Find out why pint doesn't allow registry mixing

[seanpearsonuk]

We can ask pint in github

We think it might be to enforce consistent mixing of units (e.g. GB vs US gallons)

[hpohekar]

@seanpearsonuk @dnwillia @ansaboutin

ValueError: Cannot operate with Unit and Unit of different registries.

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Sharing response given by hgrecco, he has written Pint from scratch.

hgrecco/pint#1692

[dnwillia-work]

So reading that it does allow some comparisons but it’s not considering their units equivalent for some reason. It’s still weird, and their answers don’t really help me.

[hpohekar]

@seanpearsonuk @dnwillia-work

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Here, one approach is mentioned using __init__.py changes and another is using set_application_registry().

https://pint.readthedocs.io/en/0.7.2/tutorial.html#using-pint-in-your-projects

hgrecco/pint#445

[dnwillia-work]

Right, so you can instantiate multiple registries, but they isolate the units to a registry and explicitly disallow combining them perhaps because of this design decision: "There are no global units in Pint". The behaviour does seem to be as in your screenshots (below) but I'm still not sure why they have imposed this limitation other than just establishing it as a convention (i.e. this is just a rule we have and it's the way it is).

>>> from pint import UnitRegistry
>>> q1 = UnitRegistry().meter
>>> q2 = UnitRegistry().meter
>>> q1 + q1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'Unit' and 'Unit'
>>> q1
<Unit('meter')>
>>> q2
<Unit('meter')>
>>> q1 == q2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dnwillia/Developer/CubicEosLib/venv/lib/python3.11/site-packages/pint/facets/plain/unit.py", line 188, in __eq__
    if self._check(other):
       ^^^^^^^^^^^^^^^^^^
  File "/Users/dnwillia/Developer/CubicEosLib/venv/lib/python3.11/site-packages/pint/util.py", line 987, in _check
    raise ValueError(
ValueError: Cannot operate with Unit and Unit of different registries.
>>> q3 = q1 + q2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'Unit' and 'Unit'

[hpohekar]

Yes, right.