I saw this in someone's code. What does it mean?
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
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.