API: change Index set ops sort=True -> sort=None

doc/source/whatsnew/v0.24.1.rst

@@ -15,10 +15,34 @@ Whats New in 0.24.1 (February XX, 2019)
 These are the changes in pandas 0.24.1. See :ref:`release` for a full changelog
 including other versions of pandas.
 
+.. _whatsnew_0241.api:
+
+API Changes
+~~~~~~~~~~~
+
+Changing the ``sort`` parameter for :class:`Index` set operations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The default ``sort`` value for :meth:`Index.union` has changed from ``True`` to ``None`` (:issue:`24959`).
+The default *behavior*, however, remains the same: the result is sorted, unless
+
+1. ``self`` and ``other`` are identical
+2. ``self`` or ``other`` is empty
+3. ``self`` or ``other`` contain values that can not be compared (a ``RuntimeWarning`` is raised).
+
+This change will allow to preserve ``sort=True`` to mean "always sort" in a future release.
+
+The same change applies to :meth:`Index.difference` and :meth:`Index.symmetric_difference`, which
+would do not sort the result when the values could not be compared.
+
+For :meth:`Index.intersection` the option of ``sort=True`` is also renamed
+to ``sort=None`` (but for :meth:`Index.intersection` it is not the default), as
+the result is not sorted when ``self`` and ``other`` were identical.
+
 .. _whatsnew_0241.regressions:
 
 Fixed Regressions
-^^^^^^^^^^^^^^^^^
+~~~~~~~~~~~~~~~~~
 
 - Bug in :meth:`DataFrame.itertuples` with ``records`` orient raising an ``AttributeError`` when the ``DataFrame`` contained more than 255 columns (:issue:`24939`)
 - Bug in :meth:`DataFrame.itertuples` orient converting integer column names to strings prepended with an underscore (:issue:`24940`)
@@ -28,7 +52,7 @@ Fixed Regressions
 .. _whatsnew_0241.enhancements:
 
 Enhancements
-^^^^^^^^^^^^
+~~~~~~~~~~~~
 
 
 .. _whatsnew_0241.bug_fixes:

pandas/_libs/lib.pyx

@@ -233,10 +233,11 @@ def fast_unique_multiple(list arrays, sort: bool=True):
             if val not in table:
                 table[val] = stub
                 uniques.append(val)
-    if sort:
+    if sort is None:
         try:
             uniques.sort()
         except Exception:
+            # TODO: RuntimeWarning?
             pass
 
     return uniques

pandas/core/indexes/api.py

@@ -112,7 +112,7 @@ def _get_combined_index(indexes, intersect=False, sort=False):
     elif intersect:
         index = indexes[0]
         for other in indexes[1:]:
-            index = index.intersection(other, sort=sort)
+            index = index.intersection(other)
     else:
         index = _union_indexes(indexes, sort=sort)
         index = ensure_index(index)

pandas/core/indexes/base.py

@@ -2245,18 +2245,37 @@ def _get_reconciled_name_object(self, other):
             return self._shallow_copy(name=name)
         return self
 
-    def union(self, other, sort=True):
+    def _validate_sort_keyword(self, sort):
+        if sort not in [None, False]:
+            raise ValueError("The 'sort' keyword only takes the values of "
+                             "None or False; {0} was passed.".format(sort))
+
+    def union(self, other, sort=None):
         """
         Form the union of two Index objects.
 
         Parameters
         ----------
         other : Index or array-like
-        sort : bool, default True
-            Sort the resulting index if possible
+        sort : bool or None, default None
+            Whether to sort the resulting Index.
+
+            * None : Sort the result, except when
+
+              1. `self` and `other` are equal.
+              2. `self` or `other` has length 0.
+              3. Some values in `self` or `other` cannot be compared.
+                 A RuntimeWarning is issued in this case.
+
+            * False : do not sort the result.
 
             .. versionadded:: 0.24.0
 
+            .. versionchanged:: 0.24.1
+
+               Changed the default `sort` from True to None (without
+               change in behaviour).
+
         Returns
         -------
         union : Index
@@ -2269,6 +2288,7 @@ def union(self, other, sort=True):
         >>> idx1.union(idx2)
         Int64Index([1, 2, 3, 4, 5, 6], dtype='int64')
         """
+        self._validate_sort_keyword(sort)
         self._assert_can_do_setop(other)
         other = ensure_index(other)
 
@@ -2319,7 +2339,7 @@ def union(self, other, sort=True):
             else:
                 result = lvals
 
-            if sort:
+            if sort is None:
                 try:
                     result = sorting.safe_sort(result)
                 except TypeError as e:
@@ -2342,8 +2362,12 @@ def intersection(self, other, sort=False):
         Parameters
         ----------
         other : Index or array-like
-        sort : bool, default False
-            Sort the resulting index if possible
+        sort : False or None, default False
+            Whether to sort the resulting index.
+
+            * False : do not sort the result.
+            * None : sort the result, except when `self` and `other` are equal
+              or when the values cannot be compared.
 
