In addition, in your module, you redefine all culumns where _amount_all is called. For v7:

_columns  = {

        'amount_untaxed': fields.function(_amount_all, digits_compute=dp.get_precision('Account'), string='Subtotal', track_visibility='always',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line'], 20),
                'account.invoice.tax': (_get_invoice_tax, None, 20),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 20),
            },
            multi='all'),

        'amount_tax': fields.function(_amount_all, digits_compute=dp.get_precision('Account'), string='Tax',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line'], 20),
                'account.invoice.tax': (_get_invoice_tax, None, 20),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 20),
            },
            multi='all'),

        'amount_total': fields.function(_amount_all, digits_compute=dp.get_precision('Account'), string='Total',
            store={
                'account.invoice': (lambda self, cr, uid, ids, c={}: ids, ['invoice_line'], 20),
                'account.invoice.tax': (_get_invoice_tax, None, 20),
                'account.invoice.line': (_get_invoice_line, ['price_unit','invoice_line_tax_id','quantity','discount','invoice_id'], 20),
            },
            multi='all'),

}

The method will be called.