Bad import of the VQE class when importing from algorithms

[pfargas]

Environment

• Qiskit Terra version:
• Python version:
• Operating system:

What is happening?

If you import VQE from qiskit.algorithms, it returns the deprecated version, found in minimum_eigen_solvers

How can we reproduce the issue?

from qiskit.algorithms import VQE

What should happen?

The import should get the VQE class found in minimum_eigensolvers, not minimum_eigen_solvers

Any suggestions?

No response

[woodsp-ibm]

This is as intended since if the import was switched it would break a lot of existing code that used the older VQE that is based on using Quantum Instance. Also at the moment that VQE is pending deprecated, not yet deprecated, which should happen in this upcoming release. There is a migration guide, in progress, that covers this #9557 To use the new VQE, based on the new Estimator primitives, it has to be imported explicitly from the new location. At some future point we can consider having the import from qiskit.algorithms point to the new VQE, but its way to soon to do that.

[gorkamunoz]

Hi! Maybe it would be useful to have a warning when importing from minimum_eigen_solvers stating that it is deprecated and that the VQE from minimum_eigensolvers should be used instead?

[woodsp-ibm]

This will happen in the upcoming release. Up until now it was pending deprecation so unless you enabled seeing such warnings you would normally not notice. https://github.com/Qiskit/qiskit-terra/blob/7dc7a1cc7111b80f6cb7eea6de867e36db3ab1a8/qiskit/algorithms/minimum_eigen_solvers/vqe.py#L130-L136

As from 0.24.0 you will see a deprecated message. You can see this change in #9883 which switches the warning type to deprecated. If you look for VQE among the files changed you will see if refers the user to the new one and the migration guide. (as per below from the decorator args there)

        additional_msg=(
            "Instead, use the class ``qiskit.algorithms.minimum_eigensolvers.VQE``. "
            "See http://qisk.it/algo_migration for a migration guide."
        ),
        since="0.24.0",