Add optional sort parameter to difference method in subclasses

doc/source/whatsnew/v0.24.0.txt

@@ -194,6 +194,7 @@ Other Enhancements
 - :meth:`Index.to_frame` now supports overriding column name(s) (:issue:`22580`).
 - New attribute :attr:`__git_version__` will return git commit sha of current build (:issue:`21295`).
 - Compatibility with Matplotlib 3.0 (:issue:`22790`).
+- :meth:`Index.difference` now has an optional ``sort`` parameter to specify whether the results should be sorted if possible (:issue:`17839`)
 
 .. _whatsnew_0240.api_breaking:
 

pandas/core/indexes/multi.py

@@ -2764,10 +2764,18 @@ def intersection(self, other):
             return MultiIndex.from_arrays(lzip(*uniq_tuples), sortorder=0,
                                           names=result_names)
 
-    def difference(self, other):
+    def difference(self, other, sort=True):
         """
         Compute sorted set difference of two MultiIndex objects
 
+        Parameters
+        ----------
+        other : MultiIndex
+        sort : bool, default True
+            Sort the resulting MultiIndex if possible
+
+            .. versionadded:: 0.24.0
+
         Returns
         -------
         diff : MultiIndex
@@ -2780,11 +2788,13 @@ def difference(self, other):
 
         if self.equals(other):
             return MultiIndex(levels=self.levels,
-                              labels=[[]] * self.nlevels,
-                              names=result_names, verify_integrity=False)
+                                labels=[[]] * self.nlevels,
+                                names=result_names, verify_integrity=False)
+
+        difference = set(self._ndarray_values) - set(other._ndarray_values)
 
-        difference = sorted(set(self._ndarray_values) -
-                            set(other._ndarray_values))
+        if sort:
+            difference = sorted(difference)
 
         if len(difference) == 0:
             return MultiIndex(levels=[[]] * self.nlevels,

pandas/tests/indexes/test_base.py

@@ -1047,15 +1047,17 @@ def test_iadd_string(self):
 
     @pytest.mark.parametrize("second_name,expected", [
         (None, None), ('name', 'name')])
-    def test_difference_name_preservation(self, second_name, expected):
+    @pytest.mark.parametrize("sort", [
+        (True, False)])
+    def test_difference_name_preservation(self, second_name, expected, sort):
         # TODO: replace with fixturesult
         first = self.strIndex[5:20]
         second = self.strIndex[:10]
         answer = self.strIndex[10:20]
 
         first.name = 'name'
         second.name = second_name
-        result = first.difference(second)
+        result = first.difference(second, sort)
 
         assert tm.equalContents(result, answer)
 
@@ -1064,18 +1066,22 @@ def test_difference_name_preservation(self, second_name, expected):
         else:
             assert result.name == expected
 
-    def test_difference_empty_arg(self):
+    @pytest.mark.parametrize("sort", [
+        (True, False)])
+    def test_difference_empty_arg(self, sort):
         first = self.strIndex[5:20]
         first.name == 'name'
-        result = first.difference([])
+        result = first.difference([], sort=sort)
 
         assert tm.equalContents(result, first)
         assert result.name == first.name
 
-    def test_difference_identity(self):
+    @pytest.mark.parametrize("sort", [
+        (True, False)])
+    def test_difference_identity(self, sort):
         first = self.strIndex[5:20]
         first.name == 'name'
-        result = first.difference(first)
+        result = first.difference(first, sort)
 
         assert len(result) == 0
         assert result.name == first.name
@@ -1124,13 +1130,15 @@ def test_symmetric_difference_non_index(self):
         assert tm.equalContents(result, expected)
         assert result.name == 'new_name'
 
-    def test_difference_type(self):
+    @pytest.mark.parametrize("sort", [
+        (True, False)])
+    def test_difference_type(self, sort):
         # GH 20040
         # If taking difference of a set and itself, it
         # needs to preserve the type of the index
         skip_index_keys = ['repeats']
         for key, index in self.generate_index_types(skip_index_keys):
-            result = index.difference(index)
+            result = index.difference(index, sort)
             expected = index.drop(index)
             tm.assert_index_equal(result, expected)
 
@@ -2344,13 +2352,15 @@ def test_intersection_different_type_base(self, klass):
         result = first.intersection(klass(second.values))
         assert tm.equalContents(result, second)
 
-    def test_difference_base(self):
+    @pytest.mark.parametrize("sort", [
+        (True, False)])
+    def test_difference_base(self, sort):
         # (same results for py2 and py3 but sortedness not tested elsewhere)
         index = self.create_index()
         first = index[:4]
         second = index[3:]
 
-        result = first.difference(second)
+        result = first.difference(second, sort)
         expected = Index([0, 1, 'a'])
         tm.assert_index_equal(result, expected)