Automatically initialize regridders with loaded weights in the Regridder class

[mnurisso]

PR description:

I tested a Regridder alone workflow such as:

regridder = Regridder(cfg_regrid, data=data, loglevel=args.loglevel, src_grid_name=src_grid_name)
regridder.weights(tgt_grid_name=resolution, regrid_method="con", reader_kwargs=reader_kwargs)
data_r = regridder.regrid(data)

In the current implementation it gives error:

2026-02-18 16:23:22 :: Regridder :: ERROR    -> Regridder for vertical coordinate 2d not found.
2026-02-18 16:23:22 :: Regridder :: ERROR    -> Cannot regrid variable tprate

This because the self.initialize method is not used. I can add it manually in my code but this is exactly the example from the documentation so I think a fix should be inserted in the code and this is what I am proposing

People involved:

@oloapinivad

• Changelog is updated.

[codecov]

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[mnurisso]

Hi @jhardenberg could you check if you find any problem in this additional step?
I had problem running a script in which the Regridder class alone had to generate weights and immediately use them because they were not initialized. I think in the Reader the initialization is handled somewhere else so I'm not sure if I'm adding an overhead.

[jhardenberg]

The reason why the initialization of the regridders was delayed in the initialize() method is to allow the application of the datamodel and of the fixers on the weights, see lines 322-326 of the reader. So in the reader the weights generation is called, the weights are fixed and only then the initialization of the regridders occurs.
As you notice, this breaks the usage of the Regridder class alone.

The example should be fixed anyway, because regridder.weights() norw actually returns the weights.

We could either just change the docs saying that one needs to call initialize after computing the weights (but I admit that for a user this looks complicated) or introduce a 'double' behaviour of the weights() method by adding a initialize key as an argument which will be True by default and which will be used with False by the reader instead.
This way we could preserve the behaviour described in the docs and at the meantime leave the reader the opportunity to apply the fixer/datamodel .....

[mnurisso]

The reason why the initialization of theregridders was delayed in the initialize() method is to allow the application of the datamodel and of the fixers on the weights, see lines 322-326 of the reader. So in the reader the weights generation is called, the weights are fixed and only then the initialization of the regridders occurs. As you notice, this breaks the usage of the Regridder class alone.

The example should be fixed anyway, because regridder.weights() norw actually returns the weights.

We could either just change the docs saying that one needs to call initialize after computing the weights (but I admit that for a user this looks complicated) or introduce a 'double' behaviour of the weights() method by adding a initialize key as an argument which will be True by default and which will be used with False by the reader instead. This way we could preserve the behaviour described in the docs and at the meantime leave the reader the opportunity to apply the fixer/datamodel .....

Maybe this can be discussed on monday if there is extra time, I have no problem in calling explicitly the initialize method but indeed I'm not a beginner user and the documentation was wrong about this specific case.

[mnurisso]

Add initialize=True by default for Regridder call alone and initialize=False to be added in the Reader to shut down the automatic call of the initialize method