Python - add key-value pair to dictionary in array

Question

Question is in title. I can't make it work:

>>> data = [{}] * 2
>>> data[1].update({3:4})
>>> data
[{3: 4}, {3: 4}]

key-value pair adds to all elements of array. I expected to receive that:

[{}, {3: 4}]

Answer 1

The problem is that

data = [{}] * 2

Creates a list that has the same dictionary in it twice.

To illustrate this, let's look at id(data[0]) and id(data[1]):

>>> data = [{}] * 2
>>> data
[{}, {}]
>>> id(data[0])
4490760552
>>> id(data[1])
4490760552            

Note that id(data[0]) and id(data[1]) are the same because both entries in the list refer to the same object

What you probably want is

>>> d2 = [{} for i in range(2)]
>>> d2[0][4] = 'a'
>>> d2
[{4: 'a'}, {}]

Answer 2

The problem is, that you added the same dictionary twice to your list.

The trouble is already in your first statement:

data = [{}] * 2

This creates a list with two times the same entry. When you update one of those, the update will be visible in both entries of the list.

Try this:

data = [ {} for i in range(2) ]

When you now do your update, you will get your expected result.

Answer 3

Instead of using:

data = [{}] * 2
data[1].update({3:4})
print data

which adds the same dictionary to your list, use this instead:

data = [{}, {}]
data[1].update({3:4})
print data

data[1] now means the second {} in your list. The result of this program is:

[{}, {3: 4}]

Why doesn't data = [{}] * 2 work? Because it you are making the same thing twice, which means you are forced to replace both {} with {3:4} which gives you the result you had. With data = [{}, {}], you can only change the second dictionary since data[0] is the first one and data[1] is the next item and so on.