Amm using odoo 12 and have a custom python function that have one parametre and i want to call it in my report using _get_report_value,  here is my model 

from odoo import  models,api

class ReportInvoiceWithoutPayment(models.AbstractModel):
    _name = 'report.account.report_invoice'
    _description = 'Account report without payment lines'

    def chiffreen(self, num):

        muz = (' ', 'Onze', 'Douze', 'Treize',
               'Quatorze', 'Quinze', 'Seize', 'Dix-Sept', 'Dix-Huit', 'Dix-Neuf')

        to_19_fr = ('Zéro', 'Un', 'Deux', 'Trois', 'Quatre', 'Cinq', 'Six',
                    'Sept', 'Huit', 'Neuf', 'Dix', 'Onze', 'Douze', 'Treize',
                    'Quatorze', 'Quinze', 'Seize', 'Dix-Sept', 'Dix-Huit', 'Dix-Neuf')
        tens_fr = (
            'Vingt', 'Trente', 'Quarante', 'Cinquante', 'Soixante', 'Soixante-Dix', 'Quatre-Vingt', 'Quatre-Vingt Dix')
        denom_fr = ('',
                    'Mille', 'Million(s)', 'Milliards', 'Billions', 'Quadrillions',
                    'Quintillion', 'Sextillion', 'Septillion', 'Octillion', 'Nonillion',
                    'Décillion', 'Undecillion', 'Duodecillion', 'Tredecillion', 'Quattuordecillion',
                    'Sexdecillion', 'Septendecillion', 'Octodecillion', 'Icosillion', 'Vigintillion')

        def _convert_nn_fr(val):
            """ convertion des valeurs < 100 en Français
            """
            if val < 20:
                return to_19_fr[val]
            for (dcap, dval) in ((k, 20 + (10 * v)) for (v, k) in enumerate(tens_fr)):
                if dval + 10 > val:
                    if val % 10:
                        if (val > 70 and val <= 79):
                            dcap = 'Soixante'
                            return dcap + '-' + muz[val % 10]

                        if (val > 90 and val <= 99):
                            dcap = 'Quatre-vingt'
                            return dcap + '-' + muz[val % 10]
                        else:
                            return dcap + '-' + to_19_fr[val % 10]

                    return dcap

        def _convert_nnn_fr(val):
            """ convert a value < 1000 to french

                special cased because it is the level that kicks
                off the < 100 special case.  The rest are more general.  This also allows you to
                get strings in the form of 'forty-five hundred' if called directly.
            """
            word = ''
            (mod, rem) = (val % 100, val // 100)
            b = val // 100
            if rem > 0:
                if b == 1:
                    word = 'Cent'
                else:
                    word = to_19_fr[rem] + ' Cent'
            if mod > 0:
                word += ' '
            if mod > 0:
                word += _convert_nn_fr(mod)
            return word

        def french_number(val):
            if val < 100:
                return _convert_nn_fr(val)
            if val < 1000:
                return _convert_nnn_fr(val)
            for (didx, dval) in ((v - 1, 1000 ** v) for v in range(len(denom_fr))):
                if dval > val:
                    mod = 1000 ** didx
                    l = val // mod
                    r = val - (l * mod)
                    if (l == 1) and (denom_fr[didx] == 'Mille'):
                        ret = denom_fr[didx]
                    else:
                        ret = _convert_nnn_fr(l) + ' ' + denom_fr[didx]
                    if r > 0:
                        ret = ret + ' ' + french_number(r)
                    return ret

        def amount_to_text_fr(number):
            import math
            number = '%.2f' % number
            units_name = ' '
            list = str(number).split('.')
            muzamil = (french_number(abs(int(list[0]))))
            start_word = muzamil
            end_word = ''
            # french_number(int(list[1]))
            cents_number = int(list[1])
            cents_name = (cents_number > 1) and ' Francs' or ' Franc'
            final_result = start_word + ' ' + units_name + ' ' + end_word + ' ' + cents_name
            return final_result

        def IntOuDouble(num):
            double = True if type(num) == float else False
            if double == True:
                number = str(num)
                premier = number.split('.')[0]
                deuxieme = number.split('.')[1]
                # print(premier)
                # print(deuxieme)
                premier = amount_to_text_fr(int(premier))
                deuxieme = amount_to_text_fr(int(deuxieme))
                premier = premier[:len(premier) - 6]
                deuxieme = deuxieme[:len(deuxieme) - 6] + 'Dinare Algerien'
                # print(premier)
                # print(deuxieme)
                if deuxieme == 'Zéro    Dinare Algerien':
                    res = premier + 'Dinare Algerien'
                else:
                    res = premier + 'virgule ' + deuxieme
                resultat = ''
                for rec in res.split():
                    resultat = resultat + ' ' + rec
                res = resultat
                return res
            else:
                return amount_to_text_fr(num)

        return IntOuDouble(num)

    @api.model
    def _get_report_values(self, docids, data=None):
        docs=self.env['account.invoice'].browse(docids)
        return {
            'doc_ids': docids,
            'doc_model': 'account.invoice',
            'docs':docs ,
            'chiffreen':self.chiffreen(docs.amount_total),
            'report_type': data.get('report_type') if data else '',

        }