gistfile1.py

from root_numpy import tree2rec
from root_numpy.testdata import get_filepath

from rootpy.io import root_open
import rootpy.compiled as C

C.register_code("""

    double mult(double x, double y)
    {
        return x * y;
    }

""", "mult".split())

assert C.mult(2, 2) == 4

filename = get_filepath('test.root')

with root_open(filename) as rfile:
    print tree2rec(rfile.tree,
        branches=['x', 'y', 'mult(x, y)'],
        selection='z > 0',
        start=0, stop=10, step=2)