fix inconsistent index naming with union/intersect #35847

doc/source/user_guide/merging.rst

@@ -154,6 +154,14 @@ functionality below.
    frames = [ process_your_file(f) for f in files ]
    result = pd.concat(frames)
 
+.. note::
+
+   When concatenating DataFrames with named axes, pandas will attempt to preserve
+   these index/column names whenever possible. In the case where all inputs share a
+   common name, this name will be assigned to the result. When the input names do
+   not all agree, the result will be unnamed. The same is true for :class:`MultiIndex`,
+   but the logic is applied separately on a level-by-level basis.
+
 
 Set logic on the other axes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~

doc/source/whatsnew/v1.2.0.rst

@@ -157,6 +157,26 @@ Alternatively, you can also use the dtype object:
    behaviour or API may still change without warning. Expecially the behaviour
    regarding NaN (distinct from NA missing values) is subject to change.
 
+.. _whatsnew_120.index_name_preservation:
+
+Index/column name preservation when aggregating
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When aggregating using :meth:`concat` or the :class:`DataFrame` constructor, Pandas
+will attempt to preserve index (and column) names whenever possible (:issue:`35847`).
+In the case where all inputs share a common name, this name will be assigned to the
+result. When the input names do not all agree, the result will be unnamed. Here is an
+example where the index name is preserved:
+
+.. ipython:: python
+
+    idx = pd.Index(range(5), name='abc')
+    ser = pd.Series(range(5, 10), index=idx)
+    pd.concat({'x': ser[1:], 'y': ser[:-1]}, axis=1)
+
+The same is true for :class:`MultiIndex`, but the logic is applied separately on a
+level-by-level basis.
+
 .. _whatsnew_120.enhancements.other:
 
 Other enhancements

pandas/core/indexes/api.py

@@ -4,12 +4,12 @@
 from pandas._libs import NaT, lib
 from pandas.errors import InvalidIndexError
 
-import pandas.core.common as com
 from pandas.core.indexes.base import (
     Index,
     _new_Index,
     ensure_index,
     ensure_index_from_sequences,
+    get_unanimous_names,
 )
 from pandas.core.indexes.category import CategoricalIndex
 from pandas.core.indexes.datetimes import DatetimeIndex
@@ -57,7 +57,7 @@
     "ensure_index_from_sequences",
     "get_objs_combined_axis",
     "union_indexes",
-    "get_consensus_names",
+    "get_unanimous_names",
     "all_indexes_same",
 ]
 
@@ -221,9 +221,9 @@ def conv(i):
         if not all(index.equals(other) for other in indexes[1:]):
             index = _unique_indices(indexes)
 
-        name = get_consensus_names(indexes)[0]
+        name = get_unanimous_names(*indexes)[0]
         if name != index.name:
-            index = index._shallow_copy(name=name)
+            index = index.rename(name)
         return index
     else:  # kind='list'
         return _unique_indices(indexes)
@@ -267,30 +267,6 @@ def _sanitize_and_check(indexes):
         return indexes, "array"
 
 
-def get_consensus_names(indexes):
-    """
-    Give a consensus 'names' to indexes.
-
-    If there's exactly one non-empty 'names', return this,
-    otherwise, return empty.
-
-    Parameters
-    ----------
-    indexes : list of Index objects
-
-    Returns
-    -------
-    list
-        A list representing the consensus 'names' found.
-    """
-    # find the non-none names, need to tupleify to make
-    # the set hashable, then reverse on return
-    consensus_names = {tuple(i.names) for i in indexes if com.any_not_none(*i.names)}
-    if len(consensus_names) == 1:
-        return list(list(consensus_names)[0])
-    return [None] * indexes[0].nlevels
-
-
 def all_indexes_same(indexes):
     """
     Determine if all indexes contain the same elements.

pandas/core/indexes/base.py

@@ -1,5 +1,6 @@
 from copy import copy as copy_func
 from datetime import datetime
+from itertools import zip_longest
 import operator
 from textwrap import dedent
 from typing import (
@@ -11,6 +12,7 @@
     List,
     Optional,
     Sequence,
+    Tuple,
     TypeVar,
     Union,
 )
@@ -2525,7 +2527,7 @@ def _get_reconciled_name_object(self, other):
         """
         name = get_op_result_name(self, other)
         if self.name != name:
-            return self._shallow_copy(name=name)
+            return self.rename(name)
         return self
 
     def _union_incompatible_dtypes(self, other, sort):
@@ -2633,7 +2635,9 @@ def union(self, other, sort=None):
         if not self._can_union_without_object_cast(other):
             return self._union_incompatible_dtypes(other, sort=sort)
 
