Retrieve units sorted by [dimension]

[garrosh]

Hello you lovely people!

So I wanted to make a simple GUI to convert values with each other. The user selects some type of unit, say 'length', and the interface would then display two comboboxes with the various choices to do the conversion. I had a version working that I made using purpose-built enums.

class UnitCategory(Enum):
    LENGTH = 'length'
    FORCE = 'force'
    PRESSURE = 'pressure'
    DENSITY = 'density'
    MOMENT = 'moment'

class LengthUnits(Enum):
    METER = 'm'
    CENTIMETER = 'cm'
    MILLIMETER = 'mm'
    KILOMETER = 'km'
    INCH = 'in'
    FOOT = 'ft'

class DefaultUnits(Enum):
    LENGTH = 'in'
    FORCE = 'lbf'
    PRESSURE = 'psi'
    DENSITY = 'lbf/in³'
    MOMENT = 'lbf*in'

def get_units_for_category(category):
    units = None
    if category == UnitCategory.LENGTH.value:
        units = LengthUnits
    elif ...
    return units

def get_default_units_index_for_category(category, default = None):
    unit_list = [ureg(unit).units for unit in category]
    print(unit_list)
    if default is None:
        return unit_list.index(DefaultUnits[category].value)
    else:
        default_unit = ureg(f"{default}").units
        return unit_list.index(default_unit)

But then I thought I could just wrap Pint itself, no? I've read through the documentation, looked at members, all these issues, and I think I have to develop something new, but that's surprising to me.

The best option I have so far would be to iterate over UnitRegistry._units, collecting references and then do .is_compatible_with() to get the list of compatible units. Did I miss something?

[garrosh]

Trying to use get_root_units('[length]') fails with pint.errors.UndefinedUnitError: '[length]' is not defined in the unit registry, which I did not expect. I also tried get_root_units('length') which fails in a similar way. I also can't find a function called get_preferred_units().

So I really have to iterate over every single unit in the registry?

[andrewgsavage]

Yes, this is new. It's been known as kind or quantitykind in previous issues.

It requires more information than is currently stored in the definitions file as the dimensions don't have units associated with them (ie Nm is a unit for torque, not energy)

I had a try here:
#1967
but it did not include a list of units for each kind as you're asking for

[garrosh]

All right, I'm going to take a bit of time next week to finish off my little converter app. Maybe I'll open an issue to talk about what we see here:

Specifically how sometimes there are unit exponents that are formated as floats and others that are not.

If there is an interest, I could also publish it, but for now, it's more of a personal use thing.