Dynamically changing val_mapping on existing parameters does not produce expected behaviour.

Dynamically changing val_mapping on existing parameters does not produce expected behaviour.

Steps to reproduce

1. create a parameter on an instrument instance (here named: 'vna') (or have an existing one)

vna.add_parameter(
    name="sweep_type",
    get_cmd='SWPT?',
    set_cmd='SWPT {}',
    val_mapping={
        "Linear frequency": "LINF",
        "Power": "POWE"
    }
) 

2. update val_mapping

vna.sweep_type.val_mapping = {'Linear frequency': 'LINF', 'Logarithmic frequency': 'LOGF'}

3. set parameter to a: removed or b: added value or c: get value from instrument (instrument replying with: 'LOGF')
   a: vna.sweep_type('Power')
   b: vna.sweep_type('Logarithmic fequency')
   c: vna.sweep_type()

Expected behaviour

a: ValueError: ("'Power' is not in {'Logarithmic frequency', 'Linear frequency'}; ...)
b: parameter set to: 'Logarithmic frequency' without error
c: return 'Logarithmic frequency'

Actual behaviour

a: KeyError: ('Power', 'setting vna_sweep_type to Power')
b: ValueError: ("'Logarithmic fequency' is not in {'Power', 'Linear frequency'}; ...
c: KeyError: ("'LOGF' not in val_mapping", 'getting vna_sweep_type')

System

It would be helpful to provide such information:

Windows 10

If you are using a released version of qcodes (recommended):
0.34.1 (from ...\qcodes\_version.py)

Remarks

I'm trying to wirte an instrument driver and can change vals or unit without issue.
But maybe I am using the wrong approach, and Qcodes actully works as intended?

Reproduce the issue with an instrument parameter created through add_parameter and then change its val_mapping as shown. Trace the parameter's set and get behavior for removed, added, and instrument-returned values; done means all three cases produce the expected ValueError, mapped setting, and mapped return value.