It may be raised because you tried to write to more than one record at the same time.

If it works for record with single record, Try this code:

@api.depends('projet.projetbc_ligne_ids')

def _total(self):

        for variant in self.projet.projetbc_ids:

            for variant_one in variant:

                variant_one.quantite_encoure = sum(line.quantite for line in self.projet.projetbc_ligne_ids)

hi thank you for your replys in fact i solve it yestrday after posting the quation and yes @IT LIbertqs you re right . i just add the first loop for and it works there 's my code :

@api.depends('projet.projetbc_ligne_ids')

    def _total(self):

        for record in self :

            for variant in record.projet.projetbc_ids:

                variant.quantite_encoure = sum(line.quantite for line in record.projet.projetbc_ligne_ids)

Not sure what projetbc_ids & projetbc_ligne_ids meant.  However, there is a strong error in your code. You are using the decorator api@multi (the default decorator if you haven't specified any). Thus, you should iterate over self.

@api.depends('projet.projetbc_ligne_ids')
def _total(self):
    for object in self:
        for variant in object.projet.projetbc_ids:
            variant.quantite_encoure = sum(line.quantite for line in object.projet.projetbc_ligne_ids)