Appending a dictionary to a list – I see a pointer like behavior

Question

I tried the following in the python interpreter:

>>> a = []
>>> b = {1:'one'}
>>> a.append(b)
>>> a
[{1: 'one'}]
>>> b[1] = 'ONE'
>>> a
[{1: 'ONE'}]

Here, after appending the dictionary b to the list a, I’m changing the value corresponding to the key 1 in dictionary b. Somehow this change gets reflected in the list too. When I append a dictionary to a list, am I not just appending the value of dictionary? It looks as if I have appended a pointer to the dictionary to the list and hence the changes to the dictionary are getting reflected in the list too.

I do not want the change to get reflected in the list. How do I do it?

Asked By: neo29

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Source

Answer 1

You are correct in that your list contains a reference to the original dictionary.

a.append(b.copy()) should do the trick.

Bear in mind that this makes a shallow copy. An alternative is to use copy.deepcopy(b), which makes a deep copy.

Answered By: NPE

Answer 2

use copy and deep copy

http://docs.python.org/library/copy.html

Answered By: Mohammad Efazati

Answer 3

Also with dict

a = []
b = {1:'one'}

a.append(dict(b))
print a
b[1]='iuqsdgf'
print a

result

[{1: 'one'}]
[{1: 'one'}]

Answered By: eyquem

Appending a dictionary to a list – I see a pointer like behavior

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