Move quantity to a separate sub-package - #1595

I have been doing a lot of

from astropy import units as u
x = u.Quantity(...)

Is there a backward-compatibility option so the above won't break? Or will that introduce circular import between new quantity sub-package and units?

For myself, this backward compatibility is not critical. But other developers who build on astropy might also be doing the same, and this move will break their codes.

@eteq - I think you may have misunderstood my comment: I had indeed been considering whether quantity should have its own package, but I thought @astrofrog's suggestion was even better: just update the documentation, and call the documentation page "Quantities and Units". The reason for that is that I find that in practice I almost always define quatities through units, i.e., by

import astropy.units as u
v = 15. * u.km /u.s

and in the few times, I do use it directly, I do u.Quantity, like @pllim. So, if we split the package, the net effect is that I would have to import one package more, rather than one fewer.

So, overall, I'd say keep Quantity in units, update the documentation as suggested by @astrofrog in #1591, and introduce the flux sub-package that you suggested (I do think flux is a basic astronomy-related item that does not really belong in a quantity package).

👍 to all of @mhvk's comment. I think it's mainly a matter of increasing prominence of quantity within units. Quantity isn't really anything separate from units, it's just a (value, unit) container.

Ah, I didn't see #1591 until you mentioned it here @mhvk. So this is an alternative to that.

The advantage of a separate subpackage is that it then provides a possible place for special quantity subclasses (e.g. Distance, Angle, etc.), without mixing it in with the other unit items. That ship may have sailed already given that those already are in other subclasses, though.

Having seen #1591 and thought a bit more, I actually like that better as a solution... But I'll leave this for a little while in case others disagree. This can be closed once #1591 is implemented/merged.

@eteq: I think you meant #1591 in your above comment, but didn't want to edit it in case I'm just confused...

whoops, yeah, sorry - just edited it now. They're only one digit off, so it's easy to get confused :)

I also think we should not separate them - for one, most users don't even use Quantity directly!

I actually use Quantity directly all the time but I am already used to it being inside astropy.units. 😱

At 2013-10-18 hangout, it was agreed we would follow the approach in #1591 instead of this. Closing.