python中关键词with的使用

http://stackoverflow.com/questions/3012488/what-is-the-python-with-statement-designed-for

非常好的回答，值得花时间粘过来，粗浅理解就是with利用context managers来为我们要执行的一些操作做预处理和最后的收尾，也就是把try和finally的操作放在了里面，我们再用with调用的时候，就简化了代码。

如最常用的例子

with open('test.txt', 'r') as f:

print f.read()

到这里，f已经close掉了，是因为open自身就是个context manager

后面这个回答者写的几个例子也非常简单明了。也让我第一次感受到装饰器的用处。

1. I believe this has already been answered by other users before me, so I only add it for the sake of completeness: the with statement simplifies exception handling by encapsulating common preparation and cleanup tasks in so-called context managers. More details can be found in PEP 343. For instance, the open statement is a context manager in itself, which lets you open a file, keep it open as long as the execution is in the context of the with statement where you used it, and close it as soon as you leave the context, no matter whether you have left it because of an exception or during regular control flow. The with statement can thus be used in ways similar to the RAII pattern in C++: some resource is acquired by the withstatement and released when you leave the with context.

2. Some examples are: opening files using with open(filename) as fp:, acquiring locks using with lock: (where lock is an instance of threading.Lock). You can also construct your own context managers using the contextmanager decorator from contextlib. For instance, I often use this when I have to change the current directory temporarily and then return to where I was:

   from contextlib import contextmanagerimport os@contextmanagerdef working_directory(path):    current_dir = os.getcwd()    os.chdir(path)    try:        yield    finally:        os.chdir(current_dir)with working_directory("data/stuff"):    # do something within data/stuff# here I am back again in the original working directory

   Here's another example that temporarily redirects sys.stdin, sys.stdout and sys.stderr to some other file handle and restores them later:

   from contextlib import contextmanagerimport sys@contextmanagerdef redirected(**kwds):    stream_names = ["stdin", "stdout", "stderr"]    old_streams = {}    try:        for sname in stream_names:            stream = kwds.get(sname, None)            if stream is not None and stream != getattr(sys, sname):                old_streams[sname] = getattr(sys, sname)                setattr(sys, sname, stream)        yield    finally:        for sname, stream in old_streams.iteritems():            setattr(sys, sname, stream)with redirected(stdout=open("/tmp/log.txt", "w")):     # these print statements will go to /tmp/log.txt     print "Test entry 1"     print "Test entry 2"# back to the normal stdoutprint "Back to normal stdout again"

   And finally, another example that creates a temporary folder and cleans it up when leaving the context:

   from tempfile import mkdtempfrom shutil import rmtree@contextmanagerdef temporary_dir(*args, **kwds):    name = mkdtemp(*args, **kwds)    try:        yield name    finally:        shutil.rmtree(name)with temporary_dir() as dirname:    # do whatever you want