BUG: RangeIndex.difference with mismatched step (#38028)

* BUG: return RangeIndex from RangeIndex.difference

* test coverage

* BUG: RangeIndex.difference with mismatched step

* whatsnew

Author: jbrockmendel

doc/source/whatsnew/v1.2.0.rst

@@ -772,6 +772,7 @@ Other
 - Bug in :meth:`Index.union` behaving differently depending on whether operand is an :class:`Index` or other list-like (:issue:`36384`)
 - Passing an array with 2 or more dimensions to the :class:`Series` constructor now raises the more specific ``ValueError`` rather than a bare ``Exception`` (:issue:`35744`)
 - Bug in ``dir`` where ``dir(obj)`` wouldn't show attributes defined on the instance for pandas objects (:issue:`37173`)
+- Bug in :meth:`RangeIndex.difference` returning :class:`Int64Index` in some cases where it should return :class:`RangeIndex` (:issue:`38028`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/indexes/range.py

@@ -670,13 +670,17 @@ def difference(self, other, sort=None):
         if not isinstance(overlap, RangeIndex):
             # We wont end up with RangeIndex, so fall back
             return super().difference(other, sort=sort)
+        if overlap.step != first.step:
+            # In some cases we might be able to get a RangeIndex back,
+            #  but not worth the effort.
+            return super().difference(other, sort=sort)
 
         if overlap[0] == first.start:
             # The difference is everything after the intersection
             new_rng = range(overlap[-1] + first.step, first.stop, first.step)
-        elif overlap[-1] == first.stop:
+        elif overlap[-1] == first[-1]:
             # The difference is everything before the intersection
-            new_rng = range(first.start, overlap[0] - first.step, first.step)
+            new_rng = range(first.start, overlap[0], first.step)
         else:
             # The difference is not range-like
             return super().difference(other, sort=sort)

pandas/tests/indexes/ranges/test_setops.py

@@ -247,21 +247,38 @@ def test_difference(self):
 
         result = obj.difference(obj)
         expected = RangeIndex.from_range(range(0), name="foo")
-        tm.assert_index_equal(result, expected)
+        tm.assert_index_equal(result, expected, exact=True)
 
         result = obj.difference(expected.rename("bar"))
-        tm.assert_index_equal(result, obj.rename(None))
+        tm.assert_index_equal(result, obj.rename(None), exact=True)
 
         result = obj.difference(obj[:3])
-        tm.assert_index_equal(result, obj[3:])
+        tm.assert_index_equal(result, obj[3:], exact=True)
 
         result = obj.difference(obj[-3:])
-        tm.assert_index_equal(result, obj[:-3])
+        tm.assert_index_equal(result, obj[:-3], exact=True)
+
+        result = obj[::-1].difference(obj[-3:])
+        tm.assert_index_equal(result, obj[:-3][::-1], exact=True)
+
+        result = obj[::-1].difference(obj[-3:][::-1])
+        tm.assert_index_equal(result, obj[:-3][::-1], exact=True)
 
         result = obj.difference(obj[2:6])
         expected = Int64Index([1, 2, 7, 8, 9], name="foo")
         tm.assert_index_equal(result, expected)
 
+    def test_difference_mismatched_step(self):
+        obj = RangeIndex.from_range(range(1, 10), name="foo")
+
+        result = obj.difference(obj[::2])
+        expected = obj[1::2]._int64index
+        tm.assert_index_equal(result, expected, exact=True)
+
+        result = obj.difference(obj[1::2])
+        expected = obj[::2]._int64index
+        tm.assert_index_equal(result, expected, exact=True)
+
     def test_symmetric_difference(self):
         # GH#12034 Cases where we operate against another RangeIndex and may
         #  get back another RangeIndex