dvc.api.params_show: More helpful error handling

[dberenbaum]

Bug Report

Description

Doesn't show useful errors.

Reproduce

1. git clone git@github.com:iterative/pipeline-conifguration.git
2. cd pipeline-conifguration
3. Start Python shell:

>>> import dvc.api
>>> dvc.api.params_show()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dave/Code/dvc/dvc/api/params.py", line 267, in params_show
    return _postprocess(params)
  File "/Users/dave/Code/dvc/dvc/api/params.py", line 256, in _postprocess
    return processed[first(processed)]
KeyError: None

Expected

I'm guessing it fails because there's no dvc.yaml or params.yaml in the project root where the shell is started, but the error message doesn't help me understand this or how to fix it.

[daavoo]

The error, in this case, can be avoided. Do you think it would be more useful to return an empty dict or rather raise an exception like No params found?

[dberenbaum]

I think an error in this case? I doubt there's much reason to call params_show if you expect an empty dict.

What would happen if there was a params.yaml but no dvc.yaml found? What about the inverse?

[daavoo]

I think an error in this case? I doubt there's much reason to call params_show if you expect an empty dict.

I was thinking about returning an empty dict instead of raising an exception because dvc {X} show commands don't fail when there is nothing to return.

What would happen if there was a params.yaml but no dvc.yaml found?

The contents of params.yaml will be returned.

What about the inverse?

The KeyError point will be reached.

[dberenbaum]

I was thinking about returning an empty dict instead of raising an exception because dvc {X} show commands don't fail when there is nothing to return.

Good point. I think we err on the side of not throwing errors in the CLI to avoid breaking CI scripts. If you feel the symmetry is important, I think it's fine to return an empty dict.

IMO, it's more likely to want an error here, especially for the intended use case of reading stage params into user code. I would much rather break early with a helpful error message than have to debug that params_show() returned an empty dict.

[dberenbaum]

A warning might be reasonable also, so at least it's not silently returning the empty dict?