Examples of DefineSlot and RedefineSlot for python

[acampove]

Explain what you would like to see improved and how.

I am having again problems with multithreading in python because I seem to be using Define and Redefine, which are not thread safe. There seem to be thread safe versions of these methods called DefineSlot and RedefineSlot. However I am struggling to make it work and I cannot find any example of their usage for python.

ROOT version

6.38

Installation method

mamba

Operating system

almalinux9

Additional context

No response

[ferdymercury]

A (kind of) similar request: https://root-forum.cern.ch/t/random-number-generation-with-rdataframe/64583/3?u=ferhue
and https://root-forum.cern.ch/t/is-there-an-example-of-defineslot-with-rdataframe/34226

It seems (Re)DefineSlot does not work yet with Python

[ferdymercury]

There seem to be thread safe versions of these methods called DefineSlot and RedefineSlot

DefineSlot and RedefineSlot seem unfortunately not yet supported in Python, but luckily they are just a shortcut for Define, so you can get the same functionality in the meantime using the full expression. For example a thread-safe multi-core Gaussian random number generator:

import ROOT
ROOT.gInterpreter.Declare('double myfunctor(UInt_t rdfslot) { static thread_local TRandom3 r3(rdfslot); return r3.Gaus();}')
rdf = ROOT.RDataFrame(90000000)
rdf_x = rdf.Define("x", "myfunctor(rdfslot_)")
h = rdf_x.Histo1D("x")
h.Draw()

What DefineSlot would do, if it worked with python, would be just that you would have it more compact (no need to spell the first variable rdfslot), so is just a shortcut, nothing more fancy. Side note: You still need to pass though more variables if you have extra arguments in your functor, after rdfslot.

rdf_x = rdf.DefineSlot("x", "myfunctor")

Same with RedefineSlot vs Redefine

[jainoshika]

Hi @ferdymercury, I went through this issue and the forum blog you shared. I want to work on a PR for this and wanted to check with you first if I should add documentation / Python examples for DefineSlot and RedefineSlot as part of the PR, or is there anything specific you would like me to focus on?
Thanks

[003-kalki]

Hi @ferdymercury,
I’d like to work on this issue. Could you please assign it to me if no one else is currently working on it?

[gayatripadalia]

Hi @ferdymercury,
I've been looking into this issue and the existing workaround using rdfslot_ with Define. I understand that DefineSlot may not be supported in Python yet.
Would it be useful if I contribute a clear Python example and documentation demonstrating the thread-safe pattern using rdfslot_, as a practical alternative to DefineSlot for thread-safe column definitions?
I can start with that, and also explore whether partial support for string-based expressions is feasible - but happy to follow your guidance on what would be most useful.