plot_ppc modifies coords argument

[erocarrera]

If this is the expected behavior it'd probably be good to mention it in the docstring. However I am inclined to think that the dictionary passed as coords should be left unmodified, it seems to be the behavior of a handful of plot_* functions that I've checked.

arviz/arviz/plots/ppcplot.py

Line 298 in f4a8d66

coords[key] = np.where(np.in1d(observed_data[key], coords[key]))[0]

[OriolAbril]

They should not be modified, we should copy them before modifying, like we do with kwargs before we add default values

[tspeng]

Hi @erocarrera and @OriolAbril, I am a new contributor. Based on your suggestions, I think it could be fixed by modifying the if statement in lines 270-271, so the original mutable object coords will not be modified. Since line 298 is an assignment operation, shallow copy should be enough.

if coords is None:
    coords = {}
else:
    coords = coords.copy()

[OriolAbril]

Yes, that sounds good, thanks!