Failing instrument creation adds instrument to list of instruments

[AdriaanRol]

Steps to reproduce

1. Create an instrument that fails (e.g. timeout, invalid arguments etc)
2. The name of the instrument gets added to the list of existing instruments
3. This prevents creating an instrument with the same name that does not fail.
4. This is a problem because you want to create an instrument with that name when you fixed e.g. your invalid input argument/address or driver bug.

Expected behaviour

The name of the instrument does not get added to the list of current instruments

Actual behaviour

The name of the instrument gets added to the list of instruments preventing any other instrument from being created with that name, even though no instrument with the name actually exists.

Workaround

As suggested by @alexcjohnson use Instrument.find_instrument(name).close() to close the non-existing instrument.

[giulioungaretti]

this was introduced with #323, correct?

[alexcjohnson]

It would have existed prior to #323 as well, in the instance list that each subclass maintains for itself (to enable finding instruments to test) but since we don't have a lot of driver tests (yet) noone has been impacted by this aspect of it.

I think there's a relatively simple solution to this using a metaclass - the standard metaclass, type, is what actually calls __init__, so if we make our own that first uses type to create the object, then we record the instance from there, it will never get to the record call if __init__ throws an error. I will try this out...

[giulioungaretti]

If the goal was to keep qcodes maintainable and not too black boxy ( reason to use python in the first place, then it sounds way too complex to me, but let's see! 🍝