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

Alex Martelli aleaxit at gmail.com
Mon Feb 6 22:03:26 CET 2006

On 2/6/06, Aahz <aahz at pythoncraft.com> wrote: ...

> def areclose(x, y, relativeerr = 1.e-5, absoluteerr=1.e-8): > diff = abs(x - y) > ave = (abs(x) + abs(y))/2 > return diff < absoluteerr or diff/ave < relativeerr_ _> > Also, separating the two terms with 'or' rather than '+' makes the > two error terms mean more what they are named. The '+' mixes the two > effects and even though the result is basically the same, it makes it > difficult to explain when the test will be true.

Yes, that's a big help. I was a bit concerned that this would have no utility for numbers with large magnitude. Alex, given your focus on Python readability, I'm a bit surprised you didn't write this to start with!

As I said, I was just copying the definition in Numeric, which is well-tried by long use. Besides, this "clear expression" could present problems, such as possible overflows or divisions by zero when ave is 0 or very small; much as I care about readability, I care about correctness even more.

Once it comes to readability, I prefer Numeric's choice to call the two terms "tolerances", rather than (as here) "errors"; maybe that depends on my roots being in engineering, where an error means a mistake (like it does in real life), while tolerance's a good and useful thing to have (ditto), rather than some scientific discipline where terms carry different nuances.

Alex

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