[skip-ci][tutorials] refactor roofit tutorial

[siliataider]

This failure appeared when testing the python wheels in #19600:

    legend2.SetFillColor("kWhite")
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
TypeError: none of the 2 overloaded methods succeeded. Full details:
  void TAttFill::SetFillColor(TColorNumber) =>
    TypeError: could not convert argument 1
  short conversion expects an integer object

[vepadulano]

@guitargeek I vaguely remember some Pythonizations in RooFit that converted strings with the same name as ROOT constants to the actual constant value to allow for situations similar to the one of this tutorial. Maybe this particular overload is not Pythonized? I could also be misremembering.

[vepadulano]

Thanks a lot! While we understand the situation of the Pythonizations better I leave two very minor comments.

[guitargeek]

Thanks a lot! I'd just suggest so refactor the tutorial such that we don't rely on Pythons variable scoping.

[guitargeek]

@guitargeek I vaguely remember some Pythonizations in RooFit that converted strings with the same name as ROOT constants to the actual constant value to allow for situations similar to the one of this tutorial. Maybe this particular overload is not Pythonized? I could also be misremembering.

This looks like a bug in the implicit conversion of cppyy. It should work, because TColorNumber can be constructed from a string:
https://github.com/root-project/root/blob/master/core/base/inc/TColor.h#L145

So we can merge this PR to make things work, but also open a GitHub issue about this and revert the change once the string version works again (probably with the cppyy update by @Vipul-Cariappa and @aaronj0). I will investigate and open such an issue.

[guitargeek]

Ok, it has nothing to do with cppyy, I just didn't know that two implicit conversions at once are not allowed in C++ 🙂

So this can be fixed on the C++ side: #20271

Let's see how the CI responds to that PR, but if it works In don't think we need to change from "kWhite" back to the more verbose ROOT.kWhite. It would still be worth it to do the refactoring to avoid relying on Python-specific variable scoping though!

Thanks a lot for catching this in any case

[siliataider]

Sure, then I'll wait for the status of that PR before changing this one

[guitargeek]

Thank you very much!