Why is only the latest data added to the dictionary?

Question

Python

When you try to add data to the dictionary using the put method, only the last entered value is saved (when the count method is displayed, it will give 1, although there are many more surnames). Plus, the __str__ method doesn't output anything at all. What could be the problem? Thanks in advance.

class Vedomost(object):
    def __init__(self, dis, group):
        self.__dis = dis
        self.__group = group
        self.__ved = {}
    @property
    def ved(self):
        return self.__ved
    @property
    def dis(self):
        return self.__dis
    @property
    def group(self):
        return self.__group
    def __str__(self):
        return '{0:10} <=> {1:10} <=> {2:10}'.format(self.__dis, self.__group, self.__ved)
    def put(self, fam, mark):
        self.__ved.update(fam = mark)
    def get(self, fam):
        return self.__ved.get(fam)
    def change(self, fam, mark):
        self.__ved[fam] = mark
    def delete(self, fam):
        del self.ved[fam]
    def result(self):
        exc = 0
        good = 0
        nb = 0
        bad = 0
        nz = 0
        for key in self.ved:
            if self.__ved[key] == 'отл':
                exc += 1
            elif self.__ved[key] == 'хор':
                good += 1
            elif self.__ved[key] == 'уд':
                nb += 1
            elif self.__ved[key] == 'неуд':
                bad += 1
            elif self.__ved[key] == 'н':
                nz += 1
        return (exc, good, nb, bad, nz)
    def count(self):
        return len(self.__ved)
    def names(self):
        return self.__ved.keys()
his = Vedomost(dis = 'История', group = 'BI 1-1')
his.put(fam = 'Иванов', mark = 'уд')
his.put(fam = 'Артемов', mark = 'хор')
his.put(fam = 'Назаров', mark = 'отл')
his.put(fam = 'Иванчук', mark = 'неуд')
his.put(fam = 'Сидоров', mark = 'уд')
his.put(fam = 'Павлов', mark = 'отл')
his.put(fam = 'Рожков', mark = 'отл')
his.put(fam = 'Шефчук', mark = 'н')
his.put(fam = 'Морозов', mark = 'хор')
print(his.result())
print(his.count())

Answer 1

def __str__(self):
    return '{:10s} <=> {:10s} <=> {:s}'.format(self.__dis, self.__group, repr(self.__ved))

def put(self, fam, mark):
    self.__ved[fam] = mark