Custom Widget docs: Adding a list of variable names to avoid

[jasonho-lynx]

Link: https://github.com/jupyter-widgets/ipywidgets/blob/master/docs/source/examples/Widget%20Custom.ipynb

Problem

Hi - would like to suggest adding some detail in the documentation relating to widget variable name collisions.

Using certain widget variables can cause errors. In my case, I used 'style', which returned the following error from DOMWidgetView:

Uncaught (in promise) TypeError: Cannot read property 'then' of undefined
    at l.t.create_view (manager-base.js:85)
    at EmailView.e.create_child_view (widget.js:604)
    at widget.js:774

Looking into the widget.ts file, it does show that the 'style' variable is already used there for DOMWidgetView.

Steps I took to get this error message:

1. Followed the Custom Widget tutorial, but replaced every occurrence of the value variable with style instead.
2. In Jupyter Notebook, instantiated the widget using email = Email(value='john.doe@domain.com', disabled=False) and email
3. F12 to see the error in console

It didn't seem to affect functionality.

Suggested Improvement

One solution might be to add into the documentation a list of some variables to avoid using because of potential collisions.

[ianhi]

Thanks for bringing this up @jasonho-lynx - I think adding more docs for this is great idea. Would you like to make a PR? It could either go into the existing tutorial or into a new page that would have miscellaneous information for making a custom widget.

We could also consider changing the name like we did in #2944 and #3122

[jasonho-lynx]

Sure, will take a stab at it when I have the time!