QuantumCircuit.save_statevector: Inform user when Aer backend is not initialized

[rht]

Summary

The QuantumCircuit class doesn't have the save_statevector until it is monkey-patched during the Aer backend initialization. Adding a placeholder method (by this PR) gives an informative error message.

Fixes #8325. (And maybe #6346)

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[rht]

I'm not sure why the test said that QiskitError was not raised. I tested by manually adding the save_statevector() method to a local installed version of qiskit-terra (from PyPI), and I got the expected QiskitError. Later, I will prepare an environment to actually build from Git from scratch.

[rht]

I have set up a fresh Qiskit Terra from my Git branch on Docker environment (using the Rust official Docker image, tagged 1-bullseye). I used Python 3.7.13, as it is the one that was failing in the CI.
However, the test ran successfully

$ tox -epy37 -- -n test.python.circuit.test_initializer.TestInitialize.test_save_statevector
...
{0} test.python.circuit.test_initializer.TestInitialize.test_save_statevector [0.005662s] ... ok

I can't reproduce the CI failure.

[Cryoris]

Maybe that's because the CI does have Aer installed and never calls the method you added 🤔

[rht]

That could be the cause. The tests are likely not run in isolation (even though they are parallelized with stestr), and in one of the tests, the Aer backend is initialized, before test_save_statevector is executed.

[rht]

I misread your statement. You meant that I don't have qiskit-aer installed in my fresh setup. I tried pip install qiskit-aer, and then did

tox -epy37 -- -n test.python.circuit.test_initializer.TestInitialize

so that test_save_statevector is not the only one being tested, nor that it is the first one. No parallelization. I still got no error.

I'm testing the full test stestr run next.

[rht]

I ran stestr run on my laptop. But it crashed my system with OOM. I was able to capture this error

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/moun/test/python/circuit/test_initializer.py", line 85, in test_save_statevector
    self.assertRaises(QiskitError, qc.save_statevector)

      File "/root/.pyenv/versions/3.7.13/lib/python3.7/unittest/case.py", line 756, in assertRaises
    return context.handle('assertRaises', args, kwargs)

      File "/root/.pyenv/versions/3.7.13/lib/python3.7/unittest/case.py", line 178, in handle
    callable_obj(*args, **kwargs)

      File "/root/.pyenv/versions/3.7.13/lib/python3.7/unittest/case.py", line 201, in __exit__
    self.obj_name))

      File "/root/.pyenv/versions/3.7.13/lib/python3.7/unittest/case.py", line 135, in _raiseFailure
    raise self.test_case.failureException(msg)

    AssertionError: QiskitError not raised by save_statevector

[rht]

I have tried undoing the monkey patch in my latest commit. It works locally with tox, but fails in CI via stestr.

There are 2 course of actions:

1. Define original_save_statevector, and then define save_statevector as an alias to the former. Test only the former.
2. Separate test_save_statevector to its own file and test separately. Though I doubt this will fix the issue, because the Python instance is long-running and doesn't get shut down for each file.

Should I do point 1?

[jakelishman]

I'm not sure we should add this at all, sorry. Aer's design of monkey-patching methods onto QuantumCircuit is certainly not desirable, and it does lead to some weird error messages, but Terra shouldn't be in the business of correcting Aer's weirdnesses, because that's just bloating the Terra code. There's also a lot more methods than just save_statevector. Within the current monkey-patch setup, it would be better to document Aer-specific methods with a .. note:: in the built documentation (that would need to be done in the Aer repo), so it displays a big warning box letting people know.

Another reason to not have these messages is that it stymies code-completion workflows. If we add stubs that just throw errors, then you can't tell if save_<x> is available by normal IDE/Jupyter tab-completion, because we've stubbed it out with a method whose only purpose is to raise errors.

[rht]

I agree with the save_statevector monkey patching being confusing. I got tripped by it and hence I made this PR.
This PR is not an ideal solution, but at least it is a temporary solution that will prevent people from having to dig the issues in search engine results / GH issues.

[jakelishman]

I don't believe there is a need for any modification to Terra here, temporary or not. A correct solution can be done immediately, and that's to add documentation to the relevant methods within Aer that says they can only be used once Aer is loaded. Thanks for the interest in contributing - if you'd like to take this further, I'd suggest making the documentation changes as a PR to Aer. There's a bit more discussion from another Terra maintainer here: #6346 (comment), from a while ago.

With the release of Aer 0.11, we're going to start moving away from the namespace packaging for Aer, which will help ease the confusion caused by the lazy-loading qiskit.Aer object - after import qiskit_aer, the methods will be immediately available. (That's not possible yet - we're still untangling the namespace packaging.)

I'm going to close this PR now, because there's no code changes necessary to Terra. Sorry about the delay between you opening it and this - I've been on leave for a month.

[rht]

I appreciate that I finally received a feedback!