BUG: fix date_range inclusive filtering with periods (#46331)

When date_range was called with start+periods or end+periods (without
the other endpoint), the inclusive parameter failed to filter the
unspecified endpoint because Timestamp(None) produced NaT, causing the
endpoint comparison to always fail.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jbrockmendel

doc/source/whatsnew/v3.1.0.rst

@@ -159,6 +159,7 @@ Datetimelike
 ^^^^^^^^^^^^
 - Bug in :class:`Timestamp` constructor where passing ``np.str_`` objects would fail in Cython string parsing (:issue:`48974`)
 - Bug in :class:`Timestamp` constructor, :class:`Timedelta` constructor, :func:`to_datetime`, and :func:`to_timedelta` with non-round ``float`` input and ``unit`` failing to raise when the value is just outside the representable bounds (:issue:`57366`)
+- Bug in :func:`date_range` where ``inclusive`` parameter failed to filter endpoints when only ``start`` and ``periods`` or ``end`` and ``periods`` were specified (:issue:`46331`)
 - Bug in :func:`to_datetime` and :func:`to_timedelta` on ARM platforms where round ``float`` values outside the int64 domain (e.g. ``float(2**63)``) could silently produce incorrect results instead of raising (:issue:`64619`)
 - Bug in :func:`to_datetime` and :func:`to_timedelta` where ``uint64`` values greater than ``int64`` max silently overflowed instead of raising :class:`OutOfBoundsDatetime` or :class:`OutOfBoundsTimedelta` (:issue:`60677`)
 - Bug in :meth:`DatetimeArray.isin` and :meth:`TimedeltaArray.isin` where mismatched resolutions could silently truncate finer-resolution values, leading to false matches (:issue:`64545`)

pandas/core/arrays/datetimes.py

@@ -551,11 +551,11 @@ def _generate_range(
         if start == end:
             if not left_inclusive and not right_inclusive:
                 i8values = i8values[1:-1]
-        else:
-            start_i8 = Timestamp(start)._value
-            end_i8 = Timestamp(end)._value
-            if not left_inclusive or not right_inclusive:
-                if not left_inclusive and len(i8values) and i8values[0] == start_i8:
+        elif not left_inclusive or not right_inclusive:
+            if len(i8values):
+                start_i8 = Timestamp(start)._value if start is not None else i8values[0]
+                end_i8 = Timestamp(end)._value if end is not None else i8values[-1]
+                if not left_inclusive and i8values[0] == start_i8:
                     i8values = i8values[1:]
                 if not right_inclusive and len(i8values) and i8values[-1] == end_i8:
                     i8values = i8values[:-1]

pandas/tests/indexes/datetimes/test_date_range.py

@@ -743,6 +743,38 @@ def test_range_where_start_equal_end(self, inclusive_endpoints_fixture):
 
         tm.assert_index_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "inclusive, expected_values",
+        [
+            ("both", ["2020-06-01", "2020-06-02", "2020-06-03", "2020-06-04"]),
+            ("left", ["2020-06-01", "2020-06-02", "2020-06-03"]),
+            ("right", ["2020-06-02", "2020-06-03", "2020-06-04"]),
+            ("neither", ["2020-06-02", "2020-06-03"]),
+        ],
+    )
+    def test_inclusive_with_periods_and_start(self, inclusive, expected_values):
+        # GH#46331 - inclusive should filter endpoints even when
+        # only start+periods is provided (end is not specified)
+        result = date_range(start="2020-06-01", periods=4, inclusive=inclusive)
+        expected = DatetimeIndex(expected_values, freq="D")
+        tm.assert_index_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "inclusive, expected_values",
+        [
+            ("both", ["2020-06-01", "2020-06-02", "2020-06-03", "2020-06-04"]),
+            ("left", ["2020-06-01", "2020-06-02", "2020-06-03"]),
+            ("right", ["2020-06-02", "2020-06-03", "2020-06-04"]),
+            ("neither", ["2020-06-02", "2020-06-03"]),
+        ],
+    )
+    def test_inclusive_with_periods_and_end(self, inclusive, expected_values):
+        # GH#46331 - inclusive should filter endpoints even when
+        # only end+periods is provided (start is not specified)
+        result = date_range(end="2020-06-04", periods=4, inclusive=inclusive)
+        expected = DatetimeIndex(expected_values, freq="D")
+        tm.assert_index_equal(result, expected)
+
     def test_freq_dateoffset_with_relateivedelta_nanos(self):
         # GH 46877
         freq = DateOffset(hours=10, days=57, nanoseconds=3)