Initializing a TBranch in PyROOT with a custom class in Python

ROOT
1

Hello everyone,

I have a question about how I can make a custom class written in Python known to ROOT so that I can store an object of that class in a ROOT TBranch.

My custom Python class is a simple class that looks like this:

class Event:
    def __init___(self):
        self.event_data = {} # Python dictionary with 36-64 keys
        self.event_num = 0 # Int
    def load_data (self, event_num, data):
        self.event_data = data
        self.event_num = event_num

I then try to load this event into a TBranch as below

with ROOT.TFile("test_tree.root", "RECREATE") as infile:
    tree = ROOT.TTree("Event_Tree", "Event_Tree")
    for times in range(0, 5):
        dummy_event = Event()
        dummy_event.load_data(1, {1: [1, 2, 3], 2:[5, 6, 7]})
        branch_name = "Event_obj"
        tree.Branch(branch_name, dummy_event)
        tree.Fill()
        tree.Write()
    print("Done!")

However, running this code gives me the error “TypeError: Template method resolution failed:
none of the 8 overloaded methods succeeded.” I realize that my custom class “isn’t known to ROOT” as it says on the ROOT CERN TTree documentation page.

How do I make my class known to ROOT so that I can put objects of the Event class in branches of my TTree? For context, I am relatively new to C++ so I would appreciate some pointers here if there is some C++ required!

Thanks!

_ROOT Version: 6.32.02
_Platform: Ubuntu 22.04.5 LTS
_Compiler: linux8664gcc

2

Hello @RobinS,

welcome to the ROOT Forum! You have first to generate the dictionary for your class and then you can use it from Python too. See this example for a vector of vectors structure.

Cheers,
Monica

3

What does generating a dictionary for my class mean in this context? Do you mean that I have to write a C++ file redefining my Python class and use an std::map<int, std::vector>?

If so, when I’m reading my stored events back from my tree, how would I do this? Do I simply create an object of my class first and then read into it like so?

temp_event = Event()
tree.SetBranchAddress("event1", Event)

Will ROOT understand how to link the stored data back into my object structure?

4

Do you mean that I have to write a C++ file redefining my Python class

If you need store a class in a TTree, you will indeed need a C++ (interpreted via Cling and gInterpreter or compiled via the generation of a dictionary).

However you might need to store class and you might be able to just store the individual component of your Event each in their own branch.