repro: enforce targets (and update desc) when -R is present? [qa]

[jorgeorpinel]

The -R option of dvc repro has no effect when no targets that are directories are also given. For example, dvc repro -R is the same as just dvc repro. For UI and consistency improvement I propose

1. • targets are enforced when -R is used, or at least WARN msg is printed otherwise so the user is aware he might be expecting something that's not being done.
2. • Even if targets are given, at least one of them should be an existing directory path, or the above enforcement/warning will happen.
3. (This may already be the case) if the target directory has no .dvc files or dvc.yaml, print a separate warning about that?

UPDATE: See #4292 (comment) below for an alternative solution — maybe easier.

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THE REMAINING TEXT HAS BEEN EXTRACTED TO #4393 (so stop reading)

Separately (but related), the targets param accepted by dvc repro is described (in its help output) as Stages to reproduce. 'dvc.yaml' by default. This message is confusing as dvc.yaml is not a stage. In docs we're using Stage or .dvc file to reproduce but that's not correct either because apparently paths to dvc.yaml files are also accepted.

• Should we use Stage or path to dvc.yaml or .dvc file to reproduce instead?
• ^ still doesn't cover the case of accepting directory paths with -R, but that we can explain in that argument's description:

-  -R, --recursive       Reproduce all stages in the specified directory.
+  -R, --recursive       Reproduce all stages in the specified directories.
+                        These can be given as `targets` arguments.

and I'll explain this special case further in the cmd ref. Options.

[jorgeorpinel]

1. For UI and consistency improvement I propose...

An alternative to this, as I discussed with @efiop, is to make -R accept an argument, instead of relying on targets. His answer was that it's still basically a target so why not just use the command's argument(s), which was convincing. However, I just noticed that the -c option does accept a <path> argument (specifically for directories), so maybe after all (for consistency) the best option is to add an arg to -R, instead of points 1-3 above (and apply the logic to that arg).

[efiop]

However, I just noticed that the -c option does accept a argument (specifically for directories), so maybe after all (for consistency) the best option is to add an arg to -R, instead of points 1-3 above (and apply the logic to that arg).

-c is a whole different thing, it is in no way related to targets. It just makes dvc chdir to that directory and work from there.

[jorgeorpinel]

True, you could combine the args like dvc repo -c some/path stage1 stage2. OK, never mind about the alternative then.

P/s I wonder what happens if you supply a path to a dvc.yaml file as target along with a -c, is it made relative to the -c path arg?

[jorgeorpinel]

P/s I wonder what happens if you supply a path to a dvc.yaml file as target along with a -c, is it made relative to the -c arg?

Yes, it is. So, strangely, dvc repro = dvc repro -c subdir/ ../dvc.yaml 🤷

UPDATE: Added quick note about this in iterative/dvc.org@d30bc63.