Type not JSON serializable

Dash Python
1

I’ve seen this come up before, but so far, no answer seems to be given.

In my case, I want to use ‘type’ Objects as values for a Dropdown menu, which results in the old JSON error. Is there a way to achieve this? Maybe it would be a good addition in general, as passing type can make a lot of calls easier. Of course, I could also use type.__name__ and then have an additional if statement checking for that, but it just occured to me that a general workaround or the allowance of more value types would be nice.

Is there an outlook on this issue or any plans? Even ways to contribute?

2

Do you mean passing objects that are custom classes into the options list?

If so, have you considered giving your classes a property or method that just returns their relevant contents as a dictionary object? Passing myObject.d seems quite clear and clean.

It’s either that, or work out how to pass a customized superclass of jsonEncoder into the dash library, which sounds comparatively cumbersome.

3

I might be misunderstanding the error, but the issue occured when setting the ‘value’ of a dropdown option to a type object. This would in theory result in the type object being passed as the value of the callback to my evaluating function, where i could then call a classmethod or the constructor of that type, get the classes signature etc…

I managed to make it work passing type.__name__ instead and later evaluating that against class.__subclasses__().__name__ and calling a constructor on that subclass.

As in:

if inputValue is not None:
   for subclass in Class.__subclasses__():
      if subclass.__name__ == inputValue:
         sign = [p for p in inspect.signature(subclass.__init__).parameters]

EDIT
What i would have liked to do:

if inputValue is not None:
   sign = [p for p in inspect.signature(inputValue.__init__).parameters]
4

Sorry, you’ve lost me a little. I can’t quite track how you want your data to be stored and where you want it to go.

Do you have a class containing a list (‘parameters’) of objects that themselves have attributes/ class variables of “label” and “value”, and you want to be able to bulk compose them into a list of dropdown options?

5

Yeah, there is a dash dropdown component

https://dash.plot.ly/dash-core-components/dropdown

and mine looks like this:

dcc.Dropdown(
                options=self._options,
                id='filter_unit_dropdown_{0}_{1}_{2}'.format(self._deviceNumber, self._servoNumber, self._filterIndex),
                className='three columns'
            )

with self._options being

self._options = [{'label': 'None', 'value': None}]
        for filter in self._servoController.getFilterOptions():
            self._options.append({'label': filter.__name__, 'value': filter.__name__})

and getFilterOptions() works in the way I previously described:

def getFilterOptions(self):
        return [filter for filter in Filter.__subclasses__()]

As for the callback structure

def _createFieldVisibCallback(self, dropdownID, filterFieldID):
        def callback(inputValue):
            return self._servoController.filterFieldVisibility(filterFieldID, inputValue, self._deviceNumber, self._servoNumber, self._filterIndex)
        return callback

def setCallbacks(self):
        dropdown = 'some_id'.format(self._deviceNumber, self._servoNumber, self._filterIndex)
        frequencyField = 'another_id'.format(self._deviceNumber, self._servoNumber, self._filterIndex)

        dynamically_generated_function = self._createFieldVisibCallback(dropdown, frequencyField)
        self._app.callback(
            Output(frequencyField, 'style'),
            [Input(dropdown, 'value')]
        )(dynamically_generated_function)

and lastly the functionality of

_servoController.filterFieldVisibility(xxx) is what I described in the post before.

6

Well, I’m tapped out for useful input then. It sure looks like in all that structure, the difference between

item.__name__
and
item.stringProperty_Set_AtInstantiation_or_InDefinition_or_OnConstruction

is the least of your worries.

Additionally, passing an object directly into the DCC renderer and having it pick to use a string representation of the object’s class’ name (if i follow what you’re describing) sounds like really non-obvious and potentially troublesome behavior.

7

Unfortunately this is not going to work. value needs to be a string. We don’t enforce this right now but we will in the future. We are passing this value over the network to the Dash front-end, which is why we can’t handle arbitrary python objects. The underlying component that we use in the front-end also expects a string.

If you want to keep things dynamic, then use a dictionary that maps strings to your objects:

values = {
    some_object: 'value-1',
    another_object: 'value-2'
}

dcc.Dropdown(value=values[some_object])
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