Difference between param.Parameterized and pn.viewable.Viewer?

Theom · September 28, 2023, 1:45pm

Hi,

Are there any practical differences between the two classes? Viewer inherits from Parameterized, which can already be viewed easily thanks to the panel method.

ahuang11 · May 10, 2024, 7:36pm

I believe Viewer just makes it behave like a Panel object, i.e. include show(), servable(), etc. Combine Existing Components — Panel v1.4.2

You can check out the source code here:

github.com

holoviz/panel/blob/main/panel/viewable.py#L1024-L1071


      
          class Viewer(param.Parameterized):
              """
              A baseclass for custom components that behave like a Panel object.
              By implementing __panel__ method an instance of this class will
              behave like the returned Panel component when placed in a layout,
              render itself in a notebook and provide show and servable methods.
              """
          
              def __panel__(self) -> Viewable:
                  """
                  Subclasses should return a Panel component to be rendered.
                  """
                  raise NotImplementedError
          
              def _create_view(self):
                  from .pane import panel
                  from .param import ParamMethod
          
                  if hasattr(self.__panel__, "_dinfo"):
                      view = ParamMethod(self.__panel__, lazy=True)

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Theom · May 13, 2024, 11:32am

Thanks @ahuang11

I think if you put a param.Parameterized object within a layout, I think it’s converted into a Viewer, just as a Holoviews plot is converted into a Holoviews pane. So Viewer is only useful if you need the panel object interface.

Marc · May 13, 2024, 4:47pm

There is a big difference.

With a param.Parameterized object you will be viewing its parameters via pn.Param.
With a pn.viewable.Viewer object you will be viewing what ever its __panel__ method returns.

So the first will work like a settings object you would like to put in your sidebar.
The latter can be used to display a plot, table, some widgets (settings) or a combination.

The purpose of the tutorial Reusable Components is to teach the use of Parameterized and Viewable and when to use which.

The Structure with a DataStore builds on the Reusable Components tutorial and really shows how to use the Viewable class.