metrics diff with missing old value

[dmpetrov]

If metrics file(s) has only no old version (it was just created):

$ dvc metrics diff HEAD^
WARNING: Metrics file 'metrics/eval.json' does not exist at the reference 'HEAD^'.
WARNING: Metrics file 'metrics/train.json' does not exist at the reference 'HEAD^'.
ERROR: unexpected error - 'HEAD^'

Expected the same output with no difference:

$ dvc metrics diff HEAD^
       Path             Metric       Value         Change
metrics/train.json   took            0.004   -
metrics/train.json   num_steps       2500    -
metrics/train.json   learning_rate   0.015   -
metrics/eval.json    accuracy        0.897   -

In JSON - the same output with no old values.

Also, the error message is misleading: ERROR: unexpected error - 'HEAD^'. It should be something like ERROR: the old version of the metric file is not found

Another problem is writing status to STDOUT (it should be in STDIN).

$ dvc metrics diff HEAD
No changes.

Summary:

• Handle metrics diff based on one version only
• Make sure JSON supports missing old metrics value
• Better output error message
• Make sure only the table is presented in STDOUT. All other messages (like "No changes.") should be in STDERR.

[efiop]

Make sure only the table is presented in STDOUT. All other messages (like "No changes.") should be in STDERR.

Not sure why it should be like that. Table as well as No changes. are for the user, not really meant to be parsable. For parsing we have --show-json. I don't think we do that anywhere else in the dvc.

[dmpetrov]

I'd prefer to have an empty output in this case. No need in a special message. It is a usual approach for many commands including git diff.

[efiop]

@dmpetrov But we've been talking about dvc commands being too silent, hence why we've introduced such summary messages everywhere. No changes is consistent here with the rest of the commands.

[shcheklein]

Don't have a strong opinion on this (git diff does not show anything, right? but on the other hand it outputs patch by default and DVC outputs some human-readable table - so not a direct comparison for sure).

It's better to be consistent with other DVC "diff" commands though.