[Python-Dev] Optional delta argument for assertAlmostEqual

Steven D'Aprano steve at pearwood.info
Sun Mar 28 09:06:06 CEST 2010

On Sun, 28 Mar 2010 02:54:19 pm Charles McCreary wrote:
> Perhaps not just absolute but relative tolerance, e.g.:
> def isclose(x, y, rtol=1.e-5, atol=1.e-8):
>     return abs(x-y) <= atol + rtol * abs(y)

I'm not sure why you add the tolerances like that. Surely a more 
appropriate approach is to compare the values against the tolerances 
individually, and return True if they meet either condition.

For what it's worth, here's a recipe I wrote for approximately equal, 
following someone's complaint on comp.lang.python that Python has no 
approximately-equal operator.


Comments and criticism welcome.

I've never found unittest.TestCase.assertAlmostEqual to be useful. I 
generally write my own comparison function, then call:

assert_(compare(x, y))

as needed. I'm +0.5 on Michael's suggested change -- it will make 
assertAlmostEqual marginally more useful, which is a plus, but I'm not 
sure that absolute tolerances are better than relative. But how general 
do we want to be? "Almost equal" means something different to everyone.

> On Fri, Mar 26, 2010 at 7:59 PM, Michael Foord 
<fuzzyman at voidspace.org.uk>wrote:
> > Hello all,
> >
> > A user has suggested an optional argument to
> > unittest.TestCase.assertAlmostEqual for specifying a maximum
> > difference between the expected and actual values, instead of using
> > rounding.

Steven D'Aprano

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