: from contextlib import contextmanager

In [23]: @contextmanager
    ...: def timethis(label):
    ...:     start = time.time()
    ...:     try:
    ...:         yield #yield之前的语句作为enter函数中的内容，yield之后的语句作为exit函数中的内容，如果有异常，则在yield处返回
    ...:     finally: # 如果有enter函数中有报错，下面的部分不执行
    ...:         end =time.time()
    ...:         print('{}:{}'.format(label,end-start))
    ...:

In [24]: with timethis('counting'):
    ...:     n = 100000
    ...:     while n>0:
    ...:         n-=1
    ...:
counting:0.015624046325683594