Merge #1131

1131: fix reduce dimensions when previous dimension was zeroed out r=hgrecco a=bloer

…1058

- [1058] Closes # (insert issue number)
- [+] Executed ``black -t py36 . && isort -rc . && flake8`` with no errors
- [+] The change is fully covered by automated unit tests
- [+] Documented in docs/ as appropriate
- [+] Added an entry to the CHANGES file


Co-authored-by: Ben Loer <[email protected]>
Co-authored-by: Hernan Grecco <[email protected]>

CHANGES

@@ -11,10 +11,13 @@ Pint Changelog
   units (Issue #1145)
 - Implement Dask collection interface to support Pint Quantity wrapped Dask arrays.
 - Started automatically testing examples in the documentation
+- Fixed an exception generated when reducing dimensions with three or more
+  units of the same type
 - Fixed right operand power for dimensionless Quantity to reflect numpy behavior. (Issue #1136)
 - Eliminated warning when setting a masked value on an underlying MaskedArray.
 - Add `sort` option to `formatting.formatter` to permit disabling sorting of component units in format string
 
+
 0.14 (2020-07-01)
 -----------------
 

pint/quantity.py

@@ -702,8 +702,8 @@ def to_base_units(self):
 
     def ito_reduced_units(self):
         """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.
+        dimension. This will not reduce compound units (e.g., 'J/kg' will not
+        be reduced to m**2/s**2), nor can it make use of contexts at this time.
         """
 
         # shortcuts in case we're dimensionless or only a single unit
@@ -716,6 +716,9 @@ def ito_reduced_units(self):
         # 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)

pint/testsuite/test_issues.py

@@ -693,6 +693,13 @@ def test_issue973(self):
         assert isinstance(q1, ureg.Quantity)
         assert len(q0) == len(q1) == 0
 
+    def test_issue1058(self):
+        """ verify that auto-reducing quantities with three or more units
+        of same base type succeeds """
+        q = 1 * ureg.mg / ureg.g / ureg.kg
+        q.ito_reduced_units()
+        self.assertIsInstance(q, ureg.Quantity)
+
     def test_issue1062_issue1097(self):
         # Must not be used by any other tests
         assert "nanometer" not in ureg._units