Surprising timeit result

[John O'Hagan]

On Tue, 27 Oct 2009, Paul Rubin wrote:
> "John O'Hagan" <research at johnohagan.com> writes:
> > Timer('d1.update(d2)', setup).timeit()
> > 2.6499271392822266
> >
> > Timer('if d1 != d2: d1.update(d2)', setup).timeit()
> > 1.0235211849212646
> >
> > In other words, in this case it's substantially quicker to check for
> > something and then proceed, than it is to just proceed! I'm curious
> > about the explanation.
> 
> Looks to me like in both versions, d2 is only initialized once, so
> the d1.update in the second case only gets called on the first loop.
> It's not so surprising that comparing d1 and d2 is faster than
> actually updating d1.
> 

Of course:

Timer('d1 = {"a":0}; d2 = {"a":1}\nif d1 != d2: d1.update(d2)').timeit()
1.9810600280761719

Timer('d1 = {"a":0}; d2 = {"a":1}\nd1.update(d2)').timeit()
1.7072379589080811

as expected. I wasn't quite clear about how Timer() worked, is my excuse!

Thanks,

John