feature: replace __del__ with weakref.finalize()

[maximilianluc]

Feature addition: replacing del with weakref.finalize() as called method during object garbage collection.

Issue: #3774

• renamed __del__ as __finalize, logic stays the same.
• added weakref.finalize() in __init__ in instrument.py calling upon '__finalize()' during gc.

[maximilianluc]

I think we can rename __del__ to __finalize to be called as weakref.finalize(obj, __finalize), right? With the logic in __del__ , now in __finalize(), the close() method should stay alive until the instrument is gc'd. Or am I overlooking something?

[codecov[bot]]

Codecov Report

Merging #4386 (a450af6) into master (a450af6) will not change coverage. The diff coverage is n/a.

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[jenshnielsen]

@maximilianluc Thanks for this. It looks great but I will need a little bit of time to test this manually to see how it works in practice

[jenshnielsen]

@maximilianluc Sorry for the late response. I have spent a bit of time investigating this and unfortunately its a bit more complicated as the finalize method cannot contain a reference to self since that would increase the ref count to the instrument preventing the finalizer to be called. https://docs.python.org/3/library/weakref.html#weakref.finalize

In https://github.com/QCoDeS/Qcodes/pull/5341 I have implemented a partial solution for visa instruments that do not have this issue