Deprecate `PauliList` estimator observables (#11055)

* Deprecate PauliList estimator observables

Deprecates using a `PauliList` as an observable that is implicitly converted to a `SparsePauliOp` with coefficients 1 when calling `Estimator.run`. Users should instead explicitly convert the argument using `SparsePauliOp(pauli_list)` first.

* Revert algorithms_test_case.py

* Update test/python/algorithms/algorithms_test_case.py

Co-authored-by: Takashi Imamichi <31178928+t-imamichi@users.noreply.github.com>

* Update test/python/primitives/test_estimator.py

---------

Co-authored-by: Takashi Imamichi <31178928+t-imamichi@users.noreply.github.com>
Co-authored-by: ikkoham <ikkoham@users.noreply.github.com>

qiskit/primitives/utils.py

@@ -14,6 +14,7 @@ Utility functions for primitives
 """
 from __future__ import annotations
 
+import warnings
 import sys
 import typing
 from collections.abc import Iterable
@@ -23,7 +24,7 @@ import numpy as np
 from qiskit.circuit import Instruction, ParameterExpression, QuantumCircuit
 from qiskit.circuit.bit import Bit
 from qiskit.circuit.library.data_preparation import Initialize
-from qiskit.quantum_info import SparsePauliOp, Statevector
+from qiskit.quantum_info import SparsePauliOp, Statevector, PauliList
 from qiskit.quantum_info.operators.base_operator import BaseOperator
 from qiskit.quantum_info.operators.symplectic.base_pauli import BasePauli
 
@@ -82,6 +83,15 @@ def init_observable(observable: BaseOperator | PauliSumOp | str) -> SparsePauliO
     elif isinstance(observable, BaseOperator) and not isinstance(observable, BasePauli):
         return SparsePauliOp.from_operator(observable)
     else:
+        if isinstance(observable, PauliList):
+            warnings.warn(
+                "Implicit conversion from a PauliList to a SparsePauliOp with coeffs=1 in"
+                " estimator observable arguments is deprecated as of Qiskit 0.46 and will be"
+                " in Qiskit 1.0. You should explicitly convert to a SparsePauli op using"
+                " SparsePauliOp(pauli_list) to avoid this warning.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
         return SparsePauliOp(observable)
 
 

releasenotes/notes/dep-estimator-paulilist-ef2bb2865b66f012.yaml

@@ -0,0 +1,7 @@
+---
+deprecations:
+  - |
+    Deprecates using a :class:`~.PauliList` as an observable that is implicitly
+    converted to a :class:`~.SparsePauliOp` with coefficients 1 when calling
+    :meth:`.Estimator.run`. Users should instead explicitly convert the argument
+    using ``SparsePauliOp(pauli_list)`` first.

test/python/primitives/test_estimator.py

@@ -349,7 +349,6 @@ class TestObservableValidation(QiskitTestCase):
     @data(
         ("IXYZ", (SparsePauliOp("IXYZ"),)),
         (Pauli("IXYZ"), (SparsePauliOp("IXYZ"),)),
-        (PauliList("IXYZ"), (SparsePauliOp("IXYZ"),)),
         (SparsePauliOp("IXYZ"), (SparsePauliOp("IXYZ"),)),
         (PauliSumOp(SparsePauliOp("IXYZ")), (SparsePauliOp("IXYZ"),)),
         (
@@ -360,10 +359,6 @@ class TestObservableValidation(QiskitTestCase):
             [Pauli("IXYZ"), Pauli("ZYXI")],
             (SparsePauliOp("IXYZ"), SparsePauliOp("ZYXI")),
         ),
-        (
-            [PauliList("IXYZ"), PauliList("ZYXI")],
-            (SparsePauliOp("IXYZ"), SparsePauliOp("ZYXI")),
-        ),
         (
             [SparsePauliOp("IXYZ"), SparsePauliOp("ZYXI")],
             (SparsePauliOp("IXYZ"), SparsePauliOp("ZYXI")),
@@ -378,6 +373,19 @@ class TestObservableValidation(QiskitTestCase):
         """Test obsevables standardization."""
         self.assertEqual(validation._validate_observables(obsevables), expected)
 
+    @data(
+        (PauliList("IXYZ"), (SparsePauliOp("IXYZ"),)),
+        (
+            [PauliList("IXYZ"), PauliList("ZYXI")],
+            (SparsePauliOp("IXYZ"), SparsePauliOp("ZYXI")),
+        ),
+    )
+    @unpack
+    def test_validate_observables_deprecated(self, obsevables, expected):
+        """Test obsevables standardization."""
+        with self.assertRaises(DeprecationWarning):
+            self.assertEqual(validation._validate_observables(obsevables), expected)
+
     @data(None, "ERROR")
     def test_qiskit_error(self, observables):
         """Test qiskit error if invalid input."""