Modifying class instances in Python - class

Question

I have a small class:
import pickle
import shelve
class Base:
def __init__(self, userid, username, pic_list=[]):
self.userid=userid
self.pic_list=pic_list
self.username_list=[username]
self.username=self.username_list[-1]
def pic_add(self, pic):
self.pic_list.append(pic)
if __name__ == '__main__':
path="D:/"
db = shelve.open(path+'test_base')
db['111']=Base('111','111name',[4,5,6])
db['111'].pic_add('123dsf')
print (sorted(db['111'].pic_list))
db.close()
I want to append 123dsf to pic_list of class instance "111". But the result I get is:
[4, 5, 6]
[Finished in 0.3s]
I want to receive [4, 5, 6, 123dsf]. What am I doing wrong?
Thanx.
P.S. Hint - It is something with shelve module syntax, 'cos adding 'y' works fine:
db['111']=Base('111','111name',[4,5,6])
db['111'].pic_add('123dsf')
Base.pic_add(db['111'],'123dsf')
y=Base('222','222name',[7,8,9])
y.pic_add('pis')
print (y.pic_list)
print (sorted(db['111'].pic_list))
The result is:
[7, 8, 9, 'pis']
[4, 5, 6]
[Finished in 0.4s]

Answer 1

Two ways to do it - as proposed in the documentation: https://docs.python.org/2/library/shelve.html#shelve-example
1. set writeback flag:
db = shelve.open(path+'test_base', writeback=True)
allows you mutate objects in place:
db['111'].pic_add('123dsf')
2. Retrieve copy of stored object, then mutate copy, then store copy back:
cpy = db['111']
cpy.pic_add('123dsf')
db['111'] = cpy