Multiplying two instances of TPBGroupedWeightedPauliOperator fails

[samanthavbarron]

Information

• Qiskit Aqua version: 0.6.5
• Python version: 3.7.5
• Operating system: Debian

{'qiskit-terra': '0.13.0',
 'qiskit-aer': '0.5.0',
 'qiskit-ignis': '0.3.0',
 'qiskit-ibmq-provider': '0.6.0',
 'qiskit-aqua': '0.6.5',
 'qiskit': '0.18.0'}

What is the current behavior?

Multiplying two instances of TPBGroupedWeightedPauliOperator fails.

Steps to reproduce the problem

from qiskit.aqua.operators import WeightedPauliOperator, TPBGroupedWeightedPauliOperator
from qiskit.aqua.operators.op_converter import to_tpb_grouped_weighted_pauli_operator

pauli_dict={'paulis': [{'label': 'IIII',
'coeff': {'real': -0.8288055048633435, 'imag': 0.0}},
{'label': 'IIIZ', 'coeff': {'real': 0.1625164975007994, 'imag': 0.0}},
{'label': 'IIZI', 'coeff': {'real': 0.1625164975007994, 'imag': 0.0}},
{'label': 'IZII', 'coeff': {'real': -0.19744295364673434, 'imag': 0.0}},
{'label': 'ZIII', 'coeff': {'real': -0.19744295364673434, 'imag': 0.0}},
{'label': 'IZIZ', 'coeff': {'real': 0.1172036494860493, 'imag': 0.0}},
{'label': 'IIZZ', 'coeff': {'real': 0.16583253947938562, 'imag': 0.0}},
{'label': 'XXYY', 'coeff': {'real': -0.046156695636240766, 'imag': 0.0}},
{'label': 'YXXY', 'coeff': {'real': 0.046156695636240766, 'imag': 0.0}},
{'label': 'XYYX', 'coeff': {'real': 0.046156695636240766, 'imag': 0.0}},
{'label': 'YYXX', 'coeff': {'real': -0.046156695636240766, 'imag': 0.0}},
{'label': 'ZIIZ', 'coeff': {'real': 0.16336034512229008, 'imag': 0.0}},
{'label': 'ZIZI', 'coeff': {'real': 0.1172036494860493, 'imag': 0.0}},
{'label': 'IZZI', 'coeff': {'real': 0.16336034512229008, 'imag': 0.0}},
{'label': 'ZZII', 'coeff': {'real': 0.1716978884591493, 'imag': 0.0}}]}
operator = WeightedPauliOperator.from_dict(pauli_dict)

tpb_operator = to_tpb_grouped_weighted_pauli_operator(
    operator,
    TPBGroupedWeightedPauliOperator.unsorted_grouping
)

print(tpb_operator.print_details())

tpb_operator * tpb_operator

AttributeErrorTraceback (most recent call last)
<ipython-input-1-6b6da3604543> in <module>
     27 print(tpb_operator.print_details())
     28 
---> 29 tpb_operator * tpb_operator

/usr/local/lib/python3.7/site-packages/qiskit/aqua/operators/weighted_pauli_operator.py in __mul__(self, other)
    276             return self._scaling_weight(other, copy=True)
    277         else:
--> 278             return self.multiply(other)
    279 
    280     def __neg__(self):

/usr/local/lib/python3.7/site-packages/qiskit/aqua/operators/tpb_grouped_weighted_pauli_operator.py in multiply(self, other)
    224         ret_op = super().multiply(other)
    225         # pylint: disable=no-member
--> 226         ret_op = ret_op._grouping_func(ret_op, **self._kwargs)
    227         return ret_op

AttributeError: 'WeightedPauliOperator' object has no attribute '_grouping_func'

What is the expected behavior?

The multiplication should go through and produce an instance of TPBGroupedWeightedPauliOperator.

Suggested solutions

My guess is that on Line 226, it should read maybe:

ret_op = self._grouping_func(ret_op, **self._kwargs)

so that the resulting TPBGroupedWeightedPauliOperator is produced using one of the available grouping functions (either from self or other).

https://github.com/Qiskit/qiskit-aqua/blob/0.6.5/qiskit/aqua/operators/tpb_grouped_weighted_pauli_operator.py#L226

[woodsp-ibm]

Yes, you are right! I'll change it to take the grouping from self and note that in the docstring. (The prior method seems to have the same problem too). BTW thanks for providing these samples that show the failure - they are super helpful.