degree_Celsius to_compact raises DimensionalityError (Pint 0.23)

[xolodho]

Calling to_compact on a celsius magnitude of absolute value smaller than
1 doesn't work properly.

For example, the following throws a pint.errors.DimensionalityError
exception:

  from pint import Quantity
  Quantity(0.1, "degree_Celsius").to_compact()

However, if right afterwards you re-run the to_compact line again, then
it does not throw exception, but instead returns

 <Quantity(273250.0, 'millidegree_Celsius')>,

which is numerically incorrect (that's milliKelvin?).

So calling to_compact for 0.1 degrees celsius either raises exception
(the first time), or returns an incorrect answer (the second time). It
would seem this bug doesn't happen if the magnitude is ≥1 in absolute
value, e.g. 1 and -2.5 and 5.8 are all ok, but 0.1 and 0.999
fail.

using Pint 0.23 installed from pip on Linux.

[andrewgsavage]

Interesting. This behaviour may be changed on master and the next version. There's a release candidate out you can install with pip install pint --pre