[Python-ideas] float comparison in doctes

[Erik Bray]

On Mon, Sep 15, 2014 at 7:55 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>
> On 16 Sep 2014 06:32, "Steven D'Aprano" <steve at pearwood.info> wrote:
>>
>> On Mon, Sep 15, 2014 at 12:02:20PM -0700, Kevin Davies wrote:
>>
>> > It seems that this didn't reach the list directly (see
>> > https://mail.python.org/pipermail/python-ideas/2014-August/028956.html),
>> > so I'm resending:
>>
>> >
>> > Erik Bray (the author of the +FLOAT_CMP extension in Astropy), Bruce
>> > Leban, and I had a short off-thread email discussion. Here are the
>> > points:
>> >
>> > - [Bruce]: ALMOST_EQUAL is the best flag name.
>> > - [Erik]: If there's agreement on this, Erik will develop a patch as
>> > soon as he can.
>> > - [Erik]: There's no way to adjust the tolerance because there seems
>> > to be no easy way to parameterize doctest flags. Ideas are welcome.
>>
>> With no way to choose between (at minimum) *four* different "almost
>> equal" models, and no way to specify a tolerance, I don't think doctest
>> ought to have such a directive.
>>
>> Almost equal can mean:
>>
>> - round and compare with == (as unittest does)
>> - absolute difference
>> - relative difference
>> - ULP difference
>
> I think it's OK for a doctest flag to just provide the default behaviour
> offered by unittest.TestCase.assertAlmostEqual. That aligns well with the
> originally intended use case of testing examples in documentation.

I think that sounds like a perfect compromise.  As long as it's
documented that "this is equivalent to assertAlmostEqual with the
default arguments" it should be clear, and that would handle all the
basic use cases this feature was intended for.  Naming the flag
ALMOST_EQUAL would also help make that clear.

> If folks want more precise control, they can then switch to full unit tests.
> It would be reasonable for the docs for the new flag to point that out
> explicitly.

Exactly--it's mostly a matter of making doctests more readable.  Note,
ellipses won't work at all in some cases either.  I've had cases where
we expected something like 1.0 and got 0.99999999..., for example.

Erik