How to make a TF1 that calls to python code

TF1s with PyROOT

import ROOT
class PoissonClass:
    def __init__(self, upper_range, tag = ''):
        self.function = ROOT.TF1('py_poisson' + tag, self, 0, upper_range, 2)  
        self.function.SetParName(0, 'Overall Normalization')
        self.function.SetParName(1, 'Lambda')

    def __call__(self, x, par):
        res = 0.0
        xx = x[0]
        if xx >= 0:
            try:
                res = par[0]*math.pow(par[1], xx)/\
                  (ROOT.TMath.Gamma(xx+1)*math.exp(par[1]))
            except OverflowError:
                return 0.0 
        return res

    def get_TF1(self):    
        return self.function