Hint for efficiency

[thesamovar]

I had a student today who had some code running very slowly. It turned out he was accessing M.v[i, t] for i and t in a loop, and M a StateMonitor. Turning this into a single v=M.v and then v[i,t] made it run almost instantly. I'm wondering if we can catch this sort of thing and warn about it because it might be tripping up a few people. Perhaps something like if you access M.v more than 100 times as often as you run M it gives you the warning? Not sure of the best way here, or if there's some general way to catch these sorts of issues or not.

[thesamovar]

Maybe it would just be better to cache that getattr so that if nothing has changed it doesn't need to do any work?

[mstimberg]

Hmm, not sure about a warning, I'd prefer if it would be fast in the first place :) Of course there's almost never a reason to loop over a 2D array instead of treating it like a matrix, but many users will still do it, and there is no reason to make this slow if we don't have to.
I just had a quick look into StateMonitor.__getattr__ and there's a weird thing: if you access M.v (i.e. with units), it will make a copy of the data, whereas it gives you a direct reference without a copy if you access M.v_ (i.e. without units). So the latter should already be reasonably fast even in looped access.

I could come up with reasons for a copy – e.g. related to problems when the underlying data gets resized in a second run – but I don't quite see why it should only apply to values with units... I'll try to dig into history at some point to see whether this was intentional.