[issue23517] datetime.utcfromtimestamp rounds results incorrectly

[Tim Peters]

Tim Peters added the comment:

> >>> x = float.fromhex('0x1.38f312b1b36bdp-1')
> >>> x
> 0.6112295
> >>> round(x, 6)
> 0.611229
> >>> timedelta(0, x).microseconds
> 611230
>
> but I no longer remember whether we concluded that
> timedelta got it wrong or round or both or neither. :-)

Here you go:

>>> import decimal
>>> decimal.Decimal(x)
Decimal('0.61122949999999998116351207499974407255649566650390625')

That's the exact value you're actually using.  What's "correct" depends on what's intended.

round(x, 6) actually rounds to

>>> decimal.Decimal(round(x, 6))
0.6112290000000000222968310481519438326358795166015625

and that's fine.  timedelta's result does not match what using infinite precision would deliver, but I couldn't care much less ;-)

The real lesson to take from all this, when you design your own killer language, is that using a binary floating point type for timestamps comes with many costs and surprises.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23517>
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