Problem with lists

Question

Can someone explain why the output of the code is [1, [3, 6]] not [9, [3, 6]]?

a = [1, [3,4]]
b = a[ : ]
a[0] = 9
a[1][1] = 6
print(b)

Answer 1

As i remember, only numbers, strings, booleans and None are stored specifically, others are treated as objects, so they can't be copied and return the reference when you assign it to a variable🤔

For example,

a = 3
b = a # pass by value

c = "test"
d = c # pass by value

e = [1,2,3]
f = e # pass by reference

when you use b=a[:]
you can think it like

b = [None]*len(a)
for p in range(len(a)):
b[p] = a[p]

if a[p] is an object, it gets the reference, not only its value

Answer 2

Like TurtleShell said, a[:] returns a shallow copy of a, that is, it just copies references to the objects in a. For immutable objects like integers, there's no noticeable difference, but mutable objects like lists would change whenever the original ones are changed.

So changing a[0] doesn't affect b[0], as it's an integer. But a[1] is a list, and its modifications would be reflected in b[1].

Answer 3

Nested lists don’t get deep copied I guess?

Answer 4

a[:] instead of just a deep copies the list meaning b gets a brand new list instead of a reference to a if you want a and b to share a list just remove the [:] like so:
a = [1, [3, 4]]
b = a

Answer 5

Got the picture. Thanks :)

Answer 6

Thank you all for your answers. I think we reached to the bottom of this! ;) Have a good wknd!

Answer 7

I understand that b is copy of a, but can't fathom why a[1][1] = 6 changes the copy and a[0] = 9 not.