According with the commutator documentation, it works on WeightedPauliOperator, not on numpy arrays.

According with the commutator documentation, it works on WeightedPauliOperator, not on numpy arrays.

from qiskit.aqua.operators.legacy import commutator
from qiskit.aqua.operators import WeightedPauliOperator
from qiskit.quantum_info import Pauli

pauli_a = 'X'
pauli_b = 'Z'
coeff_a = 0.5
coeff_b = 0.5
pauli_term_a = [coeff_a, Pauli.from_label(pauli_a)]
pauli_term_b = [coeff_b, Pauli.from_label(pauli_b)]
op_a = WeightedPauliOperator(paulis=[pauli_term_a])
op_b = WeightedPauliOperator(paulis=[pauli_term_b])

print(commutator(op_a, op_b))

Representation: paulis, qubits: 1, size: 1