How to RNTupleModel.MakeField in python

adrian_sev · August 6, 2025, 2:50pm

Hi! I try to create (and fill) in python a RNTuple but it is not clear to me the python syntax to add a field ..
for the c++ syntax i have the error:

./json2rntuple.py
Traceback (most recent call last):
  File "/home/adrian/work-GRID/root_json/./json2rntuple.py", line 20, in <module>
    name       = model.MakeField<str>("name");
TypeError: '<' not supported between instances of '_RNTupleModel_MakeField' and 'type'

while not using the type specification yields:

./json2rntuple.py
Traceback (most recent call last):
  File "/home.hdd/adrian/work-GRID/root_json/./json2rntuple.py", line 20, in <module>
    name       = model.MakeField("name");
  File "/usr/lib64/python3.9/site-packages/ROOT/_pythonization/_rntuple.py", line 110, in __call__
    self._original_method(*args)
TypeError: Template method resolution failed:
  Failed to instantiate "MakeField(std::string)"

Thanks a lot!

couet · August 6, 2025, 7:11pm

I guess @jblomer can help you.

jblomer · August 7, 2025, 4:44am

The template parameters are specified in brackets, like

model.MakeField["std::string"]("name")

Unfortunately, we don’t have Python tutorials yet. Meanwhile, you can use the tests as blueprint:

github.com/root-project/root

tree/ntuple/test/ntuple_basics.py

master

import unittest

import ROOT


class RNTupleBasics(unittest.TestCase):
    """Basic tests of using RNTuple from Python"""

    def test_write_read(self):
        """Can write and read a basic RNTuple."""

        model = ROOT.RNTupleModel.Create()
        model.MakeField["int"]("f")
        model.MakeField["std::string"]("mystr")

        with ROOT.RNTupleWriter.Recreate(model, "ntpl", "test_ntuple_py_write_read.root") as writer:
            entry = writer.CreateEntry()
            entry["f"] = 42
            entry["mystr"] = "string stored in RNTuple"
            writer.Fill(entry)

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github.com/root-project/root

tree/ntuple/test/ntuple_model.py

master

import unittest

import ROOT


class NTupleModel(unittest.TestCase):
    """Various tests for the RNTupleModel class"""

    def test_create_model(self):
        """A model can be created."""

        model = ROOT.RNTupleModel.Create()
        self.assertTrue(model)

    def test_create_bare_model(self):
        """A bare model can be created."""

        model = ROOT.RNTupleModel.CreateBare()
        self.assertTrue(model)

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