Quick question on iris.load callback and constraints

[larsbarring]

If I am loading a netcdf file using both a callback function and a constraint, which is done first?
Context: I want to extract a small region from a rather poorly metadata formatted large file where the x,y coordinates are off by a small fraction that I need to adjust. So can I use the callback to tweak the coordinated to what they are supposed to be and then let the constraints do its work to cut out the subdomain?

[trexfeathers]

Just spent 5mins looking through the Iris source and I'm pretty sure it's constraints first, then callbacks.

[larsbarring]

Big thanks for the superquick answer !

[pp-mo]

Actually, I just spent 20 minutes chasing this, and I think constraints come afterwards ...

The callback is invoked within the low-level loader (e.g. here, or here), but the constraints are mostly used by the "_CubeFilterCollection.from_cubes" call in iris._load_collection,
This underlies the various iris.load_xxx functions, but it works on "raw cubes" delivered by the loader.

I say "mostly", because the constraints are passed down to the low-level loaders.
But I think that is only so that "special cases" can be interpreted, to filter file data and speed up loading, notably, here, but see also recent #4176.
But those are, by definition, "exceptional" cases.

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Whatever, this is an amazingly pertinent question + we really should make this all this clearer somewhere !

[rcomer]

At #3944 @bjlittle said it’s constraints first 🤔

[larsbarring]

Well, I realise why my feeble attempts at reading the code were in vain. I did look at the documentation, but it never appeared to me to check the git issues...
BTW: The new readthedocs documentation is really a quantum leap forward!

[SimonPeatman]

At #3944 @bjlittle said it’s constraints first

However, the code I posted there implies that's not true ...

Either way, I agree with @pp-mo that this really needs to be clarified.