Circuit parameters do not respect insertion order

Environment

• Qiskit Terra version: 0.19.2
• Python version: 3.7.7
• Operating system: OSX

What is happening?

Parameters are not implemented on a parametrized circuit in the order defined by a user, but alphabetically. The circuit diagram shows the insertion order, but binding parameters is not done in the same order.

How can we reproduce the issue?

In the following example, I want to assign value [1, 2, 3] to [theta, gamma, lam]. The circuit diagram shows the order I defined, .i.e. theta, gamma , lam but the values are assigned in the alphabetical order rather than the insertion order, therefore, the parameters get wrong values.

param_keys = Parameter('theta'), Parameter('gamma'), Parameter('lam') 

ansatz = QuantumCircuit(1)
ansatz.u(*(param_keys), 0)
print(ansatz)

param_vals = np.array([1, 2, 3])
qc = ansatz.assign_parameters(param_vals)
print(qc)

   ┌────────────────────┐
q: ┤ U(theta,gamma,lam) ├
   └────────────────────┘
   ┌──────────┐
q: ┤ U(3,1,2) ├
   └──────────┘

What should happen?

The list of the parameters should be bound in the order that is given by a user.

Any suggestions?

No response

[jakelishman]

Thanks for the explanation Julien, and I’m glad this is all ok. I’ll close the issue now, but feel free to open a new one if there are bugs in the handling.

[Cryoris]

We decided against sorting by insertion because then the order in which you call the gate, a seemingly random choice, would have an effect on how the array of values will be bound. That would’ve likely been a source of bugs in particular in the applications and implementation-wise it would be more effort since it would require book-keeping of the parameter order when ever we transpile or modify the circuit. Sorting by name is a global order which doesn’t have these drawbacks 🙂