[tryton] why doesn' t this field get assigned ?

[Cato Nano]

I have these 2 fields

    numeroGiorni = fields.Function(fields.Integer('Numero 
Giorni'),'on_change_with_numeroGiorni')
    @fields.depends('dataInizio', 'dataFine')
    def on_change_with_numeroGiorni(self, name=None):
        if self.dataInizio and self.dataFine:
            return abs((self.dataFine - self.dataInizio).days) + 1
        return 0

[...]

    fattoreGiorni = fields.Function(fields.Numeric('fattoreGiorni'), 
'on_change_with_fattoreGiorni')

    @fields.depends('numeroGiorni')
    def on_change_with_fattoreGiorni(self, name=None):
        return (self.numeroGiorni - 10) / 5


fattoreGiorni DOESN'T get assigned
I can't see why

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