List getting replaced

Question

I have the following piece of code

>>> no=[]
>>> matrix = [[[] for x in xrange(4)] for x in xrange(4)] 
>>> no=[2]
>>> temp = no
>>> matrix[1][2].append(temp)
>>> no.append(3)
>>> matrix[1][2]

when I print matrix[1][2] I am getting the following output:

[[2, 3]]

But what I want is:

[2]

So basically what is happening is the list is getting changed inside the matrix list if I change it afterwards. But I don't want to get it changed

How can I do it ? And why is it happening ?

For matrices I would suggest looking into [`numpy`](http://www.numpy.org/) — James Mertz, Aug 06 '14 at 16:17
This has to be a duplicate, but I can't find the original question anywhere. I need to bookmark it because new questions about it happen all the time. — TheSoundDefense, Aug 06 '14 at 16:23

score 3 · Accepted Answer · answered Aug 06 '14 at 16:15

Make a copy of no temp = no[:].

temp = no is just a reference to the same object so any changes in no will affect temp

A reference:

In [21]: my_l = [1,2,3]

In [22]: ref_my_l = my_l

In [23]: ref_my_l
Out[23]: [1, 2, 3]

In [24]: my_l.append(4)

In [25]: my_l
Out[25]: [1, 2, 3, 4]

In [26]: ref_my_l
Out[26]: [1, 2, 3, 4]

A copy:

In [27]: my_l = [1,2,3]

In [28]: copy_my_l = my_l[:]

In [29]: my_l.append(4)

In [30]: my_l
Out[30]: [1, 2, 3, 4]

In [31]: copy_my_l
Out[31]: [1, 2, 3]

List getting replaced

1 Answers1