BUG: MultiIndex, IntervalIndex intersection with Categorical (#38653)

Author: jbrockmendel

doc/source/whatsnew/v1.3.0.rst

@@ -214,7 +214,7 @@ Strings
 
 Interval
 ^^^^^^^^
-
+- Bug in :meth:`IntervalIndex.intersection` always returning object-dtype when intersecting with :class:`CategoricalIndex` (:issue:`38653`)
 -
 -
 
@@ -238,6 +238,7 @@ MultiIndex
 
 - Bug in :meth:`DataFrame.drop` raising ``TypeError`` when :class:`MultiIndex` is non-unique and no level is provided (:issue:`36293`)
 - Bug in :meth:`MultiIndex.equals` incorrectly returning ``True`` when :class:`MultiIndex` containing ``NaN`` even when they are differntly ordered (:issue:`38439`)
+- Bug in :meth:`MultiIndex.intersection` always returning empty when intersecting with :class:`CategoricalIndex` (:issue:`38653`)
 
 I/O
 ^^^

pandas/core/dtypes/dtypes.py

@@ -1173,7 +1173,7 @@ def __from_arrow__(
     def _get_common_dtype(self, dtypes: List[DtypeObj]) -> Optional[DtypeObj]:
         # NB: this doesn't handle checking for closed match
         if not all(isinstance(x, IntervalDtype) for x in dtypes):
-            return np.dtype(object)
+            return None
 
         from pandas.core.dtypes.cast import find_common_type
 

pandas/core/indexes/base.py

@@ -2814,6 +2814,8 @@ def intersection(self, other, sort=False):
 
         elif not self._should_compare(other):
             # We can infer that the intersection is empty.
+            if isinstance(self, ABCMultiIndex):
+                return self[:0].rename(result_name)
             return Index([], name=result_name)
 
         elif not is_dtype_equal(self.dtype, other.dtype):

pandas/core/indexes/datetimelike.py

@@ -654,61 +654,16 @@ def difference(self, other, sort=None):
         new_idx = super().difference(other, sort=sort)._with_freq(None)
         return new_idx
 
-    def intersection(self, other, sort=False):
-        """
-        Specialized intersection for DatetimeIndex/TimedeltaIndex.
-
-        May be much faster than Index.intersection
-
-        Parameters
-        ----------
-        other : Same type as self or array-like
-        sort : False or None, default False
-            Sort the resulting index if possible.
-
-            .. versionadded:: 0.24.0
-
-            .. versionchanged:: 0.24.1
-
-               Changed the default to ``False`` to match the behaviour
-               from before 0.24.0.
-
-            .. versionchanged:: 0.25.0
-
-               The `sort` keyword is added
-
-        Returns
-        -------
-        y : Index or same type as self
-        """
-        self._validate_sort_keyword(sort)
-        self._assert_can_do_setop(other)
-        other, result_name = self._convert_can_do_setop(other)
-
-        if self.equals(other):
-            if self.has_duplicates:
-                return self.unique()._get_reconciled_name_object(other)
-            return self._get_reconciled_name_object(other)
-
-        elif not self._should_compare(other):
-            # We can infer that the intersection is empty.
-            return Index([], name=result_name)
-
-        return self._intersection(other, sort=sort)
-
     def _intersection(self, other: Index, sort=False) -> Index:
         """
         intersection specialized to the case with matching dtypes.
         """
+        other = cast("DatetimeTimedeltaMixin", other)
         if len(self) == 0:
             return self.copy()._get_reconciled_name_object(other)
         if len(other) == 0:
             return other.copy()._get_reconciled_name_object(self)
 
-        if not isinstance(other, type(self)):
-            result = Index.intersection(self, other, sort=sort)
-            return result
-
         elif not self._can_fast_intersect(other):
             result = Index._intersection(self, other, sort=sort)
             # We need to invalidate the freq because Index._intersection

pandas/core/indexes/multi.py

@@ -3449,8 +3449,8 @@ def equals(self, other: object) -> bool:
 
         if not isinstance(other, MultiIndex):
             # d-level MultiIndex can equal d-tuple Index
-            if not is_object_dtype(other.dtype):
-                # other cannot contain tuples, so cannot match self
+            if not self._should_compare(other):
+                # object Index or Categorical[object] may contain tuples
                 return False
             return array_equivalent(self._values, other._values)
 
@@ -3588,13 +3588,15 @@ def union(self, other, sort=None):
     def _union(self, other, sort):
         other, result_names = self._convert_can_do_setop(other)
 
-        if not is_object_dtype(other.dtype):
+        if not self._should_compare(other):
             raise NotImplementedError(
                 "Can only union MultiIndex with MultiIndex or Index of tuples, "
                 "try mi.to_flat_index().union(other) instead."
             )
 
-        uniq_tuples = lib.fast_unique_multiple([self._values, other._values], sort=sort)
+        # We could get here with CategoricalIndex other
+        rvals = other._values.astype(object, copy=False)
+        uniq_tuples = lib.fast_unique_multiple([self._values, rvals], sort=sort)
 
         return MultiIndex.from_arrays(
             zip(*uniq_tuples), sortorder=0, names=result_names
@@ -3631,47 +3633,11 @@ def _maybe_match_names(self, other):
                 names.append(None)
         return names
 
