Add check for delta unit to convert

Author: dalito

pint/facets/nonmultiplicative/registry.py

@@ -120,7 +120,7 @@ def _is_multiplicative(self, unit_name: str) -> bool:
         Raises
         ------
         UndefinedUnitError
-            If the unit is not in the registyr.
+            If the unit is not in the registry.
         """
         if unit_name in self._units:
             return self._units[unit_name].is_multiplicative
@@ -250,6 +250,13 @@ def _convert(
                 src, dst, extra_msg=f" - In destination units, {ex}"
             )
 
+        # Check for delta units and make sure src and dst are delta units.
+        src_delta = any(u.startswith("delta_") for u in src)
+        dst_delta = any(u.startswith("delta_") for u in dst)
+        if (src_delta and not dst_delta) or (dst_delta and not src_delta):
+            raise DimensionalityError(src, dst)
+
+        # convert if no offset units are present
         if not (src_offset_unit or dst_offset_unit):
             return super()._convert(value, src, dst, inplace)
 

pint/testsuite/test_unit.py

@@ -978,6 +978,9 @@ class TestConvertWithOffset(QuantityTestCase):
         (({"degC": 2}, {"kelvin": 2}), "error"),
         (({"degC": 1, "degF": 1}, {"kelvin": 2}), "error"),
         (({"degC": 1, "kelvin": 1}, {"kelvin": 2}), "error"),
+        (({"delta_degC": 1}, {"kelvin": 1}), "error"),
+        (({"delta_degC": 1}, {"degF": 1}), "error"),
+        (({"delta_degC": 1}, {"degC": 1}), "error"),
     ]
 
     @pytest.mark.parametrize(("input_tuple", "expected"), convert_with_offset)