pint.Quantity.units / pint.Unit fails

[peter-goldstein]

Steps to reproduce:

a = pint.Quantity(1, "L").units
b = pint.Unit("mL")
c = a / b

Expected behavior:
c is a dimensionless unit representing 1000

Actual behavior:
AttributeError: 'PlainUnit' object has no attribute '_get_non_multiplicative_units'

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The offending line is in pint/facets/plain/quantity.py:999:

no_offset_units_other = len(other._get_non_multiplicative_units())

other does not have this method, because other is a Unit, and not a NonMultiplicativeQuantity. It looks like the bug is coming from quantity.py:989:

if isinstance(other, self._REGISTRY.Unit):
    other = 1 * other

This is evaluating to False, so other is remaining a Unit rather than being converted to a Quantity. I suspect this isn't quite the right logic for this conditional. I believe the intention is "is this item a Unit and attached to the same registry I am," but what it is actually checking is, "was this item constructed as an instance of my registry's inner class called Unit."

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(Unnecessary) detail:

• pint.Quantity(1, "L") invokes SharedRegistryObject.__new__, which gives the created object a field _REGISTRY pointing to the application registry
• The units() property returns a self._REGISTRY.Unit, which does not have a _REGISTRY field but is an instance of an inner class of the application registry
• pint.Unit("mL") invokes SharedRegistryObject.new`, so again it has the application registry as a field, but it isn't an instance of that registry's inner Unit class

[andrewgsavage]

I'd expect to get c = Quantity(1000, "dimensionless")

Looks like you're most of the way there, I'd change that to
if isinstance(other, self._REGISTRY.Unit) or (isinstance(other, Unit) and self._REGISTRY==other._REGISTRY):
A PR with a fix is welcomed.
the only other place I see an isinstance check with a Unit is L918 in quantity.py so change that too.