Pyroot 0 0 4_ Numba Declare

This tutorial illustrates how PyROOT supports declaring C++ callables from Python callables making them, for example, usable with RDataFrame. The feature uses the numba Python package for just-in-time compilation of the Python callable and supports fundamental types and ROOT::RVec thereof.

Author: Stefan Wunsch
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, December 06, 2021 at 09:23 AM.

In [ ]:
import ROOT

To mark a Python callable to be used from C++, you have to use the decorator provided by PyROOT passing the C++ types of the input arguments and the return value.

In [ ]:
@ROOT.Numba.Declare(['float', 'int'], 'float')
def pypow(x, y):
    return x**y

The Python callable is now available from C++ in the Numba namespace. For example, we can use it from the interpreter.

In [ ]:
ROOT.gInterpreter.ProcessLine('cout << "2^3 = " << Numba::pypow(2, 3) << endl;')

Or we can use the callable as well within a RDataFrame workflow.

In [ ]:
data = ROOT.RDataFrame(4).Define('x', '(float)rdfentry_')\
                         .Define('x_pow3', 'Numba::pypow(x, 3)')\
                         .AsNumpy()

print('pypow({}, 3) = {}'.format(data['x'], data['x_pow3']))

ROOT uses the numba Python package to create C++ functions from python ones. We support as input and return types of the callable fundamental types and ROOT::RVec thereof. See the following callable computing the power of the elements in an array.

In [ ]:
@ROOT.Numba.Declare(['RVec<float>', 'int'], 'RVec<float>')
def pypowarray(x, y):
    return x**y

ROOT.gInterpreter.ProcessLine('''
ROOT::RVec<float> x = {0, 1, 2, 3};
cout << "pypowarray(" << x << ", 3) =  " << Numba::pypowarray(x, 3) << endl;
''')

and now with RDataFrame

In [ ]:
s = ROOT.RDataFrame(1).Define('x', 'ROOT::RVecF{1,2,3}')\
                      .Define('x2', 'Numba::pypowarray(x, 2)')\
                      .Sum('x2') # 1 + 4 + 9 == 14
print('sum(pypowarray({ 1, 2, 3 }, 2)) = ', s.GetValue())