<s>[INST] Explaining Python's '__enter__' and '__exit__' this is i saw in someone's code: 
    def __enter__(self):
        return self

    def __exit__(self, type, value, tb):
        self.stream.close()


from __future__ import with_statement#for python2.5 

class a(object):
    def __enter__(self):
        print 'sss'
        return 'sss111'
    def __exit__(self ,type, value, traceback):
        print 'ok'
        return False

with a() as s:
    print s


print s

 [/INST] Using these magic methods (__enter__, __exit__) allows you to implement objects which can be used easily with the with statement. 
The idea is that it makes it easy to build code which needs some 'cleandown' code executed (think of it as a try-finally block). Some more explanation here.
A useful example could be a database connection object (which then automagically closes the connection once the corresponding 'with'-statement goes out of scope):
class DatabaseConnection(object):

    def __enter__(self):
        # make a database connection and return it
        ...
        return self.dbconn

    def __exit__(self, exc_type, exc_val, exc_tb):
        # make sure the dbconnection gets closed
        self.dbconn.close()
        ...

As explained above, use this object with the with statement (you may need to do from __future__ import with_statement at the top of the file if you're on Python 2.5).
with DatabaseConnection() as mydbconn:
    # do stuff

PEP343 -- The 'with' statement' has a nice writeup as well.
 </s>