Update test imports for qiskit-aer 0.11

[wshanks]

Summary

qiskit-aer moved from a namespace package at qiskit.providers.aer to its own package qiskit_aer with version 0.11. Tests in qiskit-experiments importing from the old path fail with qiskit-aer 0.11. This PR updates those imports.

Closes #912

Details and comments

This is a follow up to #911.

For the aer change, see See Qiskit/qiskit-aer#1526.

@conradhaupt and I were running into this, but I can't add him as a reviewer.

[wshanks]

Reading #1526 more closely, I see that Qiskit/qiskit#5089 added some importing magic to redirect qiskit.providers.aer to qiskit_aer which allowed the qiskit-experiments tests to keep passing with 0.11. The actual issue requiring qiskit-experiments to update the aer imports is that pylint is not smart enough to follow the importing magic and complains about qiskit.providers.aer imports when qiskit-aer 0.11 is installed. See this run for example.

Maybe we could have satisfied pylint in a different way without updating the imports yet, but we only use aer in the tests, so it doesn't seem like there is a need to keep compatibility with qiskit-aer<0.11.

[conradhaupt]

Maybe we could have satisfied pylint in a different way without updating the imports yet, but we only use aer in the tests, so it doesn't seem like there is a need to keep compatibility with qiskit-aer<0.11.

We probably could have configured pylint to see it, but I feel like that would have been far more work than is necessary. As you said, we only use Aer in tests. My understanding of Qiskit/qiskit#5089 and Qiskit/qiskit-aer#1526 is that Qiskit Aer has officially moved to qiskit_aer, but that qiskit.provider.aer is provided so that old code still works? If that is the case, then it's better that we migrate to using qiskit_aer.

[wshanks]

My understanding of Qiskit/qiskit#5089 and Qiskit/qiskit-aer#1526 is that Qiskit Aer has officially moved to qiskit_aer, but that qiskit.provider.aer is provided so that old code still works?

Right. If aer were an actual dependency of qiskit-experiments, I think maybe we should have figured out how to get pylint to ignore it since it seems unnecessary to force users to update to keep pylint happy, but since this is just the version that gets used in tests it seems okay to require the new version.

[mtreinish]

Yeah, the new path is qiskit_aer. The import redirect hooks were added to qiskit-terra (which owns the qiskit.* namespace) for backwards compatibility. So that anyone with existing code, or code they want to support > 1 version of qiskit-aer, can continue to use the old namespace. Eventually importing from qiskit.providers.aer or qiskit.Aer will emit a deprecation warning asking you to move to qiskit_aer. It is better longer term to update the imports to the new path, but you're both correct it's not actually required. You could just add qiskit.providers.aer to the pylint module ignore list in the config to fix the failures. I guess it depends on what you want do with supporting aer versions and whether >=0.11.0 is required

[wshanks]

Since aer is only used in tests, I think it is okay to increase the minimum version. It is probably uncommon for someone to want to run the tests and not be able to update aer (in the worst case, they can make a separate environment for running qiskit-experiments tests). We could ignore the package in pylint, but then we have the small downside of pylint not checking aer function signatures. Maybe doing try/except with both import paths could support 0.10 or 0.11 and still get pylint to check the aer signatures.

I don't feel too strongly. The main thing is that we pinned aer in #911 to get tests working again and should try to get it unpinned. I defer to @coruscating on which way is best.

[itoko]

+1 to increase the minimum version (in requirements-dev.txt) since aer is only used in tests.

[coruscating]

LGTM, thanks