AttributeError when using method "to" if a custom preprocessor is in place and a string is passed as target unit

[sanzoghenzo]

I'm experiencing a problem with the new custom preprocessors feature;
trying to convert a quantity to "%" using a string instead of the quantity raises an AttributeError in the ParserHelper.from_string method, since it's not aware of the preprocessors.

UREG = UnitRegistry(
    preprocessors=[
        functools.partial(re.sub, r"(.*)%$", r"\1 percent"),
        functools.partial(re.sub, r"(.*)\u2030$", r"\1 permille"),
    ]
)
UREG.define("percent = 0.01 count = %")
UREG.define("permille = 0.001 count = \u2030")
assert UREG("count").to(UREG("%")).magnitude == 100  # this works ok
assert UREG("count").to(UREG("\u2030")).magnitude == 1000 # this also works ok
assert UREG("count").to(UREG("‰")).magnitude == 1000 # this also works ok
assert UREG("count").to("%").magnitude == 100 

The last line throws this exception:

.\venv\lib\site-packages\pint\quantity.py:561: in to
    other = to_units_container(other, self._REGISTRY)
.\venv\lib\site-packages\pint\util.py:832: in to_units_container
    return registry._parse_units(unit_like)
.\venv\lib\site-packages\pint\registry.py:1167: in _parse_units
    return super()._parse_units(input_string, as_delta)
.\venv\lib\site-packages\pint\registry.py:1045: in _parse_units
    units = ParserHelper.from_string(input_string)
.\venv\lib\site-packages\pint\util.py:574: in from_string
   ret = build_eval_tree(gen).evaluate(cls.eval_token)
AttributeError: 'NoneType' object has no attribute 'evaluate'

[jthielen]

This is (unfortunately) the expected behavior of the preprocessors feature. Given that the preprocessors exist on the registry level, they only function when using the registry-level parsers (parse_expression/registry-as-a-callable and parse_units), and not with lower-level functions using ParserHelper.from_string as you note. (See #429 (comment) and #429 (comment) for the rationale.)

However, this can definitely be better documented! (xref #972)

[sanzoghenzo]

Thanks @jthielen for the explanation, I supposed that it was related to the design of the feature (I tried to read the entire discussion but I got lost somewhere ;) ).

Documenting this is fine, but IMHO this is something that should be better handled, i.e. by issuing a warning if a string is passed and the preprocessor attribute is not empty.

[hgrecco]

But you can do the following:

>>> UREG("count").to(UREG("%"))
<Quantity(100.0, 'percent')>

[hgrecco]

Any PR to document this?