avoid destroying MultiIndex objects in dequantify (#168)

* add the test from the issue

* don't try to strip units if the variable does not contain a quantity

* changelog

* issue numbers

Author: keewis

docs/whats-new.rst

@@ -8,6 +8,8 @@ What's new
   By `Justus Magin <https://github.com/keewis>`_.
 - add support for python 3.10 (:pull:`155`)
   By `Justus Magin <https://github.com/keewis>`_.
+- preserve :py:class:`pandas.MultiIndex` objects (:issue:`164`, :pull:`168`).
+  By `Justus Magin <https://github.com/keewis>`_.
 
 0.2.1 (26 Jul 2021)
 -------------------

pint_xarray/conversion.py

@@ -282,6 +282,9 @@ def extract_unit_attributes(obj, attr="units"):
 
 
 def strip_units_variable(var):
+    if not isinstance(var.data, pint.Quantity):
+        return var
+
     data = array_strip_units(var.data)
     return var.copy(data=data)
 

pint_xarray/tests/test_accessors.py

@@ -1,4 +1,5 @@
 import numpy as np
+import pandas as pd
 import pint
 import pytest
 import xarray as xr
@@ -174,6 +175,17 @@ def test_roundtrip_data(self, example_unitless_da):
         result = quantified.pint.dequantify()
         assert_equal(result, orig)
 
+    def test_multiindex(self):
+        mindex = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=("lat", "lon"))
+
+        da = xr.DataArray(
+            np.arange(len(mindex)), dims="multi", coords={"multi": mindex}
+        )
+        result = da.pint.dequantify()
+
+        xr.testing.assert_identical(da, result)
+        assert isinstance(result.indexes["multi"], pd.MultiIndex)
+
 
 class TestPropertiesDataArray:
     def test_magnitude_getattr(self, example_quantity_da):