cli: pager functionality

[oliver-sanders]

The old pager functionality wasn't migrated to the new Python API in #3413.

All help pages should be paged e.g. cylc --help, cylc run --help, etc.

This may be slightly tricky now that the CLI has migrated away from Bash, the nice easy help | ${PAGER:-less} solution might not be an option any more.

• subprocess.PIPE might be one option.
• Otherwise a pure-python pager implementation e.g. https://pypi.org/project/pager/ might work.

Using a native shell pager would be preferable so users can set their favorite pager in the conventional way.

Pull requests welcome!

[kinow]

Click mentions something about pager method. I wonder if that can be used with cylc-flow commands now?

http://click.palletsprojects.com/en/5.x/utils/

(it looks like there is some colorama integration too...)

[oliver-sanders]

Good spot!

[hjoliver]

As I recall paging wasn't working (or wasn't implemented) in sub-commands anyway, before the click PR. Was it only working in the top level cylc --help? Pretty sure I tested that quite recently.

[oliver-sanders]

It worked fine but only for cylc help SUBCMD not cylc SUBCMD --help.

[hjoliver]

Ah, that explains it! I guess I never use cylc help SUBCMD, only cylc SUBCMD --help.

[hjoliver]

(I don't think cylc help SUBCMD conforms to standard practice, does it?)

[kinow]

(I don't think cylc help SUBCMD conforms to standard practice, does it?)

I think most programs with subcommands I've seen work the other way as you said above, cylc SUBCMD --help or cylc SUBCMD help.

[oliver-sanders]

e.g. git help remote and git remote --help.

[kinow]

Oh, good example, didn't know git allowed it.

[hjoliver]

Still, we'll want paging for the "more standard" 😁 cylc SUBCMD --help as well.

[oliver-sanders]

Attempt after #3525.