[Python-ideas] Python Numbers as Human Concept Decimal System

[Chris Angelico]

On Fri, Mar 7, 2014 at 9:39 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Fri, Mar 07, 2014 at 11:38:34AM +1100, Chris Angelico wrote:
>
>> Hmm. Is the rounding done by float.__str__() an attractive nuisance?
>
> It's not *rounding* precisely. Starting in Python 3.1, the float
> __repr__ will display the shortest decimal string that doesn't change
> the float's value.
>
> Reading the issue tracker history for this change is informative:
>
> http://bugs.python.org/issue1580
>
> To the guys who worked on that, I take my hat off to you all.

That effect, yes. Let's call it "the magic of float.__str__", because
it really is pretty amazing.

But it's still post-processing magic. It means that strings appear to
round-trip through floats, as long as you're a long way within the
available precision; but as soon as you do operations, that ceases to
be the case. I think it's great for display, but is putting that into
__repr__ (at least, they do appear to be the same) an attractive
nuisance, in that it encourages people to treat float("...") as a true
representation?

Don't get me wrong, it's a really *awesome* feature. It just happens
to have been partially responsible for this thread, which I think is
approaching 300 posts.

ChrisA