更新列表中的词典

Question

I think this will be a simple answer and I'm just missing something. I have a list which contains several dictionaries. So for a simplified example the data structure is this:

acctList = [{'acctid' : '101'},{'acctid' : '102'}, {'acctid' : '103'}]

I want update the dictionaries in the list with a new value. but when I do something like this:

acctList[0].update({'acctid' : 'aaa'})

ALL of the dictionaries in the list get updated with the new value instead of just the one at index 0. I know that lists also use update, so I'm not sure how to specify I'm trying to update the dictionary and not the list item. Any ideas? Thanks in advance!

Answer 1

You need to create your dictionaries so that you have a unique dictionary in each index of the list, eg:

acctList = [dict(acctid=str(i)) for i in range(101, 104)]

and acctList is:

[{'acctid' : '101'},{'acctid' : '102'}, {'acctid' : '103'}]

Then you can modify each one separately without affecting the others:

>>> acctList[0].update(dict(acctid='10101'))
>>> acctList
[{'acctid': '10101'}, {'acctid': '102'}, {'acctid': '103'}]

Explanation:

When you create a list where every index points to the same object, and that object is mutable, changing in one place affects the rest because they're all the same object:

>>> l = [[]]*4
>>> l[0].append('foo')
>>> l
[['foo'], ['foo'], ['foo'], ['foo']]

Since integers are immutable, you can do the following:

>>> l = [0]*4
>>> l
[0, 0, 0, 0]
>>> l[0]+=1
>>> l
[1, 0, 0, 0]

In Python, mutable objects are sometimes used to hold and increment on integers when you need a pointer that can't change, and so you can't use a plain integer.