with manager as var:
  do_something(var)

var = manager.__enter__()
try:
  do_something(var)
finally:
  manager.__exit__()

>>> class closing(object):
...  def __init__(self, obj):
...   self.obj = obj
...  def __enter__(self):
...   return self.obj
...  def __exit__(self, *args):
...   self.obj.close()
>>> with closing(open('/tmp/file', 'w')) as f:
...  f.write('the contents\n')

>>> with open('/tmp/file', 'a') as f:
...  f.write('more contents\n')

filename="my.txt"
mode="w"
writer=open(filename,mode)
writer.write("hello world")
writer.close()

writer=open(filename,mode)
try:
  writer.write("hello world")
finally:
  writer.close()

with open(filename,mode) as writer:
  writer.write("hello world")

class echo():
  def output(self):
    print "hello world"
  def __enter__(self):
    print "enter"
    return self #可以返回任何希望返回的东西
  def __exit__(self,exception_type,value,trackback):
    print "exit"
    if exception_type==ValueError:
      return True
    else:
      return Flase
 
>>>with echo as e:
  e.output()

enter
hello world
exit

def __exit__(self,exc_type,exc_value,exc_tb)

from contextlib import contextmanager
@contextmanager
def make_context():
  print 'enter'
  try:
    yield "ok"
  except RuntimeError,err:
    print 'error',err
  finally:
    print 'exit'
    
>>>with make_context() as value:
  print value

  enter
  ok
  exit

@contextlib.contextmanager
def loudLock():
  print 'Locking'
  lock.acquire()
  yield
  print 'Releasing'
  lock.release()
 
with loudLock():
  print 'Lock is locked: %s' % lock.locked()
  print 'Doing something that needs locking'
 
#Output:
#Locking
#Lock is locked: True
#Doing something that needs locking
#Releasing

with open(filename,mode) as reader:
  with open(filename1,mode1) as writer:
    writer.write(reader.read())

with contextlib.nested(open(filename,mode),open(filename1,mode1)) as (reader,writer):
  writer.write(reader.read())

with open(filename,mode) as reader,open(filename1,mode1) as writer:
  writer.write(reader.read())

import contextlib
class myclass():
  def __init__(self):
    print '__init__'
  def close(self):
    print 'close()'
   
with contextlib.closing(myclass()):
  print 'ok'

__init__
ok
close()