Btukfyl

I am really interested how Python memory allocation works.

I have written 2 pieces of code:

1.

list = 

a = [1,2,3]

list.append(a)

a = [3,2,1]

print(list)

print(a)

2.

def change(list):

    list[1] = 50

    list[2] = 70



list = 

a = [1,2,3]

list.append(a)

change(a)

print(list)

print(a)

When I compile the first code i get result [[1,2,3]] and [3,2,1].

However, when I compile the second code the result I get is [1,50,70].

In the first case I create an object "a" and an array "list". When I append my object "a" to the array, array points at it in fact. Then, when I am assigning "a" new array [3,2,1], the object [1,2,3] is saved in the array and "a" points at new array [3,2,1].

In the second case I also create an object "a" and an array "list". I append "a" to array and there's also a pointer, that points at "a". Then I'm calling a method on an array "a". After calling the function and changing the value of the elements, array "list" still points at object "a" without creating a new instance.

Can somebody explain me the way it really works?

Every variable you create in Python is a reference to an object. Lists are objects which contain multiple references to different objects.

Almost all operations in python modify these references, not the objects. So when you do list[1] = 50 you are modifying the second reference that the list item contains.

I have found that a useful tool for visualising this is Python Tutor

So for your first example

list =  # you create a reference from `list` to the the new list object you have created

a = [1,2,3] # you create a reference from `a` to the new list you have created which itself contains 3 references to three objects, `1`, `2` and `3`.

list.append(a) # this adds another reference to `list` to the object referenced by `a` which is the object `[1, 2, 3]`

a = [3,2,1] # create a reference from `a` to the new list you have created which itself contains 3 references to three objects, `3`, `2` and `1`.

print(list) # print the object referenced by `list`

print(a) # print the object referenced by `a`

For your second example

def change(list): # create a reference from `change` to a function object

    list[1] = 50 #change the second reference in the object referenced by `list` to `50`

    list[2] = 70 #change the third reference in the object referenced by `list` to `70`



list =  # create a reference from `list` to a new list object

a = [1,2,3] # create a reference from `a` to a new list object which itself contains three references to three objects, `1`, `2` and `3`.

list.append(a) # add a reference to list to the object pointed to by `a`

change(a) # call the function referenced by change

print(list) # print the object referenced by `list`

print(a) # print the object referenced by `a`