I'm using odoo 11
and I'm trying to create a module to show amount in letters (alpha) on quotes and invoices.

Module structure: 

Main folder 
- __init__.py (calling for : from . import models)

- models (folder)

-- models/__init__.py (code : import account_invoice)

-- models/account_invoice.py 

class AccountInvoice(models.Model):
    _inherit = "account.invoice"
    @api.multi
    @api.depends('amount_total')
    def get_amount_letter(self):
        amount = convertion.trad(self.amount_total,self.currency_id.name)
        return amount

-- models/convertion.py (functions)

and a view : report_views.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
	<data>
		<template id="report_invoice_document" inherit_id="account.report_invoice_document">
			<xpath expr="//div[@class='page']/p[position()=last()]" position="after">
				<p>
        			<strong>Arrêter la facture à la somme de:</strong>
        			<span t-esc="o.get_amount_letter()"/>
    			</p>
			</xpath>
		</template>
	</data>
</odoo>

I got the error :

Error to render compiling AST
AttributeError: 'account.invoice' object has no attribute 'get_amount_letter'
Template: account.report_invoice_document_with_payments
Path: /templates/t/t/div/p[4]/span
Node: <span t-esc="o.get_amount_letter()"/>