BUG: names on union and intersection for Index were inconsistent (GH9943 GH9862)

doc/source/whatsnew/v0.24.0.txt

@@ -411,7 +411,7 @@ Reshaping
 - Bug in :func:`pandas.concat` when joining resampled DataFrames with timezone aware index (:issue:`13783`)
 - Bug in :meth:`Series.combine_first` with ``datetime64[ns, tz]`` dtype which would return tz-naive result (:issue:`21469`)
 - Bug in :meth:`Series.where` and :meth:`DataFrame.where` with ``datetime64[ns, tz]`` dtype (:issue:`21546`)
--
+- 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`)
 -
 
 Build Changes
@@ -427,4 +427,3 @@ Other
 - Require at least 0.28.2 version of ``cython`` to support read-only memoryviews (:issue:`21688`)
 -
 -
--

pandas/core/indexes/base.py

@@ -61,7 +61,7 @@
 import pandas.core.sorting as sorting
 from pandas.io.formats.printing import (
     pprint_thing, default_pprint, format_object_summary, format_object_attrs)
-from pandas.core.ops import make_invalid_op
+from pandas.core.ops import make_invalid_op, get_op_result_name
 from pandas.core.strings import StringMethods
 
 __all__ = ['Index']
@@ -1200,7 +1200,7 @@ def _convert_can_do_setop(self, other):
             other = Index(other, name=self.name)
             result_name = self.name
         else:
-            result_name = self.name if self.name == other.name else None
+            result_name = get_op_result_name(self, other)
         return other, result_name
 
     def _convert_for_op(self, value):
@@ -2643,19 +2643,15 @@ def __or__(self, other):
     def __xor__(self, other):
         return self.symmetric_difference(other)
 
-    def _get_consensus_name(self, other):
+    def _get_reconciled_name_object(self, other):
         """
-        Given 2 indexes, give a consensus name meaning
-        we take the not None one, or None if the names differ.
-        Return a new object if we are resetting the name
+        If the result of a set operation will be self,
+        return self, unless the name changes, in which
+        case make a shallow copy of self.
         """
-        if self.name != other.name:
-            if self.name is None or other.name is None:
-                name = self.name or other.name
-            else:
-                name = None
-            if self.name != name:
-                return self._shallow_copy(name=name)
+        name = get_op_result_name(self, other)
+        if self.name != name:
+            return self._shallow_copy(name=name)
         return self
 
     def union(self, other):
@@ -2683,10 +2679,10 @@ def union(self, other):
         other = _ensure_index(other)
 
         if len(other) == 0 or self.equals(other):
-            return self._get_consensus_name(other)
+            return self._get_reconciled_name_object(other)
 
         if len(self) == 0:
-            return other._get_consensus_name(self)
+            return other._get_reconciled_name_object(self)
 
         # TODO: is_dtype_union_equal is a hack around
         # 1. buggy set ops with duplicates (GH #13432)
@@ -2749,11 +2745,10 @@ def union(self, other):
                                   stacklevel=3)
 
         # for subclasses
-        return self._wrap_union_result(other, result)
+        return self._wrap_setop_result(other, result)
 
-    def _wrap_union_result(self, other, result):
-        name = self.name if self.name == other.name else None
-        return self.__class__(result, name=name)
+    def _wrap_setop_result(self, other, result):
+        return self._constructor(result, name=get_op_result_name(self, other))
 
     def intersection(self, other):
         """
@@ -2783,7 +2778,7 @@ def intersection(self, other):
         other = _ensure_index(other)
 
         if self.equals(other):
-            return self._get_consensus_name(other)
+            return self._get_reconciled_name_object(other)
 
         if not is_dtype_equal(self.dtype, other.dtype):
             this = self.astype('O')
@@ -2803,7 +2798,7 @@ def intersection(self, other):
         if self.is_monotonic and other.is_monotonic:
             try:
                 result = self._inner_indexer(lvals, rvals)[0]
-                return self._wrap_union_result(other, result)
+                return self._wrap_setop_result(other, result)
             except TypeError:
                 pass
 
@@ -4072,7 +4067,7 @@ def _join_monotonic(self, other, how='left', return_indexers=False):
             return join_index
 
     def _wrap_joined_index(self, joined, other):
-        name = self.name if self.name == other.name else None
+        name = get_op_result_name(self, other)
         return Index(joined, name=name)
 
     def _get_string_slice(self, key, use_lhs=True, use_rhs=True):

pandas/core/indexes/category.py

@@ -24,6 +24,7 @@
 import pandas.core.common as com
 import pandas.core.missing as missing
 import pandas.core.indexes.base as ibase
+from pandas.core.ops import get_op_result_name
 from pandas.core.arrays.categorical import Categorical, contains
 
