Datetime and timedelta

[jetheurer]

Can pint support determining units from a timedelta? It would be useful if it could for dealing with raw data.

[hgrecco]

There is not such thing right now but it would be interesting.

[aglassdarkly]

Is this the use case?

ureg = UnitRegistry()
t = ureg.Quantity(datetime.timedelta(days=4))
str(t)
'4.0 day'

[hgrecco]

I guess that is more in the direction of interoperating datetime objects with currently supported units such as:

>>> # Currently possible
>>> 4 * ureg.hour
<Quantity(4, 'hour')>
>>> 4 * ureg.hour + 30 * ureg.min
<Quantity(4.5, 'hour')>
>>> # Not possible now
>>> datetime.datetime.now() + 4 * ureg.hour

[jetheurer]

Yes exactly. I don't think its an easy problem to solve as there are a lot of nuances with dates and times, but as long it is was properly defined/documented it would be interesting. python-dateutil has some interesting solutions.

[hgrecco]

I think we can start by providing a conversion utility between quantity and timedelta

[dopplershift]

Not sure if it's a good idea, but you could also put isinstance checks for date time in __add__, __radd__, etc.

[aglassdarkly]

I was exploring a bit and wrote a conversion between timedelta and quantity. I'll try hooking it up in the math ops and see what happens. This is an interesting issue!

[hgrecco]

I think that date rules are very similar to our offset units:

datetime - datetime = timedelta
datetime (+  or -) timedelta = datetime
timedelta (+  or -) timedelta = timedelta

and that Pint Quantities are conceptualy timedeltas. What do you think?

[aglassdarkly]

I agree that Pint quantities are conceptually timedeltas. Would it be as simple as adding something like this method to _Quantity and calling it appropriately from __add__, __sub__, etc. when the other side is a datetime?

    def to_timedelta(self):
        return datetime.timedelta(microseconds=self.to('microseconds').magnitude)

I can make a PR with these changes if this seems right.

[hgrecco]

Sounds good to me.

[hgrecco]

This is implemented in 0.8.1. Let's see how it plays out.