What does `registry=None` mean in custom formatter ?

[aulemahal]

Hi!

With pint 0.24, the internal workings of register_unit_format have changed and the registry argument of the wrapped function can now be "None". I do not see in the doc the meaning of that new possible value. Wouldn't a Unit object always be related to a registry, even if it's the default one ?

This is causing issues in cf-xarray (and downstream) for example (xarray-contrib/cf-xarray#507) where we used registry to convert the UnitContainer back to a Unit (see https://github.com/xarray-contrib/cf-xarray/blob/24dd81b2748db59c3ede68735a200b63b8a3d2d2/cf_xarray/units.py#L17-L38)

EDIT: Also, the way register_unit_format is written, wouldn't registry always be None ?

[hgrecco]

First, sorry for breaking things. The move to the new Formatter delegate allows more flexibility so I think this annoyance will pay off. Let me answer each of your questions:

Wouldn't a Unit object always be related to a registry, even if it's the default one ?
The formatter also format unit like objects (i.e. dicts mapping names to exponents) which do not necesesary require a registry.

Also, the way register_unit_format is written, wouldn't registry always be None
Yes. I was planning to try a solution but there was no need until now.

The idea would be to make this function look and instantiate the registered formatters.

    def get_formatter(self, spec: str):
        if spec == "":
            return self._formatters["D"]
        for k, v in self._formatters.items():
            if k in spec:
                return v

        try:
            orphan_fmt = REGISTERED_FORMATTERS[spec]
        except KeyError:
            return self._formatters["D"]

        try:
            fmt = orphan_fmt.__class__(self._registry)
            spec = get_attr(fmt, "spec", spec)
            self._formatters[spec] = fmt
            return fmt
        except:
            return orphan_fmt

[hgrecco]

and then also add the spec attribute to NewFormatter here

[hgrecco]

On a second thought
spec = get_attr(fmt, "spec", spec) and adding the spec attribut might not be necessary.

[aulemahal]

I think I understand, thanks! I think I can try to suggest these changes in a PR if that helps.

[hgrecco]

That would be great. I have review the code, it is necessary to add spec = get_attr(fmt, "spec", spec) and adding the spec attribute to NewFormatter. Let me know if you need help.