Step-by-Step Explanation:
Line 1:
x = [1, 2, 3, 4]
A list [1, 2, 3, 4] is created and assigned to the variable x.
In Python, lists are mutable, which means their elements can be changed after creation.
Line 2:
y = x
The variable y is assigned the reference to the same list as x.
Both x and y now point to the same memory location in Python's memory.
Line 3:
y[0] = 99
This modifies the first element (index 0) of the list y. Since y is referencing the same memory location as x, this change is reflected in x as well.
So now, the list becomes [99, 2, 3, 4].
Line 4:
print(x)
The print() function outputs the current state of x.
Because y[0] = 99 also changed the first element of x, the output will show [99, 2, 3, 4].
Output:
[99, 2, 3, 4]