ui: use global singleton UI object for printing

[skshetry]

I understand this is a bit controversial, but it seems that using this ui object globally is the best thing to do.
DVC tries to provide a sane API as well as CLI. Extending api to accomodate ui object (which is for cli) does not seem like a good thing, as it extends our API surface and makes it ugly. We'd also need to pass them to the depth of our APIs.

Similarly, we cannot just use ui object in CLIs, as our cli commands are usually just wrapper to the Repo apis. So, even if we try hard to separate them and isolate it inside the CLI, it'd be quite difficult.

Looking at it, and the purpose that ui object serves, it seems to me that it is a global singleton object anyway (an instance in a single session of CLI). And, we use loggers in a similar way, so I find it fitting as per our needs, without adding any extra efforts.

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• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[skshetry]

maybe it should be console now?

[skshetry]

Q: How important is thread-safety for us while printing? logger is thread safe, but this is not.

[pared]

So, ultimately, we are intending to use ui.write everywhere we use logger now? If so, I would say thread safety is pretty important as we already have some parts of code that utilize ThreadPoolExecutor and log something down the line (for example, _upload_plans.

[skshetry]

thanks, @pared. Will come to it as we start using it in multithreaded applications.

[efiop]

@skshetry Could you clarify how we would change verbosity or in general tweak ui in cases like erepo?

[skshetry]

Could you clarify how we would change verbosity

Not implemented, but verbosity could be changed in a similar way. Probably, changing the API of enable to configure.

ui.configure(level="success", enable=True)

But, I think we should not do that, as I am thinking ui.write to be always visible (except on --quiet). Verbosity will display logging information as well.

or in general tweak ui in cases like erepo?

I had implemented this before, but removed it to keep API simple. I am thinking of using contextvars here (requires compat package in 3.6, in stdlib since 3.7). It'll look something like following:

with ui.enabled():
  ui.write("visible")
  with ui.disabled():
    ui.write("hidden")
    with ui.enabled():
      ui.write(" visible again")
    ui.write(" hidden again")

This should be thread and async safe.

[efiop]

@skshetry Sorry for the delay. Looks pretty good. 👍

[skshetry]

Okay, merging it for now.