On Sat, Mar 8, 2014 at 11:54 AM, Guido van Rossum <guido@python.org> wrote:
>
> I'll try to respond to Mark Dickinson's second message (and nothing else that happens in the thread since last night), because (a) it concisely summarizes his position and (b) brings up a new strawman.
>
> On Sat, Mar 8, 2014 at 2:01 AM, Mark Dickinson <dickinsm@gmail.com> wrote:
>>
>> On Sat, Mar 8, 2014 at 9:43 AM, Mark Dickinson <dickinsm@gmail.com> wrote:
>>>
>>>
>>> I see three sane options for float to Decimal conversion:
>>>
>>> 1. Raise an exception.
>>> 2. Round to the nearest Decimal value using the current context for that round operation.
>>> 3. Do what we're currently doing, and do an exact conversion.
>
>
> I think you're writing this entirely from the POV of an expert in floating point. And I'm glad we have experts like you around! I don't consider myself an expert at all, but I do think I have something to bring to the table -- insight the experience of non-expert users.
>

This reminded me a discussion we had with Mark Dickinson on the bug tracker: "When a user is entering 0.6112295, she means 0.6112295, not 0x1.38f312b1b36bdp-1 or 0.61122949999999998116351207499974407255649566650390625 which are exact values of the underlying binary representation of 0.6112295."

http://bugs.python.org/issue8860#msg108601

The topic was the conversion from binary floats to timedelta which is effectively a fixed point decimal with 6 decimal places after the point.