咱们用的os模块，读取文件的时候，其实他是含有__enter__ __exit__ 。 一个是with触发的时候，一个是退出的时候。

with file('nima,'r') as f: print f.readline()

那咱们自己再实现一个标准的可以with的类。 我个人写python的时候，喜欢针对一些需要有关闭逻辑的代码，构造成with的模式 。

#encoding:utf-8class echo: def __enter__(self): print 'enter' def __exit__(self,*args): print 'exit' with echo() as e: print 'nima'

contextlib是个比with优美的东西，也是提供上下文机制的模块，它是通过Generator装饰器实现的，不再是采用__enter__和__exit__。contextlib中的contextmanager作为装饰器来提供一种针对函数级别的上下文管理机制。

from contextlib import contextmanager @contextmanagerdef make_context() : print 'enter' try : yield {} except RuntimeError, err : print 'error' , err finally : print 'exit' with make_context() as value : print value

我这里再贴下我上次写的redis分布式锁代码中有关于contextlib的用法。其实乍一看，用了with和contextlib麻烦了，但是最少让你的主体代码更加鲜明了。

from contextlib import contextmanagerfrom random import random DEFAULT_EXPIRES = 15DEFAULT_RETRIES = 5 @contextmanagerdef dist_lock(key, client): key = 'lock_%s' % key try: _acquire_lock(key, client) yield finally: _release_lock(key, client) def _acquire_lock(key, client): for i in xrange(0, DEFAULT_RETRIES): get_stored = client.get(key) if get_stored: sleep_time = (((i+1)*random()) + 2**i) / 2.5 print 'Sleeipng for %s' % (sleep_time) time.sleep(sleep_time) else: stored = client.set(key, 1) client.expire(key,DEFAULT_EXPIRES) return raise Exception('Could not acquire lock for %s' % key) def _release_lock(key, client): client.delete(key)

Context Manager API

一个上下文管理器通过with声明激活, 而且API包含两个方法。__enter__()方法运行执行流进入到with代码块内。他返回一个对象共上下文使用。当执行流离开with块时，__exit__()方法上下文管理器清除任何资源被使用。

class Context(object): def __init__(self): print '__init__()' def __enter__(self): print '__enter__()' return self def __exit__(self, exc_type, exc_val, exc_tb): print '__exit__()'with Context(): print 'Doing work in the context.'

打印结果

__init__()__enter__()Doing work in the context.__exit__()

执行上下文管理器时会调用__enter__离开时调用__exit__。

__enter__能返回任意对象，联合一个指定名字于with声明。

class WithinContext(object): def __init__(self, context): print 'WithinContext.__init__(%s)' % context def do_something(self): print 'WithinContext.do_something()' def __del__(self): print 'WithinContext.__del__'class Context(object): def __init__(self): print '__init__()' def __enter__(self): print '__enter__()' return WithinContext(self) def __exit__(self, exc_type, exc_val, exc_tb): print '__exit__()'with Context() as c: c.do_something()

打印结果

__init__()__enter__()WithinContext.__init__(<__main__.Context object at 0x7f579d8e4890>)WithinContext.do_something()__exit__()WithinContext.__del__

如果上下文管理器能处理异常,__exit__()应该返回一个True值表明这个异常不需要传播,返回False异常会在执行__exit__之后被引起。

class Context(object): def __init__(self, handle_error): print '__init__(%s)' % handle_error self.handle_error = handle_error def __enter__(self): print '__enter__()' return self def __exit__(self, exc_type, exc_val, exc_tb): print '__exit__(%s, %s, %s)' % (exc_type, exc_val, exc_tb) return self.handle_errorwith Context(True): raise RuntimeError('error message handled')printwith Context(False): raise RuntimeError('error message propagated')

打印结果

__init__(True)__enter__()__exit__(<type 'exceptions.RuntimeError'>, error message handled, <traceback object at 0x7fdfb32f8b00>)__init__(False)__enter__()__exit__(<type 'exceptions.RuntimeError'>, error message propagated, <traceback object at 0x7fdfb32f8b90>)Traceback (most recent call last): File "test.py", line 23, in <module> raise RuntimeError('error message propagated') RuntimeError: error message propagated

从生成器到上下文管理器

创建上下文管理的传统方法,通过编写一个类与__enter__()和__exit__()方法,并不困难。但有时比你需要的开销只是管理一个微不足道的上下文。在这类情况下,您可以使用contextmanager() decorat or 生成器函数转换成一个上下文管理器。

import contextlib@contextlib.contextmanagerdef make_context(): print ' entering' try: yield {} except RuntimeError, err: print ' Error:', err finally: print ' exiting' print 'Normal:'with make_context() as value: print ' inside with statement:', value printprint 'handled ereor:'with make_context() as value: raise RuntimeError('show example of handling an error')printprint 'unhandled error:'with make_context() as value: raise ValueError('this exception is not handled')

打印结果

Normal: entering inside with statement: {} exitinghandled ereor:entering Error: show example of handling an error exitingunhandled error:enteringexitingTraceback (most recent call last): File "test.py", line 30, in <module> raise ValueError('this exception is not handled') ValueError: this exception is not handled

嵌套上下文

使用nested()可以同时管理多个上下文。

import contextlib@contextlib.contextmanagerdef make_context(name): print 'entering:', name yield name print 'exiting:', namewith contextlib.nested(make_context('A'), make_context('B'), make_context('C')) as (A, B, C): print 'inside with statement:', A, B, C

打印结果

entering: Aentering: Bentering: Cinside with statement: A B Cexiting: Cexiting: Bexiting: A

因为Python 2.7和以后的版本不赞成使用nested(),因为可以直接嵌套

import contextlib@contextlib.contextmanagerdef make_context(name): print 'entering:', name yield name print 'exiting:', namewith make_context('A') as A, make_context('B') as B, make_context('C') as C: print 'inside with statement:', A, B, C

关闭open的句柄

文件类支持上下文管理器, 但是有一些对象不支持。还有一些类使用close()方法但是不支持上下文管理器。我们使用closing()来为他创建一个上下文管理器。(类必须有close方法)

import contextlibclass Door(object): def __init__(self): print ' __init__()' def close(self): print ' close()'print 'Normal Example:'with contextlib.closing(Door()) as door: print ' inside with statement' print print 'Error handling example:'try: with contextlib.closing(Door()) as door: print ' raising from inside with statement' raise RuntimeError('error message')except Exception, err: print ' Had an error:', err

打印结果

Normal Example: __init__() inside with statement close()Error handling example: __init__() raising from inside with statement close() Had an error: error message