Item assignment in a nested list

Question

I'm trying to change an item in a nested list. I thought this would be very straightforward. I have the following:

temp = [1, 2, 3, 4]
my_list = [temp for i in xrange(4)]
print "my_list = ", my_list
out: my_list = [[1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]]

So just a normal list of lists. I want to access one item:

print my_list[0][1]
out: 2

As expected. The problem comes when changing the item. I just want to change item in my_list[0][1], but I get the following:

my_list[0][1]= "A"
print my_list
out: [[1, 'A', 3, 4], [1, 'A', 3, 4], [1, 'A', 3, 4], [1, 'A', 3, 4]]

Why is it changing the four positions, instead of just one? How to avoid it?

• Kasravnd: @ajcr I just reopened the question because the suggested question was not exactly about this problem. I will be happy if you find a correct dup.
• Alex Riley: @Kasramvd: Hmm... I think stackoverflow.com/questions/240178/… is the right duplicate for explaining the problem, but I agree that the solution isn't immediately obvious from those answers (so I guess the question could be left open).

Answer 1

Since lists are mutable objects when you repeat a list inside another one you just made a list of same objects (all of them point to one memory address),for getting rid of this problem You need to copy the nested lists in each iteration :

>>> temp = [1, 2, 3, 4]
>>> my_list = [temp[:] for i in xrange(4)]
>>> my_list[0][1]= "A"
>>> print my_list
[[1, 'A', 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]]
>>> 

• Chad S.: This isn't caused by the mutability of lists, it's because everything in python is a reference.
• jonrsharpe: @ChadS. but you don't see the same outcome with immutable objects, which are also referenced.
• Alejandro: By the way... so every time I slice a list, a copy is created instead of access to the same place in memory?
• Kasravnd: @Alejandro It actually create a shallow copy of your lists read more docs.python.org/2/library/copy.html
• Alejandro: First time I post in the community, and feel overwhelmed with the fast responses, clarity and friendly vibe. Thanks for the answers. Very clear now