Referencing of custom classes in Python

Question

I understand that custom classes in Python are generally mutable. So in the example below, any changes done to the argument s in the function inc_age is reflected back at the calling section.

class Student:
    def __init__(self, name, age):
            self.name = name
            self.age = age

def inc_age(s):
    s.age += 1


s1 = Student("Test",5)
inc_age(s1)
print(s1.age) # 6

I tried to implement linked list in Python using custom class as shown below:

class ListNode:
    def __init__(self, data=0, next=None):
            self.data = data
            self.next = next

def next_node(L):
    L = L.next

end = ListNode(3)
mid = ListNode(2,end)
beg = ListNode(1,mid)

next_node(beg)
print(beg.data) # 1

My question is why the change in object L in the function next_node not observed at the calling section.

Answer 1

The short version is this:

• If you change attributes of the value in L, those will be represented in any other references to that value.
• If you just change the local variable L to be a name for a completely different value, that doesn't change anything but the local variable L.

If this seems confusing to you, you're not thinking correctly about what variables in Python are. In some languages, like C++, a variable is a location where a value lives. In other languages, like Python, a variable is just a name that you can look up in a namespace to find a value; the values live wherever they want.

So, L = doesn't do anything to whatever value L is a name for, it just makes L into a name, in the local namespace, for a different value.

But L.next = would make next into a name for a different value in the namespace of the value that L refers to. So any other name for the same value as L can see the change.