On Jan 28, 2012, at 6:42 AM, Greg Landrum wrote: > I haven't followed the evolution of context managers in Python; what's > the advantage?
The context manager reduces duplication (and generally saves a couple of lines of code) for a common idiom, and makes it easier to do resource management where a "start()" call must always be matched by an "end()" call. Context managers are syntax abstractions for the common pattern pattern of: - set things up before you run code - tear them down after you are done - possibly do something else when there's an exception The justification document is at http://www.python.org/dev/peps/pep-0343/ and the "What's New" description at http://docs.python.org/whatsnew/2.5.html#pep-343-the-with-statement Here are some examples of it in use, where I compare the old and new way. == Make sure that a file is closed, even on error # old style (works, but depends on the garbage collector for the close) f = open(filename) ... do stuff with f ... # old style (makes sure the file is always closed) f = open(filename) try: ... do stuff with f ... finally: f.close() # Make sure it's closed at the end # with a context manager (close f at the end of the block) with open(filename) as f: ... do stuff with f ... == Acquire and release a critical lock # old style write_lock = threading.Lock() ... write_lock.acquire() try: .. do critical code ... finally: write_lock.release() # new style write_lock = threading.Lock() ... with write_lock: .. do critical code == Handle a database transaction but rollback if there are errors # old style try: ... work with the database here .. except: cursor.rollback() raise else: cursor.commit() # new style with connection: ... work with the database here .. == My test code captures the value of stdout with # old style old_stdout = sys.stdout sys.stdout = cStringIO.StringIO() try: run_test() finally: output = sys.stdout.getvalue() sys.stdout = old_stdout # new style with CaptureStdout() as new_stdout: run_test() output = new_stdout.getvalue() Andrew [email protected] ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Rdkit-discuss mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/rdkit-discuss