             .. versionadded:: 0.24.0
 
@@ -2363,6 +2387,7 @@ def intersection(self, other, sort=False):
         >>> idx1.intersection(idx2)
         Int64Index([3, 4], dtype='int64')
         """
+        self._validate_sort_keyword(sort)
         self._assert_can_do_setop(other)
         other = ensure_index(other)
 
@@ -2402,7 +2427,7 @@ def intersection(self, other, sort=False):
 
         taken = other.take(indexer)
 
-        if sort:
+        if sort is None:
             taken = sorting.safe_sort(taken.values)
             if self.name != other.name:
                 name = None
@@ -2415,7 +2440,7 @@ def intersection(self, other, sort=False):
 
         return taken
 
-    def difference(self, other, sort=True):
+    def difference(self, other, sort=None):
         """
         Return a new Index with elements from the index that are not in
         `other`.
@@ -2425,11 +2450,22 @@ def difference(self, other, sort=True):
         Parameters
         ----------
         other : Index or array-like
-        sort : bool, default True
-            Sort the resulting index if possible
+        sort : False or None, default None
+            Whether to sort the resulting index. By default, the
+            values are attempted to be sorted, but any TypeError from
+            incomparable elements is caught by pandas.
+
+            * None : Attempt to sort the result, but catch any TypeErrors
+              from comparing incomparable elements.
+            * False : Do not sort the result.
 
             .. versionadded:: 0.24.0
 
+            .. versionchanged:: 0.24.1
+
+               Changed `True` to `None`, which matches the behavior of
+               pandas 0.23.4 and earlier.
+
         Returns
         -------
         difference : Index
@@ -2444,6 +2480,7 @@ def difference(self, other, sort=True):
         >>> idx1.difference(idx2, sort=False)
         Int64Index([2, 1], dtype='int64')
         """
+        self._validate_sort_keyword(sort)
         self._assert_can_do_setop(other)
 
         if self.equals(other):
@@ -2460,27 +2497,38 @@ def difference(self, other, sort=True):
         label_diff = np.setdiff1d(np.arange(this.size), indexer,
                                   assume_unique=True)
         the_diff = this.values.take(label_diff)
-        if sort:
+        if sort is None:
             try:
                 the_diff = sorting.safe_sort(the_diff)
             except TypeError:
                 pass
 
         return this._shallow_copy(the_diff, name=result_name, freq=None)
 
-    def symmetric_difference(self, other, result_name=None, sort=True):
+    def symmetric_difference(self, other, result_name=None, sort=None):
         """
         Compute the symmetric difference of two Index objects.
 
         Parameters
         ----------
         other : Index or array-like
         result_name : str
-        sort : bool, default True
-            Sort the resulting index if possible
+        sort : False or None, default None
+            Whether to sort the resulting index. By default, the
+            values are attempted to be sorted, but any TypeError from
+            incomparable elements is caught by pandas.
+
+            * None : Attempt to sort the result, but catch any TypeErrors
+              from comparing incomparable elements.
+            * False : Do not sort the result.
 
             .. versionadded:: 0.24.0
 
+            .. versionchanged:: 0.24.1
+
+               Changed `True` to `None`, which matches the behavior of
+               pandas 0.23.4 and earlier.
+
         Returns
         -------
         symmetric_difference : Index
@@ -2504,6 +2552,7 @@ def symmetric_difference(self, other, result_name=None, sort=True):
         >>> idx1 ^ idx2
         Int64Index([1, 5], dtype='int64')
         """
+        self._validate_sort_keyword(sort)
         self._assert_can_do_setop(other)
         other, result_name_update = self._convert_can_do_setop(other)
         if result_name is None:
@@ -2524,7 +2573,7 @@ def symmetric_difference(self, other, result_name=None, sort=True):
         right_diff = other.values.take(right_indexer)
 
         the_diff = _concat._concat_compat([left_diff, right_diff])
-        if sort:
+        if sort is None:
             try:
                 the_diff = sorting.safe_sort(the_diff)
             except TypeError:

pandas/core/indexes/datetimes.py

@@ -602,19 +602,21 @@ def intersection(self, other, sort=False):
         Parameters
         ----------
         other : DatetimeIndex or array-like
-        sort : bool, default True
+        sort : False or None, default False
             Sort the resulting index if possible.
 
             .. versionadded:: 0.24.0
 
             .. versionchanged:: 0.24.1
 
-               Changed the default from ``True`` to ``False``.
+               Changed the default to ``False`` to match the behaviour
+               from before 0.24.0.
 
         Returns
         -------
         y : Index or DatetimeIndex
         """
+        self._validate_sort_keyword(sort)
         self._assert_can_do_setop(other)
 
         if self.equals(other):

pandas/core/indexes/interval.py

@@ -1093,7 +1093,7 @@ def equals(self, other):
     def overlaps(self, other):
         return self._data.overlaps(other)
 
-    def _setop(op_name, sort=True):
+    def _setop(op_name, sort=None):
         def func(self, other, sort=sort):
             other = self._as_like_interval_index(other)