Remove set difference fix

Author: Dr-Irv

doc/source/whatsnew/v0.23.0.txt

@@ -838,7 +838,6 @@ Indexing
 - Bug in :class:`IntervalIndex` where set operations that returned an empty ``IntervalIndex`` had the wrong dtype (:issue:`19101`)
 - Bug in :meth:`DataFrame.drop_duplicates` where no ``KeyError`` is raised when passing in columns that don't exist on the ``DataFrame`` (issue:`19726`)
 - Bug in :func:`Index.union` and :func:`Index.intersection` where name of the ``Index`` of the result was not computed correctly for certain cases (:issue:`9943`, :issue:`9862`)
-- Bug in :func:`Index.difference` when taking difference of ``Index`` and itself as type was not preserved. (:issue:`19849`)
 
 
 MultiIndex

pandas/core/indexes/base.py

@@ -2454,13 +2454,6 @@ def intersection(self, other):
             taken.name = None
         return taken
 
-    def _create_empty_index(self, name):
-        """
-        Returns an empty index.  Overridden as necessary by
-        subclasses that have different constructors.
-        """
-        return self.__class__([], name=name)
-
     def difference(self, other):
         """
         Return a new Index with elements from the index that are not in
@@ -2489,7 +2482,7 @@ def difference(self, other):
         self._assert_can_do_setop(other)
 
         if self.equals(other):
-            return self._create_empty_index(get_op_result_name(self, other))
+            return Index([], name=self.name)
 
         other, result_name = self._convert_can_do_setop(other)
 

pandas/core/indexes/category.py

@@ -723,13 +723,6 @@ def insert(self, loc, item):
         codes = np.concatenate((codes[:loc], code, codes[loc:]))
         return self._create_from_codes(codes)
 
-    def _create_empty_index(self, name):
-        """
-        Returns an empty index using categories and ordered of self
-        """
-        return CategoricalIndex([], categories=self.categories,
-                                ordered=self.ordered, name=name)
-
     def _concat(self, to_concat, name):
         # if calling index is category, don't check dtype of others
         return CategoricalIndex._concat_same_dtype(self, to_concat, name)

pandas/core/indexes/multi.py

@@ -2733,7 +2733,7 @@ def intersection(self, other):
         other_tuples = other._ndarray_values
         uniq_tuples = sorted(set(self_tuples) & set(other_tuples))
         if len(uniq_tuples) == 0:
-            return MultiIndex(levels=self.levels,
+            return MultiIndex(levels=[[]] * self.nlevels,
                               labels=[[]] * self.nlevels,
                               names=result_names, verify_integrity=False)
         else:
@@ -2755,7 +2755,7 @@ def difference(self, other):
             return self
 
         if self.equals(other):
-            return MultiIndex(levels=self.levels,
+            return MultiIndex(levels=[[]] * self.nlevels,
                               labels=[[]] * self.nlevels,
                               names=result_names, verify_integrity=False)
 

pandas/core/indexes/period.py

@@ -963,12 +963,6 @@ def _convert_tolerance(self, tolerance, target):
                              'target index size')
         return self._maybe_convert_timedelta(tolerance)
 
-    def _create_empty_index(self, name):
-        """
-        Returns an empty index using freq of self
-        """
-        return PeriodIndex([], freq=self.freq, name=name)
-
     def insert(self, loc, item):
         if not isinstance(item, Period) or self.freq != item.freq:
             return self.astype(object).insert(loc, item)

pandas/core/indexes/range.py

@@ -479,12 +479,6 @@ def join(self, other, how='left', level=None, return_indexers=False,
     def _concat_same_dtype(self, indexes, name):
         return _concat._concat_rangeindex_same_dtype(indexes).rename(name)
 
-    def _create_empty_index(self, name):
-        """
-        Returns an empty index using step size of self
-        """
-        return RangeIndex(start=None, stop=None, step=self._step, name=name)
-
     def __len__(self):
         """
         return the length of the RangeIndex

pandas/tests/indexes/test_base.py

@@ -997,17 +997,6 @@ def test_iadd_string(self):
         index += '_x'
         assert 'a_x' in index
 
-    def test_difference_type(self):
-        # GH 9943 9862
-        # If taking difference of a set and itself, it
-        # needs to preserve the type of the index
-        skip_index_keys = ['repeats']
-        for key, id in self.indices.items():
-            if key not in skip_index_keys:
-                result = id.difference(id)
-                expected = self.create_empty_index(id)
-                tm.assert_index_equal(result, expected)
-
     def test_difference(self):
 
         first = self.strIndex[5:20]

pandas/tests/reshape/test_concat.py

@@ -2055,10 +2055,10 @@ def test_concat_categoricalindex(self):
 
         result = pd.concat([a, b, c], axis=1)
 
-        exp_idx = pd.CategoricalIndex([9, 0, 1, 2], categories=categories)
-        exp = pd.DataFrame({0: [1, 1, np.nan, np.nan],
-                            1: [np.nan, 2, 2, np.nan],
-                            2: [np.nan, np.nan, 3, 3]},
+        exp_idx = pd.CategoricalIndex([0, 1, 2, 9])
+        exp = pd.DataFrame({0: [1, np.nan, np.nan, 1],
+                            1: [2, 2, np.nan, np.nan],
+                            2: [np.nan, 3, 3, np.nan]},
                            columns=[0, 1, 2],
                            index=exp_idx)
         tm.assert_frame_equal(result, exp)