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python with statement 进行文件操作指南

Jun 16, 2016 am 08:42 AM
python with

由于之前有一个项目老是要打开文件,然后用pickle.load(file),再处理。。。最后要关闭文件,所以觉得有点繁琐,代码也不简洁。所以向python with statement寻求解决方法。

在网上看到一篇文章:http://effbot.org/zone/python-with-statement.htm是介绍with 的,参考着例子进行了理解。

如果经常有这么一些代码段的话,可以用一下几种方法改进:

代码段:

set thing up
try:
  do something
except :
  handle exception
finally:
  tear thing down
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案例1:

假如现在要实现这么一个功能,就是打开文件,从文件里面读取数据,然后打印到终端,之后关闭文件。

那么从逻辑上来说,可以抽取“打印到终端”为数据处理部分,应该可以独立开来作为一个函数。其他像打开、关闭文件应该是一起的。

文件名为:for_test.txt

方法1:

用函数,把公共的部分抽取出来。

#!/usr/bin/env python 
from __future__ import with_statement  
filename = 'for_test.txt' 
def output(content): 
  print content 
#functio solution 
def controlled_execution(func): 
  #prepare thing 
  f = None 
  try: 
    #set thing up 
    f = open(filename, 'r') 
    content = f.read() 
    if not callable(func): 
      return 
    #deal with thing  
    func(content) 
  except IOError, e: 
    print 'Error %s' % str(e) 
  finally: 
    if f:  
      #tear thing down 
      f.close() 
def test(): 
  controlled_execution(output) 
test() 
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方法2:

用yield实现一个只产生一项的generator。通过for - in 来循环。

代码片段如下:

#yield solution 
def controlled_execution(): 
  f = None 
  try: 
    f = open(filename, 'r') 
    thing = f.read() 
    #for thing in f: 
    yield thing 
  except IOError,e: 
    print 'Error %s' % str(e) 
  finally: 
    if f:  
      f.close() 
def test2(): 
  for content in controlled_execution(): 
    output(content) 
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方法3:

用类的方式加上with实现。

代码片段如下:

#class solution 
class controlled_execution(object): 
  def __init__(self): 
    self.f = None 
  def __enter__(self): 
    try: 
      f = open(filename, 'r') 
      content = f.read() 
      return content 
    except IOError ,e: 
      print 'Error %s' % str(e) 
      #return None 
  def __exit__(self, type, value, traceback): 
    if self.f: 
      print 'type:%s, value:%s, traceback:%s' % \ 
          (str(type), str(value), str(traceback)) 
      self.f.close() 
def test3(): 
  with controlled_execution() as thing: 
    if thing: 
      output(thing) 
 
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方法4:

用with实现。不过没有exception handle 的功能。

def test4(): 
  with open(filename, 'r') as f: 
    output(f.read()) 
 
  print f.read() 
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 最后一句print是用来测试f是否已经被关闭了。

    最后总结一下,写这篇文章的目的主要是受了一句话的刺激:“使用语言的好特性,不要使用那些糟糕的特性”!python真是有很多很优雅的好特性,路漫漫其修远兮,吾将上下而求索。。。

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