Name-sorted parameters in qiskit.opflow

[Cryoris]

What is the expected enhancement?

Binding parameters to a parameterized Opflow objects requires passing a dictionary with {parameter_instance: value} pairs. More often than not, this is used in the following setting

my_circuit = # ...
expression = # some opflow expression using the circuit

param_dict = dict(zip(my_circuit.parameters, my_values))
bound_expression = expression.bind_parameters(param_dict)

which is a bit cumbersome as we either have to carry along the parameters we're interested in or have to write an extra line defining the param_dict.

Instead if would be nice to allow binding the parameters using only an array. Since we introduced a global parameter order in #5759, which says that parameters are sorted by name, that would be easy to implement. Using the same ordering convention as for the circuits, we could simply write

my_circuit = # ...
expression = # some opflow expression using the circuit

bound_expression = expression.bind_parameters(values)

Since Opflow objects already implement the parameter attribute, the steps to implement this could for instance be

1. ensure the parameter attribute returns a ParameterView object with parameters in the right order
2. if an array is passed into bind_parameters, construct the parameter dictionary for the user using the parameter order from parameters

I'm labelling this as "good first issue", but some previous experience with qiskit.opflow would certainly be helpful!

[pradkrish]

Hello, is this issue still open? I am new to this project, happy to work on this with some guidance.

[jakelishman]

Sure, thanks! I'll assign you now, so others know you're working on it. Feel free to ask questions here - @Cryoris knows rather more about opflow than I do, so he's the best person to ask.

[Cryoris]

Nice to see you're interested! You can check out how the parameters are sorted in the quantum circuit
https://github.com/Qiskit/qiskit-terra/blob/d2e3340adb79719f9154b665e8f6d8dc26b3e0aa/qiskit/circuit/quantumcircuit.py#L2373

and how they are assigned if not a dictionary but only a list of values is passed
https://github.com/Qiskit/qiskit-terra/blob/d2e3340adb79719f9154b665e8f6d8dc26b3e0aa/qiskit/circuit/quantumcircuit.py#L2495

Let us know if you have any questions!

[pradkrish]

I made an attempt to construct a circuit with parameters like this

phase = Parameter("phase")
x = Parameter("x")
y = Parameter("y")
z = Parameter("z")

qc = QuantumCircuit(1, global_phase=phase)
qc.rx(x, 0)
qc.rz(z, 0)
qc.ry(y, 0)

my_values = [0.1,10,20,30]
param_dict = dict(zip(qc.parameters, my_values))

qc.bind_parameters(param_dict) <---

Is this a good working example for this issue where instead of passing param_dict, we would like to be able to pass my_values to bind_parameters?

[Cryoris]

Hi @pradkrish, for the QuantumCircuit we're already able to pass my_values. But here's a slightly modified example where we are currently not able to pass an array but would like to

from qiskit.circuit import Parameter, QuantumCircuit
from qiskit.opflow import StateFn, Z

phase = Parameter("phase")
x = Parameter("x")
y = Parameter("y")
z = Parameter("z")

qc = QuantumCircuit(1, global_phase=phase)
qc.rx(x, 0)
qc.rz(z, 0)
qc.ry(y, 0)

a = Parameter("a")

my_values = [-1, 0.1, 10, 20, 30]  # a, phase, x, y ,z

expectation = StateFn(a * Z, is_measurement=True) @ StateFn(qc)
expectation.assign_parameters(my_values)  # not possible right now!

[Cryoris]

Hi @pradkrish any news on this? 🙂

[nssensalo]

@Cryoris, Well I'd love to give it a try. I'm dying to start my first open source contribution. May I be assigned?

If not using array, can the parameters of "assign_parameters" be in a dictionary or boolean format?
Am I going down the right rabbit hole?

This snippiet is from the qiskit docs:

[Cryoris]

Hi @nssensalo, of course you can work on this, I'll assign you 🙂

For opflow we don't allow inplace right now, so the input to assign_parameters would only be one argument that is either a dictionary of parameter values (like right now) or an array of values (this would be new!).

[AbhirMehrotra]

@Cryoris Hello, my friend and I wanted to work on this issue if it is still open. This is our first project but we are interested and are happy to work on it.

[GaneshH0571]

@Cryoris Hello, my friend and I would like to work on this issue, This is our first project but we are interested and happy to work on it

[javabster]

hi @GaneshH0571 @AbhirMehrotra this issue has already been assigned to @nssensalo, there are many other open good first issues that haven't been assigned yet if you would like to take a look at those!

for @nssensalo if you're new to open source and contributing to qiskit I recommend taking a look at these resources:

• Contributing Guidelines
• GitHub / CI tips for Qiskit Contributors

Or if you are no longer interested in working on this also let us know so we can un-assign you and let someone else be assigned to it 😄

[javabster]

update: issue has been unassigned and open for other contributors to jump in 😄