SparsePauliOp.paulis setter does not propagate dimension info

[ihincks]

Environment

• Qiskit Terra version: 0.24.1
• Python version: any
• Operating system: any

What is happening?

Setting the paulis attribute (it has a @setter) on a SparsePauliOp instance does not update the BaseOperator shape attributes.

How can we reproduce the issue?

import qiskit.quantum_info as qi

pauli_x = qi.SparsePauliOp(["X"], [1])
pauli_x.paulis = ["XX"]
print(pauli_x, pauli_x.dim)

What should happen?

The above snippet produces the output

SparsePauliOp(['XX'],
              coeffs=[1.+0.j]) (2, 2)

whereas it should produce

SparsePauliOp(['XX'],
              coeffs=[1.+0.j]) (4, 4)

if changing the dimension is allowed, or raise an exception if it's not allowed.

Any suggestions?

I haven't looked closely, but it's possible that just updating _op_shape will be sufficient to fix the bug.

[atharva-satpute]

Looks like value of num_qubits gets updated from PauliList and up but does not get updated from SparsePauliOp. Correct me if I'm wrong

[jakelishman]

Personally I'd argue that those attributes shouldn't be settable by users at all, but I think the ship's sailed on that front. Given that the setter exists, I think the best solution would be to have it reject attempts to either modify the number of elements (so len(self._pauli_list) must not change) because that allows an inconsistency with self.coeffs, and also reject any attempt to modify the number of qubits because that just seems inconsistent with the rest of qiskit.quantum_info.

[ikkoham]

Agree with @jakelishman's comment.