On Tuesday 29 July 2008 15:14:13 Ivan Vilata i Balaguer wrote:

Pierre GM (el 2008-07-29 a les 12:38:19 -0400) va dir::

Relative time versus relative time ----------------------------------

This case would be the same than the previous one (absolute vs absolute). Our proposal is to forbid this operation if the time units of the operands are different.

Mmh, less sure on this one. Can't we use a hierarchy of time units, and force to the lowest ?

For example:

numpy.ones(3, dtype="t8[Y]") + 3*numpy.ones(3, dtype="t8[M]") array([15,15,15], dtype="t8['M']")

I agree that adding ns to years makes no sense, but ns to s ? min to hr or days ? In short: systematically raising an exception looks a bit too drastic. There are some simple unambiguous cases that sould be allowed (Y+M, Y+Q, M+Q, H+D...)

Do you mean using the most precise unit for operations with "near enough", different units? I see the point, but what makes me doubt about it is giving the user the false impression that the most precise unit is *always* expected. I'd rather spare the user as many surprises as possible, by simplifying rules in favour of explicitness (but that may be debated).

Let me rephrase: Adding different relative time units should be allowed when there's no ambiguity on the output: For example, a relative year timedelta is always 12 month timedeltas, or 4 quarter timedeltas. In that case, I should be able to do:

numpy.ones(3, dtype="t8[Y]") + 3*numpy.ones(3, dtype="t8[M]") array([15,15,15], dtype="t8['M']") numpy.ones(3, dtype="t8[Y]") + 3*numpy.ones(3, dtype="t8[Q]") array([7,7,7], dtype="t8['Q']")

Similarly: * an hour is always 3600s, so I could add relative s/ms/us/ns timedeltas to hour timedeltas, and get the result in s/ms/us/ns. * A day is always 24h, so I could add relative hours and days timedeltas and get an hour timedelta * A week is always 7d, so W+D -> D However: * We can't tell beforehand how much days are in any month, so adding relative days and months would raise an exception. * Same thing with weeks and months/quarters/years There'll be only a limited number of time units, therefore a limited number of potential combinations between time units. It'd be just a matter of listing which ones are allowed and which ones will raise an exception.

Note: we refused to use the ``.astype()`` method because of the additional 'time_reference' parameter that will sound strange for other typical uses of ``.astype()``.

A method would be really, really helpful, though... [...]

Yay, but what doesn't seem to fit for me is that the method would only have sense to time values.

Well, what about a .tounit(new_unit, reference=None) ? By default, the reference would be None and default to the POSIX epoch. We could also go for .totunit (for to time unit)

NumPy is pretty orthogonal in that every method and attribute applies to every type. However, if "units" were to be adopted by NumPy, the method would fit in well. In fact, we are thinking of adding a ``unit`` attribute to dtypes to support time units (being ``None`` for normal NumPy types). But full unit support in NumPy looks so far away that I'm not sure to adopt the method.

Thanks for the insights. Cheers,