CODEWITHSUNDEEP
pythonobjectpointersreference
IVR
IVR

Reputation: 1858

Objects vs References

I'm trying to understand the difference between references and objects. Please let me know if I'm not using the right terminology.

Consider the following code:

# SCENARIO 1
a = 1
b = a
a = 3
b  # still 1, no surprises there

Also consider the following code:

# SCENARIO 2
class Node:
   def __init__(self, link, value):
       self.link = link
       self.value = value
 
sll = Node(Node(None, 1), 2)
current = sll
current = current.link
current.value = 3
sll.link.value  # updated to 3!

I've seen a lot of similarly phrased questions, but I still cannot understand what makes scenarios 1 and 2 different such that in scenario 2 we can update sll by manipulating its reference, but we cannot do the same in scenario 1.

Upvotes: 2

Views: 61

Answers (1)

Jan Stránský
Jan Stránský

Reputation: 1691

The two codes are essentially different. In scenario 1, yo do just assignments. It would be the same with Nodes (below I use simple list instead):

# SCENARIO 1
a = 1
b = a
a = 3
b  # still 1, no surprises there

# SCENARIO 1
a = [1]
b = a
a = [3]
b  # still [1], no surprises there

Really does not matter what you use here.

# SCENARIO 2
sll = [1,2]
current = sll
current[0] = 3
sll[0]  # updated to 3!

This is because sll and current are same objects. Different names, but the same object, the same place in memory.

You cannot do scenario 2 with int and similar, because int (and similar types) is immutable and you cannot do anything like a[0]=..., a.value=..., you simply cannot mutate it. You can mutate a list (or a Node in OP).

Upvotes: 3

Related Questions