On Thursday, 6 December 2012 17:44:17 UTC+5:30, Chris Angelico wrote: > On Thu, Dec 6, 2012 at 10:47 PM, Steven D'Aprano > > <steve+comp.lang.pyt...@pearwood.info> wrote: > > > Not so. Which one is faster will depend on how often you expect to fail. > > > If the keys are nearly always present, then: > > > > > > try: > > > do_stuff(mydict[k]) > > > except KeyError: > > > pass > > > > > > will be faster. Setting up a try block is very fast, about as fast as > > > "pass", and faster than "if k in mydict". > > > > > > But if the key is often missing, then catching the exception will be > > > slow, and the "if k in mydict" version may be faster. It depends on how > > > often the key is missing. > > > > > > > Setting up the try/except is a constant time cost, while the > > duplicated search for k inside the dictionary might depend on various > > other factors. In the specific case of a Python dictionary, the > > membership check is fairly cheap (assuming you're not the subject of a > > hash collision attack - Py3.3 makes that a safe assumption), but if > > you were about to execute a program and wanted to first find out if it > > existed, that extra check could be ridiculously expensive, eg if the > > path takes you on a network drive - or, worse, on multiple network > > drives, which I have had occasion to do! > > > > ChrisA
Not really. I remember a bug saying that only 256 hashes were required of known texts and then the randomization becomes useless. -- http://mail.python.org/mailman/listinfo/python-list
