Fix Nullable values in write attributes procedure

[BDeliers]

Writing attributes values with JSON data is allowed in Matter, for instance with the ACL cluster we can send this payload:

Expand

[
    {
        "fabricIndex":0,
        "privilege":5,
        "authMode":2,
        "subjects":[
            112233
        ],
        "targets":null
    },
    {
        "fabricIndex":0,
        "privilege":1,
        "authMode":2,
        "subjects":[
            4444,
            5555,
            6666
        ],
        "targets":null
    },
    {
        "fabricIndex":0,
        "privilege":3,
        "authMode":3,
        "subjects":[
            123,
            456
        ],
        "targets":[
            {
                "cluster":6,
                "endpoint":null,
                "deviceType":null
            },
            {
                "cluster":null,
                "endpoint":1,
                "deviceType":null
            },
            {
                "cluster":8,
                "endpoint":2,
                "deviceType":null
            }
        ]
    }
]

When Python parses this JSON, the null value is then converted to a Python None. This None is not allowed by the Matter SDK, at the opposite of the Nullable type which is defined in the SDK to handle this kind of situation.

This pull-request is there to fix this issue: the Python None is converted to a Matter Null value of type Nullable.

[marcelveldt]

First of all thanks for the PR!
I'm sorry but this is the wrong way to fix this - it needs to be addressed in our helper that reconstructs the Cluster object from the json in the api.

It should be as easy as replacing the value assignment with our "parse_value" helper form utils.
That is already ready for the custom sdk types.

If you want I can make the adjustment and would be great if you can test if that gives the desired behavior

[BDeliers]

Thanks for your feedback. If I understand properly what you said, it can be fixed by changing Line 137 of matter_server/common/helpers/util.py from

if value is None and value_type is Nullable:
        return None

to

if value is None and value_type is Nullable:
        return Null

?

[marcelveldt]

Well, yes but we need some more. We will also need a boolean option to that util function if the custom types are allowed.
Because in this case you are going from [json --> dataclass convertor --> SDK] and then the custom SDK types are needed but when we do the same in e.g. Home Assistant or any other consumer of the lib we need to convert the values.

Want me to edit the PR to adjust the utils func so you can use it in the write command ?

[BDeliers]

It seems that you know the project way better than I do.
Yes please, do the changes and I will then test with my use case !

[marcelveldt]

I will do so monday. I'll let you know once its ready to test

[marcelveldt]

I have modified your PR with the adjustments. Now all values that are sent to the SDK will be in the SDK's custom data types such as Nullable, uint and float32

[marcelveldt]

I've tested both sending commands and write attribute and both still work.