Current logic in the frontend prevents using the default filename

[markelg]

Current logic in the frontend does not let me to use the default filename

        # some logic about reusing weights with either filename or weights args
        if reuse_weights and (filename is None) and (weights is None):
            raise ValueError('To reuse weights, you need to provide either filename or weights.')

I think it should try to load the weights from default filename instead of raising an error. Am I missing something?

The pattern used to build the default filename is good and useful. I can find some time to write a PR if you want.

[raphaeldussin]

agreed, I think something in the logic has been broken. @huard and @aulemahal any thoughts on this?
One workaround is for you to specify explicitly the name of the weights file you want to write.

PRs are always welcome!

[aulemahal]

I agree that this would make sense! I don't think it ever worked, though, seems to me that the current behaviour was introduced when reuse_weights was introduced, but I might be mistaken.

It think it would be a matter of setting self.filename prior to this check and then add a check to see if the file exists.

[markelg]

I am testing the solution proposed. Not unexpectedly, it breaks this test:

    # check fails if no weights are provided
    with pytest.raises(ValueError):
        regridder_reuse = xe.Regridder(ds_in, ds_out, method, reuse_weights=True)

Because that combination of arguments now compute and save the weights to the default file.

I am not sure about how to proceeded in order to not to break this behavior. Maybe add a special case with filename="default"?

[huard]

The filename logic dates back to the time ESMF wrote the weights to file, and xESMF had to read them from disk. About two years ago, ESMF introduced a mechanism to share the weights in memory. The current code is the result of trying to support both the older file-based and memory-based approaches without breaking legacy code.

I'm not sure I would want xESMF to automatically read from a default file name that is not directly visible from the docstring. It feels error-prone.