ENH: RangeIndex._shallow_copy can return RangeIndex

doc/source/whatsnew/v1.5.0.rst

@@ -277,7 +277,7 @@ Other enhancements
 - Allow reading compressed SAS files with :func:`read_sas` (e.g., ``.sas7bdat.gz`` files)
 - :meth:`DatetimeIndex.astype` now supports casting timezone-naive indexes to ``datetime64[s]``, ``datetime64[ms]``, and ``datetime64[us]``, and timezone-aware indexes to the corresponding ``datetime64[unit, tzname]`` dtypes (:issue:`47579`)
 - :class:`Series` reducers (e.g. ``min``, ``max``, ``sum``, ``mean``) will now successfully operate when the dtype is numeric and ``numeric_only=True`` is provided; previously this would raise a ``NotImplementedError`` (:issue:`47500`)
--
+- :meth:`RangeIndex.union` now can return a :class:`RangeIndex` instead of a :class:`Int64Index` if the resulting values are equally spaced (:issue:`47557`, :issue:`43885`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_150.notable_bug_fixes:
@@ -1008,7 +1008,7 @@ Reshaping
 - Bug in :func:`concat` with identical key leads to error when indexing :class:`MultiIndex` (:issue:`46519`)
 - Bug in :meth:`DataFrame.join` with a list when using suffixes to join DataFrames with duplicate column names (:issue:`46396`)
 - Bug in :meth:`DataFrame.pivot_table` with ``sort=False`` results in sorted index (:issue:`17041`)
--
+- Bug in :meth:`concat` when ``axis=1`` and ``sort=False`` where the resulting Index was a :class:`Int64Index` instead of a :class:`RangeIndex` (:issue:`46675`)
 
 Sparse
 ^^^^^^

pandas/core/indexes/range.py

@@ -19,6 +19,7 @@
     index as libindex,
     lib,
 )
+from pandas._libs.algos import unique_deltas
 from pandas._libs.lib import no_default
 from pandas._typing import (
     Dtype,
@@ -435,7 +436,13 @@ def _shallow_copy(self, values, name: Hashable = no_default):
 
         if values.dtype.kind == "f":
             return Float64Index(values, name=name)
-        return Int64Index._simple_new(values, name=name)
+        unique_diffs = unique_deltas(values)
+        if len(unique_diffs) == 1 and unique_diffs[0] != 0:
+            diff = unique_diffs[0]
+            new_range = range(values[0], values[-1] + diff, diff)
+            return type(self)._simple_new(new_range, name=name)
+        else:
+            return Int64Index._simple_new(values, name=name)
 
     def _view(self: RangeIndex) -> RangeIndex:
         result = type(self)._simple_new(self._range, name=self._name)

pandas/tests/indexes/ranges/test_setops.py

@@ -145,8 +145,9 @@ def test_union_noncomparable(self, sort):
         expected = Index(np.concatenate((other, index)))
         tm.assert_index_equal(result, expected)
 
-    @pytest.fixture(
-        params=[
+    @pytest.mark.parametrize(
+        "idx1, idx2, expected_sorted, expected_notsorted",
+        [
             (
                 RangeIndex(0, 10, 1),
                 RangeIndex(0, 10, 1),
@@ -157,13 +158,13 @@ def test_union_noncomparable(self, sort):
                 RangeIndex(0, 10, 1),
                 RangeIndex(5, 20, 1),
                 RangeIndex(0, 20, 1),
-                Int64Index(range(20)),
+                RangeIndex(0, 20, 1),
             ),
             (
                 RangeIndex(0, 10, 1),
                 RangeIndex(10, 20, 1),
                 RangeIndex(0, 20, 1),
-                Int64Index(range(20)),
+                RangeIndex(0, 20, 1),
             ),
             (
                 RangeIndex(0, -10, -1),
@@ -175,7 +176,7 @@ def test_union_noncomparable(self, sort):
                 RangeIndex(0, -10, -1),
                 RangeIndex(-10, -20, -1),
                 RangeIndex(-19, 1, 1),
-                Int64Index(range(0, -20, -1)),
+                RangeIndex(0, -20, -1),
             ),
             (
                 RangeIndex(0, 10, 2),
@@ -205,7 +206,7 @@ def test_union_noncomparable(self, sort):
                 RangeIndex(0, 100, 5),
                 RangeIndex(0, 100, 20),
                 RangeIndex(0, 100, 5),
-                Int64Index(range(0, 100, 5)),
+                RangeIndex(0, 100, 5),
             ),
             (
                 RangeIndex(0, -100, -5),
@@ -230,7 +231,7 @@ def test_union_noncomparable(self, sort):
                 RangeIndex(0, 100, 2),
                 RangeIndex(100, 150, 200),
                 RangeIndex(0, 102, 2),
-                Int64Index(range(0, 102, 2)),
+                RangeIndex(0, 102, 2),
             ),
             (
                 RangeIndex(0, -100, -2),
@@ -242,13 +243,13 @@ def test_union_noncomparable(self, sort):
                 RangeIndex(0, -100, -1),
                 RangeIndex(0, -50, -3),
                 RangeIndex(-99, 1, 1),
-                Int64Index(list(range(0, -100, -1))),
+                RangeIndex(0, -100, -1),
             ),
             (
                 RangeIndex(0, 1, 1),
                 RangeIndex(5, 6, 10),
                 RangeIndex(0, 6, 5),
-                Int64Index([0, 5]),
+                RangeIndex(0, 10, 5),
             ),
             (
                 RangeIndex(0, 10, 5),
@@ -274,16 +275,16 @@ def test_union_noncomparable(self, sort):
                 Int64Index([1, 5, 6]),
                 Int64Index([1, 5, 6]),
             ),
-        ]
+            # GH 43885
+            (
+                RangeIndex(0, 10),
+                RangeIndex(0, 5),
+                RangeIndex(0, 10),
+                RangeIndex(0, 10),
+            ),
+        ],
     )
-    def unions(self, request):
-        """Inputs and expected outputs for RangeIndex.union tests"""
-        return request.param
-
-    def test_union_sorted(self, unions):
-
-        idx1, idx2, expected_sorted, expected_notsorted = unions
-
+    def test_union_sorted(self, idx1, idx2, expected_sorted, expected_notsorted):
         res1 = idx1.union(idx2, sort=None)
         tm.assert_index_equal(res1, expected_sorted, exact=True)
 

pandas/tests/reshape/concat/test_index.py

@@ -409,3 +409,25 @@ def test_concat_index_keep_dtype(self, dtype):
             [[0, 1, 1.0], [0, 1, np.nan]], columns=Index([1, 2, 3], dtype=dtype)
         )
         tm.assert_frame_equal(result, expected)
+
+    def test_concat_axis_1_sort_false_rangeindex(self):
+        # GH 46675
+        s1 = Series(["a", "b", "c"])
+        s2 = Series(["a", "b"])
+        s3 = Series(["a", "b", "c", "d"])
+        s4 = Series([], dtype=object)
+        result = concat(
+            [s1, s2, s3, s4], sort=False, join="outer", ignore_index=False, axis=1
+        )
+        expected = DataFrame(
+            [
+                ["a"] * 3 + [np.nan],
+                ["b"] * 3 + [np.nan],
+                ["c", np.nan] * 2,
+                [np.nan] * 2 + ["d"] + [np.nan],
+            ],
+            dtype=object,
+        )
+        tm.assert_frame_equal(
+            result, expected, check_index_type=True, check_column_type=True
+        )