SweepMeasurement: better design (employ the power of sweep objects)

Scope

While implementing the "sweep module", a new SweepMeasurement class has been implemented (see in PR 1107 or in qcodes/sweep/measurement.py once #1107 is merged).

The class is implemented by simply subclassing from Measurement class, and adding a register_sweep method.

Problem

The implementation is lacking design for the following reasons.

1

Thanks to the direct inheritance, register_parameter methods can be called on SweepMeasurement objects. But this is NOT desirable. The whole point of the SweepMeasurement is to get away from register_parameter, and use a sweep object for both "measurement definition" and "measurement execution". So, SweepMeasurement objects should not have register_parameter-like methods. If there is a need to add or remove parameters, that should be done through the sweep object.

2

Since SweepMeasurement make sense only for once sweep, what is the need for an explicit register_sweep method? Why not pass the sweep object directly to the constructor of the SweepMeasurement? Compare two lines

sweep_measurement = SweepMeasurement()
sweep_measurement.register_sweep(total_sweep_object)

with one line

sweep_measurement = SweepMeasurement(total_sweep_object)

3

Thanks to the sweep object infrastructure, the measurement execution code itself becomes very concise, and more importantly generic. For example:

with sweep_measurement.run() as data_saver:
    for data_point in total_sweep_object:
        data_saver.add_results(*data_point.items())

Why not implement a method on SweepMeasurement class that does that since SweepMeasurement objects are created based on a sweep object and they can contain it inside?

Since the run method is considered, it is also worth noting that the run method of a Measurement object does not actually "run" anything - it just initiates a data set and returns a context manager from within which measurement results can be saved (e.g. added to the data set). On the other hand, a run method of the SweepMeasurement class can indeed "run" the experiment, because the experiment definition is already known from the sweep object.

Proposed solution

Make a private base class _MeasurementBase (or _MeasurementWithDataSetBase).
Make Measurement class inherit from '_MeasurementBase, and implement only the register parameter methods in it (likeregister_parameter`).
Make SweepMeasurement class inherit from _MeasurementBase. Instead of implementing a register_sweep method, make the sweep object be an argument for its constructor.
Implement a run method of the SweepMeasurement such that it actually "run" the measurement. This might also require renaming of the run method of the Measurement class.

@sohailc @WilliamHPNielsen @jenshnielsen @Dominik-Vogel

microsoft / Qcodes