[Python-Dev] math.areclose ...?

Bob Ippolito bob at redivi.com
Sun Feb 5 20:16:19 CET 2006

On Feb 5, 2006, at 10:48 AM, Raymond Hettinger wrote:

>>> So I was wondering if module math (and
>>> perhaps by symmetry module cmath, too) shouldn't grow a function
>>> 'areclose' (calling it just 'close' seems likely to engender
>>> confusion, since 'close' is more often used as a verb than as an
>>> adjective; maybe some other name would work better, e.g.
>>> 'almost_equal') taking two float arguments and optional tolerances
>>> and using roughly the same specs as Numeric, e.g.:
>>> def areclose(x,y,rtol=1.e-5,atol=1.e-8):
>>>      return abs(x-y)<atol+rtol*abs(y)
> IMO, the cure is worse than the disease.  It is easier to learn about
> the hazards of floating point equality testing than to think  
> through the
> implications of tolerance testing (such as loss of transitivity) and
> learning
> how to set the right tolerance values for a given application (ones  
> that
> give the right results across the entire domain of expected inputs).
> The areclose() function can be a dangerous crutch that temporarily
> glosses over the issue.  Without some numerical sophistication, it  
> would not
> be hard create programs that look correct and pass a few test but,  
> in fact,
> contain nasty bugs (non-termination, incorrect acceptance/ 
> rejection, etc).

For those of us that already know what we're doing with floating  
point, areclose would be very convenient to have.  Especially for  
unit testing.  I could definitely throw away a bunch of ugly code  
that uses less correct arbitrary tolerance guesses if it were around.


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