#
# Use a function to instanciate a TF1 object
#
import ROOT

# define the function
def myFunc(arr, par):
    return par[0] + par[1] * arr[0] + par[2] * arr[0] * arr[0]

# create a function, need to set the number of parameters
f = ROOT.TF1('pyfunc', myFunc, -5., 5.,3)
# now set the function parameters
f.SetParameters(5., 2., 0.5)

# plot the function
c = ROOT.TCanvas()
f.Draw()