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

Paul Moore p.f.moore at gmail.com
Tue Jan 27 10:49:20 CET 2015

On 27 January 2015 at 06:24, Chris Barker <chris.barker at noaa.gov> wrote:
> If you ask the question: "are these to values close to each-other?" then the
> symmetric test makes the most sense -- obviously if a is close to b then b
> is close to a.
> whereas:
> If you ask the question: Is this value  within a defined relative difference
> of an expected value? (i.e 10% of b) then you want an asymmetric test -- you
> clearly are defining "relative" to the known value.
> However, and I've tried to say this multiple times -- and no one disagreed
> (or agreed) yet:
> I don't care much which we use, because
> IT DOES NOT MATTER in most cases which you choose.

All of that makes sense to me (I'm leaving out the detail of "it does
not matter" as I'm sort of trusting you guys with the expertise to
tell me that ;-))

But what *does* matter to me is usability and how the behaviour
matches people's intuition. Not because the mathematical results will
differ, but because it makes it easy for people to *think* about what
they are doing, and whether it's OK.

I would say that the correct approach is to make the default case as
easy to use as possible. For that, a symmetrical are_close(a,b) is a
no-brainer IMO. (Of course it has to work when either of a and b is
zero). It works either way - if one value is a known "target", or if
both values are approximations (e.g. when looking at convergence).

Once we have that as a basis, look at how people might want to tweak it:

are_close(a, b, within_abs=1e-8)  # Within a specific distance of each
other (absolute tolerance)
are_close(a, b, within_rel=0.1) # Within 10% of each other

In the relative case, I'd like "the experts" to decide for me what
precisely "within 10% of each other" means (document the details,
obviously, but don't bother me with them unless I go looking for

In either case, I'd be happy to assume that if you change the
defaults, you understand the implications (they can be explained in
the documentation) such as relative tolerances being unstable near
zero. I don't think it's a problem that the default behaviour can't be
expressed in terms of explicit settings for the tolerance arguments
(it's a wart, and could be annoying, but it's not a showstopper for me
- allow setting both explicitly to None to mean "default" if it
matters that much).

That's it. Anyone wanting to specify both parameters together, or
wanting the defaults to still apply "as well as" an explicitly
specified tolerance, is deemed an "expert" and should be looking for a
more specialised function (or writing their own).


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