[Python-Dev] Why don't range and xrange threat floats as floats?

[Guido van Rossum]

On Wed, Nov 5, 2008 at 10:26 AM, L V <somelauw at yahoo.com> wrote:
> Why don't range and xrange threat floats as floats?
> Is there any good reason range and xrange don't threat floats as floats but
> as integers?
> When I enter float arguments in a range, the floats are treated as integers.
> (+ some warning)
>
> This is how I think it should work:
>>>>range(0, 1, 0.1)
> [0.0, 0.10000000000000001, 0.20000000000000001, 0.29999999999999999,
> 0.40000000000000002, 0.5, 0.59999999999999998, 0.69999999999999996,
> 0.80000000000000004, 0.90000000000000002]
>
> I got these results by:
>>>>[x/10. for x in xrange(10)]
>
> I'm not looking for alternatives, just for the reason that it works this
> way.

The reason is that due to the rounding issues inherent to a binary
representation of floating point, it is easy to give examples where
it's ambiguous whether the end point was meant to be included or not.
This could happen e.g. when the binary representation of the end point
was rounded up a bit, while the step would be rounded down.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)