Revert "Make BitArray.{slice_bits,slice_shots,__getitem__} raise `I…

…ndexErro…" (#12929)

This reverts commit 4f09f57.

This commit reverts #12844, the original PR was making an API change as a bugfix (to make the api consistent with other similar APIs). However it is still a change in the defined stable interface of the class as documented and we shouldn't be changing that per the stability guidelines of the project. Especially in a patch version bugfix release. We can debate whether the change is acceptable for 1.2.0 independently but for a patch version it is definitely out of scope.

qiskit/primitives/containers/bit_array.py

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

test/python/primitives/containers/test_bit_array.py

@@ -527,20 +527,6 @@ 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
@@ -585,9 +571,9 @@ def test_slice_bits(self):
                 self.assertEqual(ba2.get_counts((i, j, k)), expect)
 
         with self.subTest("errors"):
-            with self.assertRaisesRegex(IndexError, "index -1 is out of bounds"):
+            with self.assertRaisesRegex(ValueError, "index -1 is out of bounds"):
                 _ = ba.slice_bits(-1)
-            with self.assertRaisesRegex(IndexError, "index 9 is out of bounds"):
+            with self.assertRaisesRegex(ValueError, "index 9 is out of bounds"):
                 _ = ba.slice_bits(9)
 
     def test_slice_shots(self):
@@ -635,9 +621,9 @@ def test_slice_shots(self):
                 self.assertEqual(ba2.get_bitstrings((i, j, k)), expected)
 
         with self.subTest("errors"):
-            with self.assertRaisesRegex(IndexError, "index -1 is out of bounds"):
+            with self.assertRaisesRegex(ValueError, "index -1 is out of bounds"):
                 _ = ba.slice_shots(-1)
-            with self.assertRaisesRegex(IndexError, "index 10 is out of bounds"):
+            with self.assertRaisesRegex(ValueError, "index 10 is out of bounds"):
                 _ = ba.slice_shots(10)
 
     def test_expectation_values(self):