DM-34778: Expose interpolant in Piff config

[arunkannawadi]

No description provided.

[jmeyers314]

Intepolant -> Interpolant

[jmeyers314]

Better to let GalSim itself determine valid names here I think (to handle to off-chance that we add a new interpolant there, for example). You can use galsim.Interpolant.from_name() and look for GalSimValueError.

[arunkannawadi]

There's some difference in the way PiFF accepts interpolant names and GalSim does. For e.g., GalSim accepts sinc but PiFF needs it to be galsim.SincInterpolant. And galsim.Interpolant.from_name('sinc').__class__ returns galsim.Interpolant.SincInterpolant, which is not accepted by PiFF. I don't foresee using anything other than Lanczos, and since the doc says we support only the ones listed there, if there are additional interpolation schemes that get added in GalSim, we can add that later. We should consider it as a technical debt at this point the very least.

[jmeyers314]

Okay.

[arunkannawadi]

Or we could mock up what PiFF does currently. We can do

import galsim
from galsim import Lanczos
...
def _validateGalSimInterpolant(name):
    try:
        eval(name)
        return True
    except AttributeError:
        return False

[jmeyers314]

Oh, interesting. Looks like Piff discourages use of anything non-Lanczos? Which is probably reasonable. I like this best.

[arunkannawadi]

On second thoughts, any executable code would pass this, even if it is not a valid interpolant name. And it'd fail later during runtime. Also, from a security standpoint, we don't want to execute an arbitrary code here.

[jmeyers314]

I guess you could add an isinstance(blah, galsim.Interpolant) check.

As for security, I don't know. GalSim and piff both contain eval statements and we use them without worry. We store piff results in the stack as pickles and I'm pretty sure you can hide arbitrary instructions in those. I kinda think the ship has sailed with regard to python eval.

[arunkannawadi]

I don't know if PiFF discourages anything non-Lanczos, but it is expecting the user to be sensible. As GalSim users, I know you and I both tried to set it to Lanczos11 which is invalid. So I think it is fair to say that other users might likely set it incorrect at first, and I'd want to capture this during config validation stage. I think I'm going to fall back to the implementation I had earlier, for reasons I mentioned in the earlier comment. Sorry Josh :-|

[arunkannawadi]

https://github.com/rmjarvis/Piff/blob/releases/1.2/piff/pixelgrid.py#L71-L73

[jmeyers314]

Yep. That's fine.