[Datetime-SIG] PEP-0500 (Alternative datetime arithmetic) Was: PEP 495 ... is ready ...

Ethan Furman ethan at stoneleaf.us
Wed Aug 19 10:17:50 CEST 2015

On 08/18/2015 10:52 PM, Tim Peters wrote:
> [Guido]
>> ...
>> This  discussion sounds overly abstract. ISTM that d(x, y) in timeline
>> arithmetic can be computed as x.timestamp() - y.timestamp(), (and converting
>> to a timedelta).
> As someone else might say, if you want timestamps, use timestamps ;-)
> I want to discourage people from thinking of it that way, because it
> only works in a theoretical framework abstracting away how arithmetic
> actually behaves.  Timestamps in Python suck in a world of
> floating-point pain that I tried hard to keep entirely out of datetime
> module semantics (although I see float operations have increasingly
> wormed their way in).
> Everyone who thinks about it soon realizes that a datetime simply has
> "too many bits" to represent faithfully as a Python float, and so also
> as a Python timestamp.  But I think few realize this isn't a problem
> confined to datetimes only our descendants will experience.  It can
> surprise people even today.  For example, here on my second try:
>>>> d = datetime.now()
>>>> d
> datetime.datetime(2015, 8, 18, 23, 8, 54, 615774)
>>>> datetime.fromtimestamp(d.timestamp())
> datetime.datetime(2015, 8, 18, 23, 8, 54, 615773)

This bug was introduced by Victor Stinner in issue 14180, and is being tracked in issue 23517.  Versions 3.3, 3.4, and soon 3.5 are affected.  Victor is refusing to fix/revert, Alexander has given up 
arguing with him, and I lack the necessary skills.

Perhaps the best path forward is to deprecate `.timestamp()` and friends.


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