Examples of DefineSlot and RedefineSlot for python #20839

Activity

1. acampove

   added

   improvement

   on Jan 11, 2026
2. dpiparo

   assigned

   siliataider

   on Jan 11, 2026
3. ferdymercury commented on Jan 14, 2026

   

   ferdymercury

   on Jan 14, 2026

   Last edited by ferdymercury

   Collaborator

   More actions

   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
4. ferdymercury

   added

   good first issue

   on Jan 15, 2026
5. ferdymercury commented on Jan 15, 2026

   

   ferdymercury

   on Jan 15, 2026

   Last edited by ferdymercury

   Collaborator

   More actions

   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
6. ferdymercury

   added

   in:RDataFrame

   in:Python Interface

   on Jan 16, 2026
7. jainoshika commented on Jan 19, 2026

   

   jainoshika

   on Jan 19, 2026

   More actions

   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
8. 003-kalki commented on Jan 24, 2026

   

   003-kalki

   on Jan 24, 2026

   Contributor

   More actions

   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?
9. gayatripadalia commented on Apr 4, 2026

   

   gayatripadalia

   on Apr 4, 2026

   More actions

   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.
10. vepadulano

    linked a pull request that will close this issueAdd RDF example on thread-safe usage of RNGs #20898on Jun 8, 2026
11. vepadulano

    closed this as completedin #20898on Jul 29, 2026