[Python-ideas] PEP 485: A Function for testing approximate equality

Paul Moore p.f.moore at gmail.com
Fri Jan 23 19:47:06 CET 2015

On 23 January 2015 at 18:10, Guido van Rossum <guido at python.org> wrote:
> On Fri, Jan 23, 2015 at 9:59 AM, Paul Moore <p.f.moore at gmail.com> wrote:
>> Guido's example of Newton iteration is a good use case (although most
>> of the time I'd expect to use a prebuilt function from a module,
>> rather than build it myself with Newton iteration, but maybe that just
>> reflects the fact that I don't do numerical programming).
> Well duh. Any algorithm that isn't already in the math module would require
> way too much code. The point of the example is that most people have
> probably seen that algorithm before, and it's only one simple step, really,
> so they won't be distracted by trying to understand the algorithm when the
> point of the example is to show how you would use is_close_to(). (And it's
> one of the simplest algorithms that gives an *approximation*, not an exact
> answer, at least not in the mathematical sense, which is also important in
> this case -- if the algorithm was exact there would be no need to use
> is_close_to().)

Sorry. What I was trying to say is that if I had a need for say a
Bessel function, or numerical integration, or a zero of a function,
I'd go hunting for a package that implemented it (something like
mpmath, maybe) rather than rolling my own numerical algorithm using

But I do agree, that implementing numerical algorithms is a good use
of is_close_to. And your example was fine, it'd make a good addition
to use cases in the PEP. (But I wonder - wouldn't it work better with
a "symmetrical" close-to function? That's probably a question for


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