Issue with Units around temperature

[slebras]

I've had issues with unit expressions around temperature. The error is thrown on this line

sample_service/lib/SampleService/core/validator/builtin.py

Line 222 in 695bb80

(1 * units).ito(req_units)

the error is:
pint.errors.OffsetUnitCalculusError: Ambiguous operation with offset unit (degree_Celsius, ). See https://pint.readthedocs.io/en/latest/nonmult.html for guidance.

The multiplication on the line shared above between units and 1 does not work with temperatures in pint. "These temperature units are expressed in a system with a reference point, and relations between temperature units include not only a scaling factor but also an offset." in their words.

[MrCreosote]

To reproduce:

In [1]: import pint                                                             

In [2]: ur = pint.UnitRegistry()                                                

In [3]: unit = ur.parse_expression('degF')                                      

In [4]: 1 *  unit                                                               
---------------------------------------------------------------------------
OffsetUnitCalculusError                   Traceback (most recent call last)
<ipython-input-4-69f3d3a309b4> in <module>
----> 1 1 *  unit
*snip*

OffsetUnitCalculusError: Ambiguous operation with offset unit (degree_Fahrenheit, ). See https://pint.readthedocs.io/en/latest/nonmult.html for guidance.

[MrCreosote]

In [7]: unit = ur.parse_expression('degK')                                      

In [8]: 1 * unit                                                                
Out[8]: 1 <Unit('kelvin')>

[MrCreosote]

How is this unit test passing? https://github.com/kbase/sample_service/blob/master/test/core/validator/builtin_test.py#L178

[MrCreosote]

Looks like the incoming units have to be absolute. Does it work with K or R?

[MrCreosote]

https://pint.readthedocs.io/en/stable/nonmult.html

[MrCreosote]

As an alternative to raising an error, pint can be configured to work more relaxed via setting the UnitRegistry parameter autoconvert_offset_to_baseunit to true. In this mode, pint behaves differently:

    Multiplication of a quantity with a single offset unit with order +1 by a number or ndarray yields the quantity in the given unit.

>>> ureg = UnitRegistry(autoconvert_offset_to_baseunit = True)
>>> T = 25.4 * ureg.degC
>>> T
<Quantity(25.4, 'degree_Celsius')>

[MrCreosote]

So a couple potential fixes, I think:

1. Only allow absolute temperature units in input, and throw a better error if they're not.
2. The option above, which probably needs to be understood better before implementation.

[slebras]

I'm leaning towards option 2 above, If we only ever use units * 1, it should not effect the incoming data. I also don't think we should alter any of the data, outside of unit conversions, within the SampleService, so that shouldn't be an issue going forward

[slebras]

PR addressing this: #388

[eapearson]

Another solution, with no other side effects, is to use the quantity api.

pint.quantity.Quantity(1, unit).to(req_unit)

which is utilized in #410