BUG: Index.difference of itself doesn't preserve type

Author: Dr-Irv

doc/source/whatsnew/v0.23.0.txt

@@ -932,6 +932,7 @@ Indexing
 - Bug in :func:`IntervalIndex.symmetric_difference` where the symmetric difference with a non-``IntervalIndex`` did not raise (:issue:`18475`)
 - 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 :meth:`Index.difference` when taking difference of an ``Index`` with itself (:issue:`20040`)
 
 
 MultiIndex

pandas/core/indexes/base.py

@@ -458,7 +458,7 @@ def _simple_new(cls, values, name=None, dtype=None, **kwargs):
         Must be careful not to recurse.
         """
         if not hasattr(values, 'dtype'):
-            if values is None and dtype is not None:
+            if (values is None or len(values) == 0) and dtype is not None:
                 values = np.empty(0, dtype=dtype)
             else:
                 values = np.array(values, copy=False)
@@ -492,6 +492,8 @@ def _shallow_copy(self, values=None, **kwargs):
             values = self.values
         attributes = self._get_attributes_dict()
         attributes.update(kwargs)
+        if len(values) == 0 and 'dtype' not in kwargs:
+            attributes['dtype'] = self.dtype
         return self._simple_new(values, **attributes)
 
     def _shallow_copy_with_infer(self, values=None, **kwargs):
@@ -512,6 +514,8 @@ def _shallow_copy_with_infer(self, values=None, **kwargs):
         attributes = self._get_attributes_dict()
         attributes.update(kwargs)
         attributes['copy'] = False
+        if len(values) == 0 and 'dtype' not in kwargs:
+            attributes['dtype'] = self.dtype
         if self._infer_as_myclass:
             try:
                 return self._constructor(values, **attributes)
@@ -2511,7 +2515,7 @@ def difference(self, other):
         self._assert_can_do_setop(other)
 
         if self.equals(other):
-            return Index([], name=self.name)
+            return self._shallow_copy([])
 
         other, result_name = self._convert_can_do_setop(other)
 

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.nlevels,
+            return MultiIndex(levels=self.levels,
                               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.nlevels,
+            return MultiIndex(levels=self.levels,
                               labels=[[]] * self.nlevels,
                               names=result_names, verify_integrity=False)
 

pandas/tests/indexes/test_base.py

@@ -1034,6 +1034,29 @@ def test_symmetric_difference(self):
         assert tm.equalContents(result, expected)
         assert result.name == 'new_name'
 
+    def test_difference_type(self):
+        # 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, id in self.indices.items():
+            if key not in skip_index_keys:
+                result = id.difference(id)
+                expected = id.drop(id)
+                tm.assert_index_equal(result, expected)
+
+    def test_intersection_difference(self):
+        # GH 20040
+        # Test that the intersection of an index with an
+        # empty index produces the same index as the difference
+        # of an index with itself.  Test for all types
+        skip_index_keys = ['repeats']
+        for key, id in self.indices.items():
+            if key not in skip_index_keys:
+                inter = id.intersection(id.drop(id))
+                diff = id.difference(id)
+                tm.assert_index_equal(inter, diff)
+
     def test_is_numeric(self):
         assert not self.dateIndex.is_numeric()
         assert not self.strIndex.is_numeric()