Calling `OmegaConf.to_container` on interpolation-to-missing errors

The behavior of the OmegaConf.to_container method has changed with respect to interpolation-to-missing nodes.

With OmegaConf v2.0.6, no exception is raised on encountering interpolation-to-missing:

>>> cfg = OmegaConf.create({"missing": "???", "x": "${missing}"})
>>> OmegaConf.to_container(cfg, resolve=True)
{'missing': '???', 'x': '???'}

Currently on the master branch, this no longer works: an InterpolationToMissingValueError is raised. This behavior change resulted from #545.

[odelalleau]

I actually disagree with to_container(..., resolve=True, throw_on_missing=False) allowing interpolations to missing values. The main reason is that this would be inconsistent with (at least) OmegaConf.select() where throw_on_missing=False doesn’t allow this, and instead one should use throw_on_resolution_failure=False (resulting in a value of None).

This is because trying to resolve an interpolation to a missing value is now considered an error (instead of being considered as yielding a missing value). This is indeed a breaking change, and it should have probably been mentioned in another news item attached to #545 (@omry should I submit a PR for this?)

I would suggest not to try and support the example given at the top unless there is a good reason for it. And if we do want to support it, I think it shouldn’t be with throw_on_missing=False. This may actually require revisiting some key decisions made in #545 so it should be considered carefully.

[omry]

This is a collateral damage from a breaking change. Not really a bug, more like something we failed to mention as a breaking change (we can add a news fragment for it now).

When introducing throw_on_missing, I agree that having it set to False should suppress the InterpolationToMissingValueError as well.