What is the python “with” statement designed for?

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 with statement and released when you leave the with context.

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 contextmanager
import os

@contextmanager
def 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 contextmanager
import sys

@contextmanager
def 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 stdout
print "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 mkdtemp
from shutil import rmtree

@contextmanager
def temporary_dir(*args, **kwds):
    name = mkdtemp(*args, **kwds)
    try:
        yield name
    finally:
        shutil.rmtree(name)

with temporary_dir() as dirname:
    # do whatever you want