Getting a DelegateParameter with scale fails if it is not initialized

[haroldmeerwaldt]

If you create a DelegateParameter and pass a scale argument, it is not possible to get its cache before it is initialized. The scale gets applied to the None that resides in the source parameter and gives a TypeError.

Steps to reproduce

from qcodes import Parameter, DelegateParameter

p = Parameter("p")
dp = DelegateParameter("dp", p, scale=1.1)

print(dp.source.cache.get(False))  # gives back None
print(dp.cache.get(False))  # fails

Expected behaviour

I expected to get None back, just as you would a plain uninitialized Parameter.

A workaround is to give nan as an initial cache value. This works:

from qcodes import Parameter, DelegateParameter
import numpy as np

p = Parameter("p")
dp = DelegateParameter("dp", p, scale=1.1, initial_cache_value=np.nan)

print(dp.source.cache.get(False))
print(dp.cache.get(False))

Actual behaviour

I got a TypeError:

Traceback (most recent call last):

  File "D:\users\hw_sw_integration\projects\low-level-software\src\lls_input\amber\scripts\example_02.py", line 14, in <module>
    print(dp.cache.get(False))

  File "D:\users\hw_sw_integration\venv-3.9\lib\site-packages\qcodes\instrument\parameter.py", line 1541, in get
    return self._parameter._from_raw_value_to_value(

  File "D:\users\hw_sw_integration\venv-3.9\lib\site-packages\qcodes\instrument\parameter.py", line 601, in _from_raw_value_to_value
    value = value / self.scale

TypeError: unsupported operand type(s) for /: 'NoneType' and 'float'

It appears that _from_raw_value_to_raw_value applies the scale on the value even if it's None. A simple solution would be to put a big if value is not None: in the method, but this may not cover all the use cases (e.g. more intricate ways of handling the scale than dividing/multiplying).

System

Windows 10
qcodes 0.30.1

[astafan8]

@haroldmeerwaldt thank you for reporting the issue!

i think your suggestion is good! but let's make the value None check only for offset and the scale, so on line https://github.com/QCoDeS/Qcodes/blob/c66a8af4ac8fa21498849a24734e4e1749952304/qcodes/instrument/parameter.py#L582 and https://github.com/QCoDeS/Qcodes/blob/c66a8af4ac8fa21498849a24734e4e1749952304/qcodes/instrument/parameter.py#L598.

Would you be willing to start a PR? and also add a few tests for this in this file https://github.com/QCoDeS/Qcodes/blob/master/qcodes/tests/parameter/test_parameter_scale_offset.py ?

[haroldmeerwaldt]

@astafan8 That sounds like a good plan! Allowing None for the parser and in the inverse_val_mapping dictionary makes sense. I'll start a PR with tests.