DatePicker.controls raises TypeError: Object of type date is not JSON serializable

[MarcSkovMadsen]

Panel 0.10.2
Bokeh 2.2.3

I'm currently working on styling all Panel widgets using https://fast.design. A very powerful tool is the .controls of each widget that allows me to quickly visualize and test different values of parameters and the looks.

For the DatePicker it raises a TypeError: Object of type date is not JSON serializable if I set the DatePicker.value up front.

from panel import widgets as pnw
from datetime import date
import panel as pn

component = pnw.DatePicker(name='Date Picker', height=200, width=200)
component.value = date(2020, 2, 18)
controls = component.controls()
pn.Column(component, controls).servable()

$ python -m panel serve 'scripts\issue_dataframe.py' --dev
2020-11-27 05:30:37,579 Starting Bokeh server version 2.2.3 (running on Tornado 6.0.4)
2020-11-27 05:30:37,580 User authentication hooks NOT provided (default user enabled)
2020-11-27 05:30:37,583 Bokeh app running at: http://localhost:5006/issue_dataframe
2020-11-27 05:30:37,583 Starting Bokeh server with process id: 22468
2020-11-27 05:30:41,695 404 GET /test_fast_bokeh_css (::1) 0.53ms
2020-11-27 05:30:43,704 Error running application handler <bokeh.application.handlers.script.ScriptHandler object at 0x00000209B290BA48>: Object of type date is not JSON serializable
File "encoder.py", line 179, in default:
raise TypeError(f'Object of type {o.__class__.__name__} ' Traceback (most recent call last):
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\bokeh\application\handlers\code_runner.py", line 197, in run
    exec(self._code, module.__dict__)
  File "C:\repos\private\panel-extensions-template\scripts\issue_dataframe.py", line 8, in <module>
    pn.Column(component, controls).servable()
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\viewable.py", line 313, in servable
    self.server_doc(title=title, location=True)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\viewable.py", line 749, in server_doc
    model = self.get_root(doc)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\viewable.py", line 482, in get_root
    root = self._get_model(doc, comm=comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\layout\base.py", line 112, in _get_model
    objects = self._get_objects(model, [], doc, root, comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\layout\base.py", line 102, in _get_objects
    child = pane._get_model(doc, root, model, comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\layout\base.py", line 112, in _get_model
    objects = self._get_objects(model, [], doc, root, comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\layout\tabs.py", line 157, in _get_objects
    child = pane._get_model(doc, root, model, comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\layout\base.py", line 112, in _get_model
    objects = self._get_objects(model, [], doc, root, comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\layout\base.py", line 102, in _get_objects
    child = pane._get_model(doc, root, model, comm)
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\widgets\base.py", line 108, in _get_model
    model = self._widget_type(**self._process_param_change(self._init_properties()))
  File "C:\repos\private\panel-extensions-template\.venv\lib\site-packages\panel\widgets\input.py", line 357, in _process_param_change
    value = json.dumps(value)
  File "C:\Program Files\Python37\lib\json\__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "C:\Program Files\Python37\lib\json\encoder.py", line
199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "C:\Program Files\Python37\lib\json\encoder.py", line
257, in iterencode
    return _iterencode(o, 0)
  File "C:\Program Files\Python37\lib\json\encoder.py", line
179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type date is not JSON serializable

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