What exactly is "exact" (was Clean Singleton Docstrings)

[Rustom Mody]

On Tuesday, July 19, 2016 at 9:12:57 AM UTC+5:30, Steven D'Aprano wrote:
> On Mon, 18 Jul 2016 08:15 pm, Chris Angelico wrote:
> 
> > On Mon, Jul 18, 2016 at 8:00 PM, Marko Rauhamaa wrote:
> >> Python programmers (among others) frequently run into issues with
> >> surprising results in floating-point arithmetics. For better or worse,
> >> Scheme has tried to abstract the concept. You don't need to explain the
> >> ideas of IEEE 64-bit floating-point numbers or tie the hands of the
> >> implementation. Instead, what you have is "reliable" arithmetics and
> >> "best-effort" arithmetics, a bit like TCP is "reliable" and UDP is
> >> "best-effort".
> > 
> > The problem with that is that failing to explain IEEE floating point
> > and just calling it "inexact" scares people off unnecessarily. I've
> > seen a lot of very intelligent people who think that you should never
> > compare floats with the == operator, because floats randomly introduce
> > "inaccuracy". 
> 
> Yes, this. "Never compare floats for equality" is a pernicious myth that
> won't die.

In this context, I asked Chris:
> I dont know what point you are trying to make
> Here is behavior.  Should one use == ?? 

As often happens between me and Chris I ask a ‘why’ (in this case ‘what’) question. And he gives a ‘how’ answer

So I again ask: You say «"Never compare floats for equality" is a pernicious
myth»

Given that for Chris’ is_equal we get
is_equal(.1+.1+.1, .3) is True
whereas for python builtin == its False

What (non)myth do you suggest for replacement?

[Note I am not arguing for the goodness of his is_equal]

Analogy: 
Mutable default parameters are a source of problem and confusion.

No A says. One can use them to simulate statics

No B says No problem as long as you make sure there is no mutation to the mutable, either inside or outside the function.

Sure thats all true. However
"Module-globals” is a simpler answer than A
And “DON’T” is simpler than B

So awaiting your preferred myth-ology…