Support passing empty lists to Pauli delete method (#10849)

* handle empty lists in delete methods, added release notes

* fixed ordering of empty list check in pauli delete method

* add proper cross-references to classes

Co-authored-by: Jake Lishman <jake@binhbar.com>

---------

Co-authored-by: Jake Lishman <jake@binhbar.com>

qiskit/quantum_info/operators/symplectic/pauli.py

@@ -363,6 +363,8 @@ def delete(self, qubits: int | list) -> Pauli:
         """
         if isinstance(qubits, (int, np.integer)):
             qubits = [qubits]
+        if len(qubits) == 0:
+            return Pauli((self._z, self._x, self.phase))
         if max(qubits) > self.num_qubits - 1:
             raise QiskitError(
                 "Qubit index is larger than the number of qubits "

qiskit/quantum_info/operators/symplectic/pauli_list.py

@@ -376,7 +376,8 @@ def delete(self, ind: int | list, qubit: bool = False) -> PauliList:
         """
         if isinstance(ind, int):
             ind = [ind]
-
+        if len(ind) == 0:
+            return PauliList.from_symplectic(self._z, self._x, self.phase)
         # Row deletion
         if not qubit:
             if max(ind) >= len(self):

qiskit/quantum_info/operators/symplectic/pauli_table.py

@@ -281,7 +281,8 @@ def delete(self, ind: int | list, qubit: bool = False) -> PauliTable:
         """
         if isinstance(ind, (int, np.integer)):
             ind = [ind]
-
+        if len(ind) == 0:
+            return PauliTable(self._array)
         # Row deletion
         if not qubit:
             if max(ind) >= self.size:

releasenotes/notes/support-empty-delete-for-pauli-16c5c5fae890c16c.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Passing an empty list to the methods :meth:`.Pauli.delete`, :meth:`.PauliList.delete`,
+    and :meth:`.PauliTable.delete` now returns a copy of the original object instead of throwing an error.

test/python/quantum_info/operators/symplectic/test_pauli.py

@@ -224,6 +224,8 @@ def test_insert(self):
     def test_delete(self):
         """Test delete method"""
         pauli = Pauli("IXYZ")
+        pauli = pauli.delete([])
+        self.assertEqual(str(pauli), "IXYZ")
         pauli = pauli.delete([0, 2])
         self.assertEqual(str(pauli), "IY")
 

test/python/quantum_info/operators/symplectic/test_pauli_list.py

@@ -1495,6 +1495,10 @@ def test_unique(self):
 
     def test_delete(self):
         """Test delete method."""
+        with self.subTest(msg="no rows"):
+            pauli = PauliList(["XX", "ZZ"])
+            self.assertEqual(pauli.delete([]), pauli)
+
         with self.subTest(msg="single row"):
             for j in range(1, 6):
                 pauli = PauliList([j * "X", j * "Y"])
@@ -1508,6 +1512,10 @@ def test_delete(self):
                 self.assertEqual(pauli.delete([1, 2]), PauliList(j * "X"))
                 self.assertEqual(pauli.delete([0, 1]), PauliList(j * "Z"))
 
+        with self.subTest(msg="no qubits"):
+            pauli = PauliList(["XX", "ZZ"])
+            self.assertEqual(pauli.delete([], qubit=True), pauli)
+
         with self.subTest(msg="single qubit"):
             pauli = PauliList(["IIX", "iIYI", "ZII"])
             value = pauli.delete(0, qubit=True)

test/python/quantum_info/operators/symplectic/test_pauli_table.py

@@ -1136,6 +1136,10 @@ def test_unique(self):
 
     def test_delete(self):
         """Test delete method."""
+        with self.subTest(msg="no rows"):
+            pauli = PauliTable.from_labels(["XX", "YY"])
+            self.assertEqual(pauli.delete([]), pauli)
+
         with self.subTest(msg="single row"):
             for j in range(1, 6):
                 with self.assertWarns(DeprecationWarning):
@@ -1151,6 +1155,10 @@ def test_delete(self):
                     self.assertEqual(pauli.delete([1, 2]), PauliTable(j * "X"))
                     self.assertEqual(pauli.delete([0, 1]), PauliTable(j * "Z"))
 
+        with self.subTest(msg="no rows"):
+            pauli = PauliTable.from_labels(["XX", "YY"])
+            self.assertEqual(pauli.delete([], qubit=True), pauli)
+
         with self.subTest(msg="single qubit"):
             with self.assertWarns(DeprecationWarning):
                 pauli = PauliTable.from_labels(["IIX", "IYI", "ZII"])