Bug fix: Measurement properties automatic padding for DynamicsBackend initialization

[arthurostrauss]

Summary

An indent was added in the reference code to avoid faulty behaviours when measurement_properties was added to the DynamicsBackend for the simulation to run properly. The bug was in the fact that the measurement schedule would be added, even though it had not been initialized properly (because the initialization was in a if statement with no else). Another check with a bool test was implemented to avoid loading an empty dictionary in case no measurement_properties was to be added.

Details and comments

Minor correction to an emerging bug

[CLAassistant]

All committers have signed the CLA.

[DanPuzzuoli]

Thanks for this correction. There are two more things that should be done with this:

• Add a test that verifies the functioning of this code block, that would have caught this bug. It probably makes the most sense to add it as a method to test.dynamics.backend.test_dynamics_backend.TestDynamicsBackend. E.g. have a check where no measurement instructions exist for any qubits, and another where some exist.
• Create a release note file describing the bug fix. The process is described in the Release Notes section of CONTRIBUTING.md. You'll only need to fill in the bugfix section.

[DanPuzzuoli]

Another minor change that can be included in this PR is to move line 207 to before the for loop, as it is just redefining the same variable each time.

[arthurostrauss]

Hi, the requested information and tests, as well as the possibility to directly load the control_channels with the from_backend() method (tests are also implemented) have been added.
Let me know if additional changes are needed.

[DanPuzzuoli]

For now I'm just reviewing the control_channel_map construction code - once that's in order we can discuss the testing/documentation.

I've added a few in-line comments. The main thing is that this code is assuming the labels for control channels are two-tuples of qubit indices. We don't want this code to assume anything about the form of the labels, as control channels are, in principle, completely arbitrary and could be labelled by anything.

Aside from what's in the in-line comments, I have the following suggestion for tightening up the code:

control_channel_map_backend = None
if hasattr(backend, "control_channels"):
    control_channel_map_backend = backend.control_channels
elif hasattr(backend.configuration(), "control_channels"):
    control_channel_map_backend = backend.configuration().control_channels

if bool(control_channel_map_backend):
    # extract indices from channels
    control_channel_map_backend = {label: channel[0].index for label, channel in control_channel_map_backend.items()}

   # insert code constructing control_channel_map here

[DanPuzzuoli]

Things are looking pretty good. I'm getting way more pedantic with my comments as I think it's good aside from minor polishing.

Aside from the comments I put directly in the code, the last ones are:

• Use self.assertTrue(x) instead of assert x and self.assertEqual(x, y) instead of assert x == y, as this is the right way to do things in unittest.
• Make sure test method doc strings are well formed sentences. There are some periods missing.

You will also need to run automatic formatting and linting, using terminal commands tox -e black and tox -e lint.

[DanPuzzuoli]

LGTM, thanks for the contribution!