Make BitArray.{slice_bits,slice_shots,__getitem__} raise `IndexErro…

…r` when indices are not valid (backport #12755) (#12844)

* Make `BitArray.{slice_bits,slice_shots,__getitem__}` raise `IndexError` when indices are not valid (#12755)

* Make BitArray.{slice_bits,slice_shots} raise IndexError when indices are out of bounds

* update __getitem__

(cherry picked from commit bfd2eea)

# Conflicts:
#	qiskit/primitives/containers/bit_array.py

* Update bit_array.py

---------

Co-authored-by: Takashi Imamichi <[email protected]>
Co-authored-by: Elena Peña Tapia <[email protected]>

qiskit/primitives/containers/bit_array.py

@@ -131,10 +131,15 @@ def __repr__(self):
 
     def __getitem__(self, indices):
         """Slices the array along an existing axis of the array."""
-        if isinstance(indices, tuple) and len(indices) >= self.ndim + 2:
-            raise ValueError(
-                "BitArrays cannot be sliced along the bits axis, see slice_bits() instead."
-            )
+        if isinstance(indices, tuple):
+            if len(indices) == self.ndim + 1:
+                raise IndexError(
+                    "BitArray cannot be sliced along the shots axis, use slice_shots() instead."
+                )
+            if len(indices) >= self.ndim + 2:
+                raise IndexError(
+                    "BitArray cannot be sliced along the bits axis, use slice_bits() instead."
+                )
         return BitArray(self._array[indices], self.num_bits)
 
     @property
@@ -428,13 +433,13 @@ def slice_bits(self, indices: int | Sequence[int]) -> "BitArray":
             A bit array sliced along the bit axis.
 
         Raises:
-            ValueError: If there are any invalid indices of the bit axis.
+            IndexError: If there are any invalid indices of the bit axis.
         """
         if isinstance(indices, int):
             indices = (indices,)
         for index in indices:
             if index < 0 or index >= self.num_bits:
-                raise ValueError(
+                raise IndexError(
                     f"index {index} is out of bounds for the number of bits {self.num_bits}."
                 )
         # This implementation introduces a temporary 8x memory overhead due to bit
@@ -455,13 +460,13 @@ def slice_shots(self, indices: int | Sequence[int]) -> "BitArray":
             A bit array sliced along the shots axis.
 
         Raises:
-            ValueError: If there are any invalid indices of the shots axis.
+            IndexError: If there are any invalid indices of the shots axis.
         """
         if isinstance(indices, int):
             indices = (indices,)
         for index in indices:
             if index < 0 or index >= self.num_shots:
-                raise ValueError(
+                raise IndexError(
                     f"index {index} is out of bounds for the number of shots {self.num_shots}."
                 )
         arr = self._array

releasenotes/notes/fix-bitarray-slice-bits-shots-c9cb7e5d907722f5.yaml

@@ -0,0 +1,11 @@
+---
+upgrade_primitives:
+  - |
+    :meth:`.BitArray.slice_bits` and :meth:`.BitArray.slice_shots`
+    will now raise ``IndexError`` when indices are out of bounds.
+    They used to raise ``ValueError`` in the case.
+  - |
+    :meth:`.BitArray.__getitem__` will now raise ``IndexError``
+    when indices are out of bounds or the number of dimensions
+    of indices does not match that of BitArray.
+    They used to raise ``ValueError`` in the case.

test/python/primitives/containers/test_bit_array.py

@@ -527,6 +527,20 @@ def test_getitem(self):
             for j in range(2):
                 self.assertEqual(ba.get_counts((0, j, 2)), ba2.get_counts(j))
 
+        with self.subTest("errors"):
+            with self.assertRaisesRegex(IndexError, "index 2 is out of bounds"):
+                _ = ba[0, 2, 2]
+            with self.assertRaisesRegex(IndexError, "index -3 is out of bounds"):
+                _ = ba[0, -3, 2]
+            with self.assertRaisesRegex(
+                IndexError, "BitArray cannot be sliced along the shots axis"
+            ):
+                _ = ba[0, 1, 2, 3]
+            with self.assertRaisesRegex(
+                IndexError, "BitArray cannot be sliced along the bits axis"
+            ):
+                _ = ba[0, 1, 2, 3, 4]
+
     def test_slice_bits(self):
         """Test the slice_bits method."""
         # this creates incrementing bitstrings from 0 to 59
@@ -571,9 +585,9 @@ def test_slice_bits(self):
                 self.assertEqual(ba2.get_counts((i, j, k)), expect)
 
         with self.subTest("errors"):
-            with self.assertRaisesRegex(ValueError, "index -1 is out of bounds"):
+            with self.assertRaisesRegex(IndexError, "index -1 is out of bounds"):
                 _ = ba.slice_bits(-1)
-            with self.assertRaisesRegex(ValueError, "index 9 is out of bounds"):
+            with self.assertRaisesRegex(IndexError, "index 9 is out of bounds"):
                 _ = ba.slice_bits(9)
 
     def test_slice_shots(self):
@@ -621,9 +635,9 @@ def test_slice_shots(self):
                 self.assertEqual(ba2.get_bitstrings((i, j, k)), expected)
 
         with self.subTest("errors"):
-            with self.assertRaisesRegex(ValueError, "index -1 is out of bounds"):
+            with self.assertRaisesRegex(IndexError, "index -1 is out of bounds"):
                 _ = ba.slice_shots(-1)
-            with self.assertRaisesRegex(ValueError, "index 10 is out of bounds"):
+            with self.assertRaisesRegex(IndexError, "index 10 is out of bounds"):
                 _ = ba.slice_shots(10)
 
     def test_expectation_values(self):