BUG: truncate has incorrect behavior when index has only one unique value (pandas-dev#42365)

Author: neelmraman

pandas/core/generic.py

@@ -9381,7 +9381,7 @@ def truncate(
         if before is not None and after is not None and before > after:
             raise ValueError(f"Truncate: {after} must be after {before}")
 
-        if len(ax) > 1 and ax.is_monotonic_decreasing:
+        if len(ax) > 1 and ax.is_monotonic_decreasing and ax.nunique() > 1:
             before, after = after, before
 
         slicer = [slice(None, None)] * self._AXIS_LEN

pandas/tests/frame/methods/test_truncate.py

@@ -148,3 +148,13 @@ def test_truncate_multiindex(self, frame_or_series):
             expected = expected["col"]
 
         tm.assert_equal(result, expected)
+
+    def test_truncate_index_only_one_unique_value(self, frame_or_series):
+        # GH 42365
+        obj = Series(0, index=date_range("2021-06-30", "2021-06-30")).repeat(5)
+        if frame_or_series is DataFrame:
+            obj = obj.to_frame(name="a")
+
+        truncated = obj.truncate("2021-06-28", "2021-07-01")
+
+        tm.assert_equal(truncated, obj)

pandas/tests/series/methods/test_truncate.py

@@ -55,3 +55,11 @@ def test_truncate_one_element_series(self):
 
         # the input Series and the expected Series are the same
         tm.assert_series_equal(result, series)
+
+    def test_truncate_index_only_one_unique_value(self):
+        # GH 42365
+        obj = Series(0, index=date_range("2021-06-30", "2021-06-30")).repeat(5)
+
+        truncated = obj.truncate("2021-06-28", "2021-07-01")
+
+        tm.assert_series_equal(truncated, obj)