From 8b8987ee3fe344077c9f28a21bacfa811375aa5b Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Wed, 23 Dec 2020 20:13:07 -0800
Subject: [PATCH] REF: share Index.difference

---
 pandas/core/indexes/base.py                   |  5 +++
 pandas/core/indexes/multi.py                  | 35 +------------------
 pandas/core/indexes/period.py                 | 10 ------
 pandas/tests/indexes/datetimes/test_setops.py |  3 +-
 pandas/tests/indexes/multi/test_setops.py     |  4 ++-
 pandas/tests/indexes/period/test_setops.py    |  3 +-
 6 files changed, 13 insertions(+), 47 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 8d48a6277d412..edb8cd57b31bd 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -2906,8 +2906,13 @@ def difference(self, other, sort=None):
         other, result_name = self._convert_can_do_setop(other)
 
         if self.equals(other):
+            # Note: we do not (yet) sort even if sort=None GH#24959
             return self[:0].rename(result_name)
 
+        if len(other) == 0:
+            # Note: we do not (yet) sort even if sort=None GH#24959
+            return self.rename(result_name)
+
         result = self._difference(other, sort=sort)
         return self._wrap_setop_result(other, result)
 
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index c7be66b596246..3c7c0fdc45d70 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -3676,42 +3676,9 @@ def _intersection(self, other, sort=False):
                 zip(*uniq_tuples), sortorder=0, names=result_names
             )
 
-    def difference(self, other, sort=None):
-        """
-        Compute set difference of two MultiIndex objects
-
-        Parameters
-        ----------
-        other : MultiIndex
-        sort : False or None, default None
-            Sort the resulting MultiIndex if possible
-
-            .. versionadded:: 0.24.0
-
-            .. versionchanged:: 0.24.1
-
-               Changed the default value from ``True`` to ``None``
-               (without change in behaviour).
-
-        Returns
-        -------
-        diff : MultiIndex
-        """
-        self._validate_sort_keyword(sort)
-        self._assert_can_do_setop(other)
+    def _difference(self, other, sort):
         other, result_names = self._convert_can_do_setop(other)
 
-        if len(other) == 0:
-            return self.rename(result_names)
-
-        if self.equals(other):
-            return MultiIndex(
-                levels=self.levels,
-                codes=[[]] * self.nlevels,
-                names=result_names,
-                verify_integrity=False,
-            )
-
         this = self._get_unique_index()
 
         indexer = this.get_indexer(other)
diff --git a/pandas/core/indexes/period.py b/pandas/core/indexes/period.py
index 4d48bc0d51912..49acb7b5592cb 100644
--- a/pandas/core/indexes/period.py
+++ b/pandas/core/indexes/period.py
@@ -635,16 +635,6 @@ def _setop(self, other, sort, opname: str):
     def _intersection(self, other, sort=False):
         return self._setop(other, sort, opname="intersection")
 
-    def difference(self, other, sort=None):
-        self._validate_sort_keyword(sort)
-        self._assert_can_do_setop(other)
-        other, result_name = self._convert_can_do_setop(other)
-
-        if self.equals(other):
-            return self[:0].rename(result_name)
-
-        return self._difference(other, sort=sort)
-
     def _difference(self, other, sort):
 
         if is_object_dtype(other):
diff --git a/pandas/tests/indexes/datetimes/test_setops.py b/pandas/tests/indexes/datetimes/test_setops.py
index 93772e2c27a82..c7632c3c5c455 100644
--- a/pandas/tests/indexes/datetimes/test_setops.py
+++ b/pandas/tests/indexes/datetimes/test_setops.py
@@ -326,7 +326,8 @@ def test_difference(self, tz, sort):
             (rng3, other3, expected3),
         ]:
             result_diff = rng.difference(other, sort)
-            if sort is None:
+            if sort is None and len(other):
+                # We dont sort (yet?) when empty GH#24959
                 expected = expected.sort_values()
             tm.assert_index_equal(result_diff, expected)
 
diff --git a/pandas/tests/indexes/multi/test_setops.py b/pandas/tests/indexes/multi/test_setops.py
index 9a7ff78bae3db..e8d5baaef42d4 100644
--- a/pandas/tests/indexes/multi/test_setops.py
+++ b/pandas/tests/indexes/multi/test_setops.py
@@ -294,7 +294,9 @@ def test_intersection(idx, sort):
     # assert result.equals(tuples)
 
 
-@pytest.mark.parametrize("method", ["intersection", "union"])
+@pytest.mark.parametrize(
+    "method", ["intersection", "union", "difference", "symmetric_difference"]
+)
 def test_setop_with_categorical(idx, sort, method):
     other = idx.to_flat_index().astype("category")
     res_names = [None] * idx.nlevels
diff --git a/pandas/tests/indexes/period/test_setops.py b/pandas/tests/indexes/period/test_setops.py
index 04acfdc65dbc0..40d1263ff70cd 100644
--- a/pandas/tests/indexes/period/test_setops.py
+++ b/pandas/tests/indexes/period/test_setops.py
@@ -318,7 +318,8 @@ def test_difference(self, sort):
             (rng7, other7, expected7),
         ]:
             result_difference = rng.difference(other, sort=sort)
-            if sort is None:
+            if sort is None and len(other):
+                # We dont sort (yet?) when empty GH#24959
                 expected = expected.sort_values()
             tm.assert_index_equal(result_difference, expected)