numpy · lysnikolaou · Mar 25, 2025 · Mar 26, 2025 · Mar 26, 2025 · Mar 26, 2025
diff --git a/benchmarks/benchmarks/bench_indexing.py b/benchmarks/benchmarks/bench_indexing.py
@@ -134,6 +134,7 @@ def setup(self):
         self.m_half = np.copy(self.m_all)
         self.m_half[::2] = False
         self.m_none = np.repeat(False, 200 * 50000)
+        self.m_index_2d = np.arange(200 * 50000).reshape((100, 100000))
 
     def time_flat_bool_index_none(self):
         self.a.flat[self.m_none]
@@ -143,3 +144,21 @@ def time_flat_bool_index_half(self):
 
     def time_flat_bool_index_all(self):
         self.a.flat[self.m_all]
+
+    def time_flat_fancy_index_2d(self):
+        self.a.flat[self.m_index_2d]
+
+    def time_flat_empty_tuple_index(self):
+        self.a.flat[()]
+
+    def time_flat_ellipsis_index(self):
+        self.a.flat[...]
+
+    def time_flat_bool_index_0d(self):
+        self.a.flat[True]
+
+    def time_flat_int_index(self):
+        self.a.flat[1_000_000]
+
+    def time_flat_slice_index(self):
+        self.a.flat[1_000_000:2_000_000]
diff --git a/doc/release/upcoming_changes/28590.improvement.rst b/doc/release/upcoming_changes/28590.improvement.rst
@@ -0,0 +1,31 @@
+Fix ``flatiter`` indexing edge cases
+------------------------------------
+
+The ``flatiter`` object now shares the same index preparation logic as
+``ndarray``, ensuring consistent behavior and fixing several issues where
+invalid indices were previously accepted or misinterpreted.
+
+Key fixes and improvements:
+
+* Stricter index validation
+
+  - Boolean non-array indices that do not match the axis's size, like
+    ``arr.flat[[True, True]]`` were incorrectly treated as
+    ``arr.flat[np.array([1, 1])]``. They now raise an index error.
+    (This is expected to not raise in the future).
+  - Float non-array indices were also cast to integer and incorrectly
+    treated as ``arr.flat[np.array([1.0, 1.0])]``. This is now deprecated
+    and will be removed in a future version.
+  - 0-dimensional boolean indices like ``arr.flat[True]`` are also
+    deprecated and will be removed in a future version.
+
+* Consistent error types:
+
+  Certain invalid `flatiter` indices that previously raised `ValueError`
+  now correctly raise `IndexError`, aligning with `ndarray` behavior.
+
+* Improved error messages:
+
+  The error message for unsupported index operations now provides more
+  specific details, including explicitly listing the valid index types,
+  instead of the generic ``IndexError: unsupported index operation``.