Fix to_reduced_units

CHANGES

@@ -9,6 +9,9 @@ Pint Changelog
 - Fix setting options of the application registry (Issue #1403).
 - Fix Quantity & Unit `is_compatible_with` with registry active contexts (Issue #1424).
 - Allow Quantity to parse 'NaN' and 'inf(inity)', case insensitive
+- Fix casting error when using to_reduced_units with array of int.
+  (Issue #1184)
+
 
 0.18 (2021-10-26)
 -----------------

pint/quantity.py

@@ -763,6 +763,21 @@ def to_base_units(self) -> Quantity[_MagnitudeType]:
 
         return self.__class__(magnitude, other)
 
+    def _get_reduced_units(self, units):
+        # loop through individual units and compare to each other unit
+        # can we do better than a nested loop here?
+        for unit1, exp in units.items():
+            # make sure it wasn't already reduced to zero exponent on prior pass
+            if unit1 not in units:
+                continue
+            for unit2 in units:
+                if unit1 != unit2:
+                    power = self._REGISTRY._get_dimensionality_ratio(unit1, unit2)
+                    if power:
+                        units = units.add(unit2, exp / power).remove([unit1])
+                        break
+        return units
+
     def ito_reduced_units(self) -> None:
         """Return Quantity scaled in place to reduced units, i.e. one unit per
         dimension. This will not reduce compound units (e.g., 'J/kg' will not
@@ -775,32 +790,27 @@ def ito_reduced_units(self) -> None:
         if len(self._units) == 1:
             return None
 
-        newunits = self._units.copy()
-        # loop through individual units and compare to each other unit
-        # can we do better than a nested loop here?
-        for unit1, exp in self._units.items():
-            # make sure it wasn't already reduced to zero exponent on prior pass
-            if unit1 not in newunits:
-                continue
-            for unit2 in newunits:
-                if unit1 != unit2:
-                    power = self._REGISTRY._get_dimensionality_ratio(unit1, unit2)
-                    if power:
-                        newunits = newunits.add(unit2, exp / power).remove([unit1])
-                        break
+        units = self._units.copy()
+        new_units = self._get_reduced_units(units)
 
-        return self.ito(newunits)
+        return self.ito(new_units)
 
     def to_reduced_units(self) -> Quantity[_MagnitudeType]:
         """Return Quantity scaled in place to reduced units, i.e. one unit per
         dimension. This will not reduce compound units (intentionally), nor
         can it make use of contexts at this time.
         """
 
-        # can we make this more efficient?
-        newq = copy.copy(self)
-        newq.ito_reduced_units()
-        return newq
+        # shortcuts in case we're dimensionless or only a single unit
+        if self.dimensionless:
+            return self.ito({})
+        if len(self._units) == 1:
+            return None
+
+        units = self._units.copy()
+        new_units = self._get_reduced_units(units)
+
+        return self.to(new_units)
 
     def to_compact(self, unit=None) -> Quantity[_MagnitudeType]:
         """ "Return Quantity rescaled to compact, human-readable units.

pint/testsuite/test_quantity.py

@@ -635,6 +635,13 @@ def test_nan(self, nan_str, has_unit):
         assert math.isnan(test.magnitude)
         assert ref != test
 
+    @helpers.requires_numpy
+    def test_to_reduced_units(self):
+        q = self.Q_([3, 4], "s * ms")
+        helpers.assert_quantity_equal(
+            q.to_reduced_units(), self.Q_([3000.0, 4000.0], "ms**2")
+        )
+
 
 class TestQuantityToCompact(QuantityTestCase):
     def assertQuantityAlmostIdentical(self, q1, q2):