Fix issue where quantify doesn't work on DataFrames containing string…

… columns (#217)

* update values to iloc to avoid casting array to obj type

* add test for PR217

* run pre-commit tests to pass lint test

---------

Co-authored-by: Nicholas Hemenway <nhemenway@mercmarine.com>

pint_pandas/pint_array.py

@@ -967,7 +967,7 @@ def quantify(self, level=-1):
 
         df_new = DataFrame(
             {
-                i: PintArray(df.values[:, i], unit)
+                i: PintArray(df.iloc[:, i], unit)
                 if unit != NO_UNIT
                 else df.values[:, i]
                 for i, unit in enumerate(units.values)

pint_pandas/testsuite/test_issues.py

@@ -229,3 +229,16 @@ def test_dequantify(self):
         )
         result = df.pint.dequantify()
         tm.assert_frame_equal(expected, result)
+
+
+class TestIssue217(BaseExtensionTests):
+    def test_roundtrip(self):
+        df = pd.DataFrame(
+            {
+                "power": pd.Series([1.0, 2.0, 3.0], dtype="pint[W]"),
+                "torque": pd.Series([4.0, 5.0, 6.0], dtype="pint[N*m]"),
+                "fruits": pd.Series(["apple", "pear", "kiwi"]),
+            }
+        )
+        df1 = df.pint.dequantify().pint.quantify(level=-1)
+        tm.assert_equal(df1.power.pint.m, df.power.pint.m)