[Python-Dev] Change the repr for datetime.timedelta (was Re: Asynchronous context manager in a typical network server)

[Alexander Belopolsky]

On Sun, Dec 20, 2015 at 9:00 PM, Guido van Rossum <guido at python.org> wrote:

> but I would really like to see a change in the repr of negative timedeltas:
>>
>> >>> timedelta(minutes=-1)
>> datetime.timedelta(-1, 86340)
>>
>> And str() is not much better:
>>
>> >>> print(timedelta(minutes=-1))
>> -1 day, 23:59:00
>>
>> The above does not qualify as a human readable representation IMO.
>>
>
> I'm sure that one often catches people by surprise. However, I don't think
> we can fix that one without also fixing the values of the attributes
>

I don't see why we have to change td.days for say td =
 timedelta(minutes=-1) if we change its repr to "timedelta(minutes=-1)".
For me an important invariant is td == eval(repr(td)) which will be
preserved.


> -- in that example days is -1 and seconds is 86340 (which will *also*
> catch people by surprise). And changing that would be much, much harder for
> backwards compatibility reasons-- we'd have to set days to 0 and seconds to
> -60, and suddenly we have a much murkier invariant, instead of the crisp
>
>     0 <= microseconds < 1000000
>     0 <= seconds < 60
>
> (There is no such invariant for days -- they hold the sign bit.)
>
> In essence, you'd have to look at all three attributes to figure out on
> which side of 0 is was (or think of the right way to do it, which is to
> compare to timedelta(0)). I might still go for it, if it wasn't too late by
> over a decade (as Tim says).
>
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