Support an "eval" attribute to evaluate widget values in @interact

[jdemeyer]

Let W be a Text widget (or another widget, but Text is a natural use case). If that is used for an interactive (@interact) function, the function receives the text from the input box, i.e. W.value as a string. I propose to support an additional evaluation step such that the interactive function could receive f(W.value) as argument for some callable f.

In the current implementation, this is done with a W.eval attribute. If that attribute exists, then W.eval(W.value) is passed to the interactive function.

The current branch deals with the internal implementation only. At this point, there is no clean user-interface for this (you have to manually set W.eval = f or define a custom widget type with an eval attribute), but that can easily be added later. I also want to avoid bikeshedding about the user-interface, I wanted feedback about the general idea first.

Context: this is to support analogous functionality from SageNB to help with OpenDreamKit/OpenDreamKit#94.

[SylvainCorlay]

Is this a public API at the moment in Sage?

[jdemeyer]

Is this a public API at the moment in Sage?

Yes, it is public: https://github.com/sagemath/sagenb/blob/master/sagenb/notebook/interact.py#L2758

In SageNB, the attribute is called type and it behaves differently from the eval I am proposing here. However, the SageNB type can easily be implemented using this eval.

[SylvainCorlay]

ok although it is surprising to have to reserve an attribute name like eval on all widgets that can be used with interact.

[jdemeyer]

Hang on, I'll propose a slightly different API.

[jdemeyer]

I think that #725 is a better idea.