 _index_doc_kwargs = dict(ibase._index_doc_kwargs)
@@ -298,6 +299,10 @@ def itemsize(self):
         # Size of the items in categories, not codes.
         return self.values.itemsize
 
+    def _wrap_setop_result(self, other, result):
+        name = get_op_result_name(self, other)
+        return self._shallow_copy(result, name=name)
+
     def get_values(self):
         """ return the underlying data as an ndarray """
         return self._data.get_values()

pandas/core/indexes/datetimes.py

@@ -37,6 +37,7 @@
 
 from pandas.core.indexes.base import Index, _index_shared_docs
 from pandas.core.indexes.numeric import Int64Index, Float64Index
+from pandas.core.ops import get_op_result_name
 import pandas.compat as compat
 from pandas.tseries.frequencies import to_offset, get_period_alias, Resolution
 from pandas.core.indexes.datetimelike import (
@@ -1018,6 +1019,10 @@ def union(self, other):
         y : Index or DatetimeIndex
         """
         self._assert_can_do_setop(other)
+
+        if len(other) == 0 or self.equals(other) or len(self) == 0:
+            return super(DatetimeIndex, self).union(other)
+
         if not isinstance(other, DatetimeIndex):
             try:
                 other = DatetimeIndex(other)
@@ -1119,7 +1124,7 @@ def _maybe_utc_convert(self, other):
         return this, other
 
     def _wrap_joined_index(self, joined, other):
-        name = self.name if self.name == other.name else None
+        name = get_op_result_name(self, other)
         if (isinstance(other, DatetimeIndex) and
                 self.freq == other.freq and
                 self._can_fast_union(other)):
@@ -1193,11 +1198,11 @@ def _fast_union(self, other):
                               end=max(left_end, right_end),
                               freq=left.freq)
 
-    def _wrap_union_result(self, other, result):
-        name = self.name if self.name == other.name else None
+    def _wrap_setop_result(self, other, result):
+        name = get_op_result_name(self, other)
         if not timezones.tz_compare(self.tz, other.tz):
             raise ValueError('Passed item and index have different timezone')
-        return self._simple_new(result, name=name, freq=None, tz=self.tz)
+        return self._shallow_copy(result, name=name, freq=None, tz=self.tz)
 
     def intersection(self, other):
         """
@@ -1213,6 +1218,10 @@ def intersection(self, other):
         y : Index or DatetimeIndex
         """
         self._assert_can_do_setop(other)
+
+        if self.equals(other):
+            return self._get_reconciled_name_object(other)
+
         if not isinstance(other, DatetimeIndex):
             try:
                 other = DatetimeIndex(other)

pandas/core/indexes/interval.py

@@ -27,6 +27,7 @@
 from pandas.core.indexes.base import (
     Index, _ensure_index,
     default_pprint, _index_shared_docs)
+from pandas.core.ops import get_op_result_name
 
 from pandas._libs import Timestamp, Timedelta
 from pandas._libs.interval import (
@@ -1432,7 +1433,7 @@ def func(self, other):
                 raise TypeError(msg.format(op=op_name))
 
             result = getattr(self._multiindex, op_name)(other._multiindex)
-            result_name = self.name if self.name == other.name else None
+            result_name = get_op_result_name(self, other)
 
             # GH 19101: ensure empty results have correct dtype
             if result.empty:

pandas/core/indexes/numeric.py

@@ -18,7 +18,7 @@
 from pandas.util._decorators import Appender, cache_readonly
 import pandas.core.dtypes.concat as _concat
 import pandas.core.indexes.base as ibase
-
+from pandas.core.ops import get_op_result_name
 
 _num_index_shared_docs = dict()
 
@@ -187,7 +187,7 @@ def _convert_scalar_indexer(self, key, kind=None):
                 ._convert_scalar_indexer(key, kind=kind))
 
     def _wrap_joined_index(self, joined, other):
-        name = self.name if self.name == other.name else None
+        name = get_op_result_name(self, other)
         return Int64Index(joined, name=name)
 
     @classmethod
@@ -264,7 +264,7 @@ def _convert_index_indexer(self, keyarr):
         return keyarr
 
     def _wrap_joined_index(self, joined, other):
-        name = self.name if self.name == other.name else None
+        name = get_op_result_name(self, other)
         return UInt64Index(joined, name=name)
 
