#
# use a class instanciating a TF1 object
#
import ROOT

# class definition
class polyFunc:
     def __call__(self,arr, par):
         return par[0] + par[1] * arr[0] + par[2] * arr[0] * arr[0]

# Instanciate object
myPol = polyFunc()

# Instanciate TF1 object, need to set the number of parameter
f = ROOT.TF1('pyfunc', myPol, -5., 5.,3)
# Now set the function parameters
f.SetParameters(5., 2., 0.5)

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