little h issue with to/from_astropy

[brittonsmith]

• unyt version: v1.0.7+73.ge9bbdf5
• Python version: 3.7.1
• Operating System: Mac OS 10.12.6

Description

This may not qualify as an issue, but perhaps just a note for the future. In which case, feel free to close.

Adding the Hubble constant, little h, to a UnitRegistry produces some unintended behavior when converting to astropy and back from astropy. This seems to be because "h" in astropy.units is already defined to mean hour. I bring this up because using little h as the Hubble constant is now quite common in yt and other places with cosmological datasets.

What I Did

from unyt import UnitRegistry, unyt_quantity
from unyt.dimensions import dimensionless

ur = UnitRegistry()
ur.add('h', 0.7, dimensionless)

L = unyt_quantity(1, 'Mpc/h', registry=ur)
L2 = L.from_astropy(L.to_astropy())

print (L)
print (L2)

This gives:

1 Mpc/h
1.0 Mpc/hr

Maybe there is nothing that can be done about this, but I thought I'd make it known in case it wasn't previously.