Assigning lists to eachother in Python

Question

Can someone explain me why this python code prints 6 instead of -10?

Why does s2/list2 not change throughout this piece of code?

def f(s1,s2):
 s2 = s1
 s1[2] = -7
 s1 = s2
 s2[2] = -10
list1 = [1,2,3]
list2 = [4,5,6]
f(list1,list2)
print(list2[2])

• Ilja Everilä: Because the first thing you do in f is that you re-bind the name s2 to the list referenced by s1. The latter s1 = s2 is effectively no-op. Could you please explain a bit more why and what did you expect.
• Yonas Kassa: because python lists are passed by reference. google passing by reference vs passing by value
• Ilja Everilä: You should read nedbatchelder.com/text/names.html
• Alexander Ameye: @IljaEverilä hey that's interesting, thanks!

Answer 1

With s2 = s1 you just lost the local reference to s2 (i.e list2). From there on, you're altering s1 (list1), assigning -7 to list1[2] and then -10.

You aren't changing list2 in any way. Use a different name in the assignment to get it to work, s = s1, for example.

Answer 2

After the line s2 = s1 inside the function, the s2 you passed in is no longer relevant inside the function.

That line assigns the value (i.e. list and contents) to the variable s2.