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Date: Thu, 6 Dec 2012 23:14:17 +1100
Subject: Re: Confused compare function :)
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:34379

On Thu, Dec 6, 2012 at 10:47 PM, Steven D'Aprano
<steve+comp.lang.python@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