Unable to use custom formatting for dimensionless units

[aulemahal]

Hi!

Some time ago, cf-xarray implemented a CF-compliant unit formatter using pint.register_unit_format. It works well except for dimensionless quantities. The current code for format_unit hardcodes what happens with dimensionless units. It's either "" if there was a ~ flag or:

pint/pint/formatting.py

Lines 397 to 401 in 7394cd9

	if not unit:
	if spec.endswith("%"):
	return ""
	else:
	return "dimensionless"

Which makes the custom formatter unable to modify the behaviour. In our case here, the CF version would be "" (and sometimes "1" when not attached to a magnitude).

Could we simply remove the code I linked? Pass the container to the formatting function, as with other units?

Tagging @dcherian that might be interested in this issue.

[huard]

I'm interested in seeing this move forward. Looking at the code, it's not obvious to me where the logic for the if not unit case should live though.

[keewis]

As far as I understand it, % is some kind of unit modifier? If that's correct, I think we should try to treat it the same way as the other modifiers (and document it in the "string formatting" guide). That way, #1448 would allow you to customize its behavior.