a list of similar objects

Question

I needed to create a list of None objects and used this form

a = [None] *2
a[0] = 0

the value of a was:

[1, None]

however, when i tried the same thing with:

a = [{'elem' : 0}] * 2

and after that

a[1]['elem'] = 0

the value of a became:

[{'elem': 1}, {'elem': 1}]

can anyone help me understand the mechanism behind this?

@MartijnPieters But that question doesn't explain the problem using this with mutable objects, right? — thefourtheye, Jan 23 '14 at 13:24
@thefourtheye: lists are mutable objects just as much as dicts are. — Martijn Pieters, Jan 23 '14 at 13:25
@thefourtheye: We'd be better off closing this as a dupe of [Unexpected feature in a Python list of lists](http://stackoverflow.com/q/240178) then. — Martijn Pieters, Jan 23 '14 at 13:25

score 1 · Answer 1 · answered Jan 23 '14 at 13:26

1

a = [{'elem' : 0}] * 2

'a' contains two references to the same dictionary. The equivalent of the first case would be:

a[0] = {'elem' : 1}

answered Jan 23 '14 at 13:26

amsh

score 0 · Answer 2 · answered Jan 23 '14 at 13:25

By using multiplication, you created list of references to the same dictionary

In [11]: a = [{'elem' : 0}] * 2
In [14]: for elem in a:
    print id(elem)
   ....:     
142676420
142676420

To create list of 2 independent dictionaries

In [15]: a = [{'elem' : 0} for _ in range(2)]

In [16]: for elem in a:
    print id(elem)
   ....:     
142695116
142695932

2 Answers2