-        return self._union(other, sort=sort)
+        result = self._union(other, sort=sort)
+
+        return self._wrap_setop_result(other, result)
 
     def _union(self, other, sort):
         """
@@ -2655,10 +2659,10 @@ def _union(self, other, sort):
         Index
         """
         if not len(other) or self.equals(other):
-            return self._get_reconciled_name_object(other)
+            return self
 
         if not len(self):
-            return other._get_reconciled_name_object(self)
+            return other
 
         # TODO(EA): setops-refactor, clean all this up
         lvals = self._values
@@ -2700,12 +2704,16 @@ def _union(self, other, sort):
                         stacklevel=3,
                     )
 
-        # for subclasses
-        return self._wrap_setop_result(other, result)
+        return self._shallow_copy(result)
 
     def _wrap_setop_result(self, other, result):
         name = get_op_result_name(self, other)
-        return self._shallow_copy(result, name=name)
+        if isinstance(result, Index):
+            if result.name != name:
+                return result.rename(name)
+            return result
+        else:
+            return self._shallow_copy(result, name=name)
 
     # TODO: standardize return type of non-union setops type(self vs other)
     def intersection(self, other, sort=False):
@@ -2775,15 +2783,12 @@ def intersection(self, other, sort=False):
             indexer = algos.unique1d(Index(rvals).get_indexer_non_unique(lvals)[0])
             indexer = indexer[indexer != -1]
 
-        taken = other.take(indexer)
-        res_name = get_op_result_name(self, other)
+        result = other.take(indexer)
 
         if sort is None:
-            taken = algos.safe_sort(taken.values)
-            return self._shallow_copy(taken, name=res_name)
+            result = algos.safe_sort(result.values)
 
-        taken.name = res_name
-        return taken
+        return self._wrap_setop_result(other, result)
 
     def difference(self, other, sort=None):
         """
@@ -5968,3 +5973,22 @@ def _maybe_asobject(dtype, klass, data, copy: bool, name: Label, **kwargs):
         return index.astype(object)
 
     return klass(data, dtype=dtype, copy=copy, name=name, **kwargs)
+
+
+def get_unanimous_names(*indexes: Index) -> Tuple[Label, ...]:
+    """
+    Return common name if all indices agree, otherwise None (level-by-level).
+
+    Parameters
+    ----------
+    indexes : list of Index objects
+
+    Returns
+    -------
+    list
+        A list representing the unanimous 'names' found.
+    """
+    name_tups = [tuple(i.names) for i in indexes]
+    name_sets = [{*ns} for ns in zip_longest(*name_tups)]
+    names = tuple(ns.pop() if len(ns) == 1 else None for ns in name_sets)
+    return names

pandas/core/indexes/datetimelike.py

@@ -719,33 +719,29 @@ def intersection(self, other, sort=False):
         """
         self._validate_sort_keyword(sort)
         self._assert_can_do_setop(other)
-        res_name = get_op_result_name(self, other)
 
         if self.equals(other):
             return self._get_reconciled_name_object(other)
 
         if len(self) == 0:
-            return self.copy()
+            return self.copy()._get_reconciled_name_object(other)
         if len(other) == 0:
-            return other.copy()
+            return other.copy()._get_reconciled_name_object(self)
 
         if not isinstance(other, type(self)):
             result = Index.intersection(self, other, sort=sort)
             if isinstance(result, type(self)):
                 if result.freq is None:
                     # TODO: no tests rely on this; needed?
                     result = result._with_freq("infer")
-            result.name = res_name
             return result
 
         elif not self._can_fast_intersect(other):
             result = Index.intersection(self, other, sort=sort)
             # We need to invalidate the freq because Index.intersection
             #  uses _shallow_copy on a view of self._data, which will preserve
             #  self.freq if we're not careful.
-            result = result._with_freq(None)._with_freq("infer")
-            result.name = res_name
-            return result
+            return result._with_freq(None)._with_freq("infer")
 
         # to make our life easier, "sort" the two ranges
         if self[0] <= other[0]:
@@ -759,11 +755,13 @@ def intersection(self, other, sort=False):
         start = right[0]
 
         if end < start:
-            return type(self)(data=[], dtype=self.dtype, freq=self.freq, name=res_name)
+            result = type(self)(data=[], dtype=self.dtype, freq=self.freq)
         else:
             lslice = slice(*left.slice_locs(start, end))
             left_chunk = left._values[lslice]
-            return type(self)._simple_new(left_chunk, name=res_name)
+            result = type(self)._simple_new(left_chunk)
+
+        return self._wrap_setop_result(other, result)
 
