pint does not parse well unit with exponents

[rcaneill]

Some units consists of multiple set of parenthesis, with exponents -1 (e.g. next line). However they are not parsed correctly by pint.

gsw-xarray/gsw_xarray/_attributes.py

Line 651 in 08a1341

"units": "(kg/m^3)(g/kg)^-1",

import cf_xarray.units  # must be imported before pint_xarray
import pint_xarray
from pint_xarray import unit_registry as ureg

ureg('(kg/m^3)*(g/kg)^-1')

outputs (what we want):

1.0 kilogram2/(gram meter3)

but without the star

ureg('(kg/m^3)(g/kg)^-1')

output

1.0 meter3/gram

I opened an issue in pint about this: hgrecco/pint#1730. In the meantime, we should replace these unit with either a '*' between the parenthesis, or with an extra set of parenthesis.
Any opinion on which solution we should implement?

[dcherian]

cc @keewis @aulemahal

[DocOtak]

Maybe (kg/m^3)/(g/kg) and just avoid the exponent?

udunits2 thinks all these options are equal to 1000 m^-3 kg i.e. cancels the g/kg to scaling constant.

[rcaneill]

So I confirm that this is an issue from pint, that was already described (hgrecco/pint#1485). The take home message for us is that we should not use parenthesis without the * sign outside of them.