Preserve original dtype when accessing MultiIndex levels (#7393)

* return multiindex levels as original dtype (#7250)

* typing in test, contributor attribution

* rst is not markdown

Author: itcarroll

doc/whats-new.rst

@@ -45,6 +45,8 @@ Bug fixes
 - add a ``keep_attrs`` parameter to :py:meth:`Dataset.pad`, :py:meth:`DataArray.pad`,
   and :py:meth:`Variable.pad` (:pull:`7267`).
   By `Justus Magin <https://github.com/keewis>`_.
+- Preserve original dtype on accessing MultiIndex levels (:issue:`7250`,
+  :pull:`7393`). By `Ian Carroll <https://github.com/itcarroll>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/indexing.py

@@ -1531,8 +1531,12 @@ def __init__(
         self.level = level
 
     def __array__(self, dtype: DTypeLike = None) -> np.ndarray:
+        if dtype is None:
+            dtype = self.dtype
         if self.level is not None:
-            return self.array.get_level_values(self.level).values
+            return np.asarray(
+                self.array.get_level_values(self.level).values, dtype=dtype
+            )
         else:
             return super().__array__(dtype)
 

xarray/tests/test_indexes.py

@@ -697,3 +697,10 @@ def test_safe_cast_to_index_datetime_datetime():
     actual = safe_cast_to_index(np.array(dates))
     assert_array_equal(expected, actual)
     assert isinstance(actual, pd.Index)
+
+
+@pytest.mark.parametrize("dtype", ["int32", "float32"])
+def test_restore_dtype_on_multiindexes(dtype: str) -> None:
+    foo = xr.Dataset(coords={"bar": ("bar", np.array([0, 1], dtype=dtype))})
+    foo = foo.stack(baz=("bar",))
+    assert str(foo["bar"].values.dtype) == dtype