ObservablesArray tolist() doesn't return list

[jyu00]

Environment

• Qiskit version: 1.2.4
• Python version: 3.11
• Operating system:

What is happening?

The tolist() method in ObservablesArray returns a dictionary instead of a list when there is only 1 observable.

How can we reproduce the issue?

from qiskit.primitives.containers.observables_array import ObservablesArray

oa = ObservablesArray.coerce("Z")
print(type(oa.tolist()))

What should happen?

Both the method name and type hint suggests it should return a list

Any suggestions?

No response

[gadial]

It's not clear whether this is indeed a bug or the expected behavior, since tolist is a numpy method which generates a nested list with ndim levels.

In the example above, the observable array is of dimension 0, so returning a scalar is actually expected. Compare with

from qiskit.primitives.containers.observables_array import ObservablesArray

oa = ObservablesArray.coerce("Z")
print(type(oa.tolist()))

oa = ObservablesArray.coerce(["Z"])
print(type(oa.tolist()))

Where the second print is indeed a list.

So we have two possible choices:

1. Keep the current behavior as it corresponds to how in general tolist behaves.
2. Do the one-line fix such that both the examples above return a one-dimensional list.

I tend to view 1 as the better possibility, to keep our usage of tolist as standard as possible; what is your opinion? I guess this issue arose from some specific problem in primitives/runtime?

[jyu00]

This was prompted by some partner trying to implement an Estimator-like interface.

I'm not against option 1, but if that's the approach, the type hint should be list | dict, and docstring should mention it could return a scalar (dictionary) if the array is 0d (similar to the docstring for numpy.tolist)