     def _can_fast_intersect(self: _T, other: _T) -> bool:
         if self.freq is None:
@@ -858,7 +856,7 @@ def _fast_union(self, other, sort=None):
             # The can_fast_union check ensures that the result.freq
             #  should match self.freq
             dates = type(self._data)(dates, freq=self.freq)
-            result = type(self)._simple_new(dates, name=self.name)
+            result = type(self)._simple_new(dates)
             return result
         else:
             return left
@@ -883,8 +881,8 @@ def _union(self, other, sort):
                 result = result._with_freq("infer")
             return result
         else:
-            i8self = Int64Index._simple_new(self.asi8, name=self.name)
-            i8other = Int64Index._simple_new(other.asi8, name=other.name)
+            i8self = Int64Index._simple_new(self.asi8)
+            i8other = Int64Index._simple_new(other.asi8)
             i8result = i8self._union(i8other, sort=sort)
             result = type(self)(i8result, dtype=self.dtype, freq="infer")
             return result

pandas/core/indexes/datetimes.py

@@ -25,7 +25,7 @@
 
 from pandas.core.arrays.datetimes import DatetimeArray, tz_to_dtype
 import pandas.core.common as com
-from pandas.core.indexes.base import Index, maybe_extract_name
+from pandas.core.indexes.base import Index, get_unanimous_names, maybe_extract_name
 from pandas.core.indexes.datetimelike import DatetimeTimedeltaMixin
 from pandas.core.indexes.extension import inherit_names
 from pandas.core.tools.times import to_time
@@ -405,6 +405,10 @@ def union_many(self, others):
                 this = this._fast_union(other)
             else:
                 this = Index.union(this, other)
+
+        res_name = get_unanimous_names(self, *others)[0]
+        if this.name != res_name:
+            return this.rename(res_name)
         return this
 
     # --------------------------------------------------------------------

pandas/core/indexes/interval.py

@@ -1023,7 +1023,7 @@ def intersection(
         if sort is None:
             taken = taken.sort_values()
 
-        return taken
+        return self._wrap_setop_result(other, taken)
 
     def _intersection_unique(self, other: "IntervalIndex") -> "IntervalIndex":
         """

pandas/core/indexes/multi.py

@@ -47,7 +47,12 @@
 from pandas.core.arrays.categorical import factorize_from_iterables
 import pandas.core.common as com
 import pandas.core.indexes.base as ibase
-from pandas.core.indexes.base import Index, _index_shared_docs, ensure_index
+from pandas.core.indexes.base import (
+    Index,
+    _index_shared_docs,
+    ensure_index,
+    get_unanimous_names,
+)
 from pandas.core.indexes.frozen import FrozenList
 from pandas.core.indexes.numeric import Int64Index
 import pandas.core.missing as missing
@@ -3426,7 +3431,7 @@ def union(self, other, sort=None):
         other, result_names = self._convert_can_do_setop(other)
 
         if len(other) == 0 or self.equals(other):
-            return self
+            return self.rename(result_names)
 
         # TODO: Index.union returns other when `len(self)` is 0.
 
@@ -3468,7 +3473,7 @@ def intersection(self, other, sort=False):
         other, result_names = self._convert_can_do_setop(other)
 
         if self.equals(other):
-            return self
+            return self.rename(result_names)
 
         if not is_object_dtype(other.dtype):
             # The intersection is empty
@@ -3539,7 +3544,7 @@ def difference(self, other, sort=None):
         other, result_names = self._convert_can_do_setop(other)
 
         if len(other) == 0:
-            return self
+            return self.rename(result_names)
 
         if self.equals(other):
             return MultiIndex(
@@ -3587,7 +3592,8 @@ def _convert_can_do_setop(self, other):
                 except TypeError as err:
                     raise TypeError(msg) from err
         else:
-            result_names = self.names if self.names == other.names else None
+            result_names = get_unanimous_names(self, other)
+
         return other, result_names
 
     # --------------------------------------------------------------------

pandas/core/indexes/range.py

@@ -539,7 +539,8 @@ def intersection(self, other, sort=False):
             new_index = new_index[::-1]
         if sort is None:
             new_index = new_index.sort_values()
-        return new_index
+
+        return self._wrap_setop_result(other, new_index)
 
     def _min_fitting_element(self, lower_limit: int) -> int:
         """Returns the smallest element greater than or equal to the limit"""

pandas/core/reshape/concat.py

@@ -23,8 +23,8 @@
     MultiIndex,
     all_indexes_same,
     ensure_index,
-    get_consensus_names,
     get_objs_combined_axis,
+    get_unanimous_names,
 )
 import pandas.core.indexes.base as ibase
 from pandas.core.internals import concatenate_block_managers
@@ -655,7 +655,7 @@ def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiInde
                 )
 
             # also copies
-            names = names + get_consensus_names(indexes)
+            names = list(names) + list(get_unanimous_names(*indexes))
 
         return MultiIndex(
             levels=levels, codes=codes_list, names=names, verify_integrity=False