Shared Reference in Python

Let, we assign a variable x to value 5, and another variable y to the variable x. Python3

x = 5
y = x

When Python looks at the first statement, what it does is that, first, it creates an object to represent the value 5. Then, it creates the variable x if it doesn't exist and made it a reference to this new object 5. The second line causes Python to create the variable y, and it is not assigned with x, rather it is made to reference that object that x does. The net effect is that the variables x and y wind up referencing the same object. This situation, with multiple names referencing the same object, is called a Shared Reference in Python. Now, if we write: Python3

x = 'Geeks'

This statement makes a new object to represent 'Geeks' and makes x to reference this new object. However, y still references the original object i.e 5. Again if we write one more statement as: Python3

y = 10

b This statement causes the creation of a new object and made y to reference this new object. The space held by the prior object is reclaimed if it is no longer referenced, that is, the object's space is automatically thrown back into the free space pool, to be reused for a future object. This automatic reclamation of object's space is known as Garbage Collection.

Shared reference and In- place changes

There are objects and operations that perform in-place object changes. For example, an assignment to an element in a list actually changes the list object itself in-place, rather than creating a new list object. For objects that support in-place changes, you need to be very careful of shared reference, as a change in one can affect others. Python3

L1 = [1, 2, 3, 4, 5]

L2 = L1

Just like x and y, L1 and L2, after statement 2, will refer to the same object. If we change the 0th place value in L1, Now think what will happen, whether it will change only L1 or both L1 and L2 ? Python3

L1 = [1, 2, 3, 4, 5]
L2 = L1

L1[0] = 0

print(L1)
print(L2)

Output:

[0, 2, 3, 4, 5]
[0, 2, 3, 4, 5]

Thus, the change in L1 reflects back to L2. Rather than creating a new object for L1, it overwrites the part of the list object at that place. This is an in-place change. If we still want to maintain a separate copy for L2 such that any changes in L1 doesn't reflect in L2, then we can request Python to create a copy of the list L1 for L2. Python3

L1 = [1, 2, 3, 4, 5]
L2 = L1[:]

L1[0] = 0

print(L1)
print(L2)

Output:

[0, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

Note: This slicing technique doesn't work for dictionaries and sets. Because of Python's reference model, there are two different ways to check for equality in Python Program. Python3

L1 = [1, 2, 3, 4, 5]
L2 = L1

print(L1 == L2)
print(L1 is L2)

Output:

True
True

The first method, the == operator tests whether the referenced objects have the same values, if they have the same values, it returns True, otherwise False. The second method, the is operator, instead tests for object identity - it returns True only if both names point to the exact same object, so basically it is a much stronger form of equality testing. It serves as a way to detect shared references in your code if required. It returns False if the names point to an equivalent but different objects. Now, here comes a tricky part: Look at the following code, Python3

L1 = [1, 2, 3, 4, 5]
L2 = [1, 2, 3, 4, 5]

print(L1 == L2)
print(L1 is L2)

Output:

True
False

What will happen if we perform the same operation on small numbers: Python3

a = 50
b = 50

print(a == b)
print(a is b)

Output:

True
True

Because small integers and strings are cached and reused, therefore they refer to a same single object. And since, you cannot change integers or strings in-place, so it doesn't matter how many references there are to the same object.