Expose private attribute _config in Figure/FigureWidget class as public property

Hello Dev!

I have been using Plotly's Python library and encountered an issue when trying to set the figure config while preserving on_change callbacks on the same figure object.

The root cause is that _config is currently a private attribute in the BaseFigure class, and the only api way to modify it is through the fig.show(config=config) method call. However, show() rerenders the figure and won't carry existing on_change listeners.

Therefore, in theory, using __setattr__ to modify the _config attribute directly instead of using .show(config=config) would make both setting on_change() and setting config (e.g. scrollZoom = True) work well at the same fig, and this has been proven to be the case in testing.

So I suggest to make _config a public attribute that users can directly set it without calling show() or using setattr.

Many thanks for your supper awesome work on Plotly!

阅读 packages/python/plotly/plotly/basedatatypes.py，并追踪 BaseFigure._config、fig.show(config=...) 和 on_change 回调。确定公共配置 setter 应如何工作而不丢失现有回调；完成标准是可以更改 Figure 和 FigureWidget 的配置，同时回调仍保持附加状态。