CompositeUnit has no value attribute - #1610

@Cadair - this is true for all units -- they do have a scale but no value. As something similar came up before, why specifically do you ask? Maybe there is a usage that we are not supporting yet, but should? Or where we should point more clearly to the "right" way?

Ah ok it's Quantity that has the value attribute, which makes more sense. So what I was trying to do was this:

>>> u.m.to_system(u.cgs)[0]
Unit("100 cm")

How do I extract the 100?!

That would be .scale, i.e.,

>>> u.m.to_system(u.cgs)[0].scale

I'd actually naively have tried to do u.m.cgs by analogy of what I do with quantities, (1*u.m).cgs, but that does not work. While I can see the reason for not wanting to have a value, the absence of a cgs property seems a bit odd -- cc: @mdboom.

Yeah -- we could add the cgs and si convenience properties. to_system is obviously more powerful and general, but if Quantity has them, Unit should probably have them to. I'll work up a pull request for this.

And just a little history on this: I've never been much of a fan of Quantity.cgs and Quantity.si, since it's possible for there to be multiple correct recompositions, and we don't (currently) specify which are more idiomatic. However, people have been using it and the sky has not fallen...

Fix attached.

Nice! And also seems a bit cleaner to have this in unit rather than in quantity -- which should know as little about the units as possible.

I looked through the changes in some detail and it all looks good to me.