咱們用的os模塊，讀取文件的時候，其實他是含有__enter__ __exit__ 。 一個是with觸發的時候，一個是退出的時候。
with file('nima,'r') as f: print f.readline()那咱們自己再實現一個標準的可以with的類。 我個人寫python的時候，喜歡針對一些需要有關閉邏輯的代碼，構造成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是個比with優美的東西，也是提供上下文機制的模塊，它是通過Generator裝飾器實現的，不再是采用__enter__和__exit__。contextlib中的contextmanager作為裝飾器來提供一種針對函數級別的上下文管理機制。
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我這里再貼下我上次寫的redis分布式鎖代碼中有關于contextlib的用法。其實乍一看，用了with和contextlib麻煩了，但是最少讓你的主體代碼更加鮮明了。
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
一個上下文管理器通過with聲明激活, 而且API包含兩個方法。__enter__()方法運行執行流進入到with代碼塊內。他返回一個對象共上下文使用。當執行流離開with塊時，__exit__()方法上下文管理器清除任何資源被使用。
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.'
打印結果
__init__()__enter__()Doing work in the context.__exit__()
執行上下文管理器時會調用__enter__離開時調用__exit__。
__enter__能返回任意對象，聯合一個指定名字于with聲明。
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()
打印結果
__init__()__enter__()WithinContext.__init__(<__main__.Context object at 0x7f579d8e4890>)WithinContext.do_something()__exit__()WithinContext.__del__
如果上下文管理器能處理異常,__exit__()應該返回一個True值表明這個異常不需要傳播,返回False異常會在執行__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')打印結果
__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
從生成器到上下文管理器
創建上下文管理的傳統方法,通過編寫一個類與__enter__()和__exit__()方法,并不困難。但有時比你需要的開銷只是管理一個微不足道的上下文。在這類情況下,您可以使用contextmanager() decorat or 生成器函數轉換成一個上下文管理器。
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')打印結果
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()可以同時管理多個上下文。
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打印結果
entering: Aentering: Bentering: Cinside with statement: A B Cexiting: Cexiting: Bexiting: A
因為Python 2.7和以后的版本不贊成使用nested(),因為可以直接嵌套
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
關閉open的句柄
文件類支持上下文管理器, 但是有一些對象不支持。還有一些類使用close()方法但是不支持上下文管理器。我們使用closing()來為他創建一個上下文管理器。(類必須有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打印結果
Normal Example: __init__() inside with statement close()Error handling example: __init__() raising from inside with statement close() Had an error: error message
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