Raise error when instantiating invalid AnyWidget subclass

[tlambert03]

in exploring the mechanics of this library, I was breaking all the rules 😂 and created an AnyWidget subclass without _esm or _modules. In which case you see the following javascript error:

Failed to create view for 'AnyView' from module 'anywidget' with model 'AnyModel' from module 'anywidget'
TypeError: Cannot read properties of undefined (reading 'startsWith')
    at is_href (http://localhost:8888/nbextensions/anywidget/index.js?v=20230201171813:7:14)
    at load_esm (http://localhost:8888/nbextensions/anywidget/index.js?v=20230201171813:48:7)
    at AnyView.render (http://localhost:8888/nbextensions/anywidget/index.js?v=20230201171813:76:26)
    at async http://localhost:8888/nbextensions/jupyter-js-widgets/extension.js?v=20230201171813:2:742339

Obviously: one should just "rtfd" here, but this PR provides a more explicit runtime error.

ValueError: Cannot instantiate AnyWidget without defining one of the following class attributes: ('_esm', '_module')

Since _esm is effectively an abstract class attribute, another option here (as opposed to raising in the __init__), would be to use __init_subclass__:

class AnyWidget(ipywidgets.DOMWidget):
    ...
    def __init_subclass__(cls) -> None:
        if not any(hasattr(cls, attr) for attr in _ESM_CLASS_ATTRS):
            raise ValueError(
                "AnyWidget subclass must defining one of the following "
                f"class attributes: {_ESM_CLASS_ATTRS}"
            )

the main difference there is that the immediate subclass of AnyWidget must have _esm... which could possibly restrict users making their own base classes. So this PR only errors at instantiation

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[manzt]

Thanks for the PR :) I guess another option would be to provide a default for _esm that doesn't do anything but maybe render some HTML/link to docs. For example,

It's technically valid to have a widget without any frontend code I guess (the traitlets stuff will just only run on the python side without any listeners).

class MyWidget(anywidget.AnyWidget):
	value = traitlets.Int().tag(sync=True)

import ipywidgets

widget = MyWidget()
slider = ipywidgets.IntSlider()

ipywidgets.link((slider, "value"), (widget, "value"))

slider # slider will update widget.value, never need to render widget technically

The _esm isn't really essential until you try to display the class in a notebook environment (i.e., _repr_mimebundle_ is called). So either we don't let folks create a class unless _esm/_module are defined, or just show something in the front-end by default if not defined (rather than raising an obscure JS error).

[tlambert03]

Yeah I think a default no op would be good too. Can close this if you want!

[tlambert03]

ok, changed it to this:

(where the link is to the docs getting started page)

thoughts?

(the missing space after "attribute" is fixed)

[manzt]

Looks great! Just some minor comments. I think we should add a unit test.