     @classmethod

pandas/core/indexes/period.py

@@ -17,6 +17,7 @@
     pandas_dtype,
     _ensure_object)
 from pandas.core.dtypes.generic import ABCSeries
+from pandas.core.ops import get_op_result_name
 
 import pandas.tseries.frequencies as frequencies
 from pandas.tseries.frequencies import get_freq_code as _gfc
@@ -767,8 +768,8 @@ def _assert_can_do_setop(self, other):
             msg = DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
             raise IncompatibleFrequency(msg)
 
-    def _wrap_union_result(self, other, result):
-        name = self.name if self.name == other.name else None
+    def _wrap_setop_result(self, other, result):
+        name = get_op_result_name(self, other)
         result = self._apply_meta(result)
         result.name = name
         return result

pandas/core/indexes/range.py

@@ -260,7 +260,8 @@ def tolist(self):
     @Appender(_index_shared_docs['_shallow_copy'])
     def _shallow_copy(self, values=None, **kwargs):
         if values is None:
-            return RangeIndex(name=self.name, fastpath=True,
+            name = kwargs.get("name", self.name)
+            return RangeIndex(name=name, fastpath=True,
                               **dict(self._get_data_as_items()))
         else:
             kwargs.setdefault('name', self.name)
@@ -341,6 +342,10 @@ def intersection(self, other):
         -------
         intersection : Index
         """
+
+        if self.equals(other):
+            return self._get_reconciled_name_object(other)
+
         if not isinstance(other, RangeIndex):
             return super(RangeIndex, self).intersection(other)
 
@@ -421,10 +426,9 @@ def union(self, other):
         union : Index
         """
         self._assert_can_do_setop(other)
-        if len(other) == 0 or self.equals(other):
-            return self
-        if len(self) == 0:
-            return other
+        if len(other) == 0 or self.equals(other) or len(self) == 0:
+            return super(RangeIndex, self).union(other)
+
         if isinstance(other, RangeIndex):
             start_s, step_s = self._start, self._step
             end_s = self._start + self._step * (len(self) - 1)

pandas/core/indexes/timedeltas.py

@@ -27,6 +27,7 @@
 from pandas.core.base import _shared_docs
 from pandas.core.indexes.base import _index_shared_docs
 import pandas.core.common as com
+from pandas.core.ops import get_op_result_name
 import pandas.core.dtypes.concat as _concat
 from pandas.util._decorators import Appender, Substitution, deprecate_kwarg
 from pandas.core.indexes.datetimelike import (
@@ -497,6 +498,10 @@ def union(self, other):
         y : Index or TimedeltaIndex
         """
         self._assert_can_do_setop(other)
+
+        if len(other) == 0 or self.equals(other) or len(self) == 0:
+            return super(TimedeltaIndex, self).union(other)
+
         if not isinstance(other, TimedeltaIndex):
             try:
                 other = TimedeltaIndex(other)
@@ -529,7 +534,7 @@ def join(self, other, how='left', level=None, return_indexers=False,
                           sort=sort)
 
     def _wrap_joined_index(self, joined, other):
-        name = self.name if self.name == other.name else None
+        name = get_op_result_name(self, other)
         if (isinstance(other, TimedeltaIndex) and self.freq == other.freq and
                 self._can_fast_union(other)):
             joined = self._shallow_copy(joined, name=name)
@@ -589,10 +594,6 @@ def _fast_union(self, other):
         else:
             return left
 
-    def _wrap_union_result(self, other, result):
-        name = self.name if self.name == other.name else None
-        return self._simple_new(result, name=name, freq=None)
-
     def intersection(self, other):
         """
         Specialized intersection for TimedeltaIndex objects. May be much faster
@@ -607,6 +608,10 @@ def intersection(self, other):
         y : Index or TimedeltaIndex
         """
         self._assert_can_do_setop(other)
+
+        if self.equals(other):
+            return self._get_reconciled_name_object(other)
+
         if not isinstance(other, TimedeltaIndex):
             try:
                 other = TimedeltaIndex(other)

pandas/tests/indexes/conftest.py

@@ -14,6 +14,7 @@
                         tm.makeTimedeltaIndex(100),
                         tm.makeIntIndex(100),
                         tm.makeUIntIndex(100),
+                        tm.makeRangeIndex(100),
                         tm.makeFloatIndex(100),
                         Index([True, False]),
                         tm.makeCategoricalIndex(100),