[Python-Dev] math.areclose ...? (original) (raw)

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. 'almostequal') 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.

-bob

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