-    def intersection(self, other, sort=False):
-        """
-        Form the intersection of two MultiIndex objects.
-
-        Parameters
-        ----------
-        other : MultiIndex or array / Index of tuples
-        sort : False or None, default False
-            Sort the resulting MultiIndex if possible
-
-            .. versionadded:: 0.24.0
-
-            .. versionchanged:: 0.24.1
-
-               Changed the default from ``True`` to ``False``, to match
-               behaviour from before 0.24.0
-
-        Returns
-        -------
-        Index
-        """
-        self._validate_sort_keyword(sort)
-        self._assert_can_do_setop(other)
-        other, _ = self._convert_can_do_setop(other)
-
-        if self.equals(other):
-            if self.has_duplicates:
-                return self.unique()._get_reconciled_name_object(other)
-            return self._get_reconciled_name_object(other)
-
-        return self._intersection(other, sort=sort)
-
     def _intersection(self, other, sort=False):
         other, result_names = self._convert_can_do_setop(other)
 
-        if not self._is_comparable_dtype(other.dtype):
-            # The intersection is empty
-            return self[:0].rename(result_names)
-
         lvals = self._values
-        rvals = other._values
+        rvals = other._values.astype(object, copy=False)
 
         uniq_tuples = None  # flag whether _inner_indexer was successful
         if self.is_monotonic and other.is_monotonic:

pandas/core/indexes/period.py

@@ -632,26 +632,6 @@ def _setop(self, other, sort, opname: str):
         result = type(self)._simple_new(parr, name=res_name)
         return result
 
-    def intersection(self, other, sort=False):
-        self._validate_sort_keyword(sort)
-        self._assert_can_do_setop(other)
-        other, result_name = self._convert_can_do_setop(other)
-
-        if self.equals(other):
-            if self.has_duplicates:
-                return self.unique()._get_reconciled_name_object(other)
-            return self._get_reconciled_name_object(other)
-
-        elif not self._should_compare(other):
-            # We can infer that the intersection is empty.
-            return Index([], name=result_name)
-
-        elif not is_dtype_equal(self.dtype, other.dtype):
-            # i.e. object dtype
-            return super().intersection(other, sort=sort)
-
-        return self._intersection(other, sort=sort)
-
     def _intersection(self, other, sort=False):
         return self._setop(other, sort, opname="intersection")
 

pandas/tests/dtypes/cast/test_find_common_type.py

@@ -9,6 +9,8 @@
     PeriodDtype,
 )
 
+from pandas import Categorical, Index
+
 
 @pytest.mark.parametrize(
     "source_dtypes,expected_common_dtype",
@@ -156,3 +158,13 @@ def test_interval_dtype(left, right):
 
     else:
         assert result == object
+
+
+@pytest.mark.parametrize("dtype", interval_dtypes)
+def test_interval_dtype_with_categorical(dtype):
+    obj = Index([], dtype=dtype)
+
+    cat = Categorical([], categories=obj)
+
+    result = find_common_type([dtype, cat.dtype])
+    assert result == dtype

pandas/tests/indexes/multi/test_equivalence.py

@@ -10,6 +10,8 @@ def test_equals(idx):
     assert idx.equals(idx)
     assert idx.equals(idx.copy())
     assert idx.equals(idx.astype(object))
+    assert idx.equals(idx.to_flat_index())
+    assert idx.equals(idx.to_flat_index().astype("category"))
 
     assert not idx.equals(list(idx))
     assert not idx.equals(np.array(idx))

pandas/tests/indexes/multi/test_setops.py

@@ -294,6 +294,20 @@ def test_intersection(idx, sort):
     # assert result.equals(tuples)
 
 
+@pytest.mark.parametrize("method", ["intersection", "union"])
+def test_setop_with_categorical(idx, sort, method):
+    other = idx.to_flat_index().astype("category")
+    res_names = [None] * idx.nlevels
+
+    result = getattr(idx, method)(other, sort=sort)
+    expected = getattr(idx, method)(idx, sort=sort).rename(res_names)
+    tm.assert_index_equal(result, expected)
+
+    result = getattr(idx, method)(other[:5], sort=sort)
+    expected = getattr(idx, method)(idx[:5], sort=sort).rename(res_names)
+    tm.assert_index_equal(result, expected)
+
+
 def test_intersection_non_object(idx, sort):
     other = Index(range(3), name="foo")
 

pandas/tests/indexes/test_setops.py

@@ -446,3 +446,20 @@ def test_intersection_difference_match_empty(self, index, sort):
         inter = index.intersection(index[:0])
         diff = index.difference(index, sort=sort)
         tm.assert_index_equal(inter, diff, exact=True)
+
+
+@pytest.mark.parametrize("method", ["intersection", "union"])
+def test_setop_with_categorical(index, sort, method):
+    if isinstance(index, MultiIndex):
+        # tested separately in tests.indexes.multi.test_setops
+        return
+
+    other = index.astype("category")
+
+    result = getattr(index, method)(other, sort=sort)
+    expected = getattr(index, method)(index, sort=sort)
+    tm.assert_index_equal(result, expected)
+
+    result = getattr(index, method)(other[:5], sort=sort)
+    expected = getattr(index, method)(index[:5], sort=sort)
+    tm.assert_index_equal(result, expected)