詳解Python中contextlib上下文管理模塊的用法

更新時間：2016年06月28日 15:36:09 作者：cangmean

Python中一些內(nèi)建對象有了上下文管理器的支持,于是可以使用with語句來實(shí)現(xiàn)自動的文件打開關(guān)閉以及線程操作等,這里我們就從根基上來詳解Python中contextlib上下文管理模塊的用法

咱們用的os模塊，讀取文件的時候，其實(shí)他是含有__enter__ __exit__ 。一個是with觸發(fā)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(shí)現(xiàn)一個標(biāo)準(zhǔn)的可以with的類。我個人寫python的時候，喜歡針對一些需要有關(guān)閉邏輯的代碼，構(gòu)造成with的模式。

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

contextlib是個比with優(yōu)美的東西，也是提供上下文機(jī)制的模塊，它是通過Generator裝飾器實(shí)現(xiàn)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(shù)級別的上下文管理機(jī)制。

from contextlib import contextmanager
 
@contextmanager
def 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(guān)于contextlib的用法。其實(shí)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 contextmanager
from random import random
 
DEFAULT_EXPIRES = 15
DEFAULT_RETRIES = 5
 
@contextmanager
def 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(yùn)行執(zhí)行流進(jìn)入到with代碼塊內(nèi)。他返回一個對象共上下文使用。當(dāng)執(zhí)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(jié)果

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

執(zhí)行上下文管理器時會調(diào)用__enter__離開時調(diào)用__exit__。

__enter__能返回任意對象，聯(lián)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(jié)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(yīng)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(zhí)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_error

with Context(True):
  raise RuntimeError('error message handled')

print

with Context(False):
  raise RuntimeError('error message propagated')

打印結(jié)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(chuàng)建上下文管理的傳統(tǒng)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(shù)轉(zhuǎn)換成一個上下文管理器。

import contextlib

@contextlib.contextmanager
def 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
  
print
print 'handled ereor:'

with make_context() as value:
  raise RuntimeError('show example of handling an error')

print
print 'unhandled error:'

with make_context() as value:
  raise ValueError('this exception is not handled')

打印結(jié)果

Normal:
 entering
 inside with statement: {}
  exiting

handled ereor:
entering
 Error: show example of handling an error
 exiting

unhandled error:
entering
exiting
Traceback (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.contextmanager
def make_context(name):
  print 'entering:', name
  yield name
  print 'exiting:', name

with 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(jié)果

entering: A
entering: B
entering: C
inside with statement: A B C
exiting: C
exiting: B
exiting: A

因?yàn)镻ython 2.7和以后的版本不贊成使用nested(),因?yàn)榭梢灾苯忧短?/p>

import contextlib

@contextlib.contextmanager
def make_context(name):
  print 'entering:', name
  yield name
  print 'exiting:', name

with make_context('A') as A, make_context('B') as B, make_context('C') as C:
  print 'inside with statement:', A, B, C

關(guān)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(chuàng)建一個上下文管理器。(類必須有close方法)

import contextlib


class 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(jié)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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