Python Coding challenge - Day 245 | What is the output of the following Python Code?

 

Let's break down the code step by step to explain what happens in the modify_list function and why the final result of print(my_list) is [1, 2, 3, 4].

def modify_list(lst, val):

    lst.append(val)

    lst = [100, 200, 300]

my_list = [1, 2, 3]

modify_list(my_list, 4)

print(my_list)

Step-by-step Explanation:

Function Definition: The function modify_list(lst, val) accepts two arguments:

lst: a list passed by reference (so modifications within the function affect the original list unless reassigned).

val: a value that will be appended to the list lst.

Initial State of my_list: Before calling the function, the list my_list is initialized with the values [1, 2, 3].

Calling the Function:

modify_list(my_list, 4)

We pass the list my_list and the value 4 as arguments to the function.

Inside the function, lst refers to the same list as my_list because lists are mutable and passed by reference.

First Line Inside the Function:

lst.append(val)

lst.append(4) adds the value 4 to the list.

Since lst refers to the same list as my_list, this operation modifies my_list as well.

At this point, my_list becomes [1, 2, 3, 4].

Reassignment of lst:

lst = [100, 200, 300]

This line creates a new list [100, 200, 300] and assigns it to the local variable lst.

However, this reassignment only affects the local variable lst inside the function. It does not modify the original list my_list.

After this line, lst refers to the new list [100, 200, 300], but my_list remains unchanged.

End of the Function: When the function finishes execution, lst (which is now [100, 200, 300]) is discarded because it was only a local variable.

my_list retains its modified state from earlier when the value 4 was appended.

Final Output:

print(my_list)

When we print my_list, it shows [1, 2, 3, 4] because the list was modified by lst.append(val) but not affected by the reassignment of lst.

Key Takeaways:

List Mutation: The append() method modifies the list in place, and since lists are mutable and passed by reference, my_list is modified by lst.append(val).

Local Reassignment: The line lst = [100, 200, 300] only reassigns lst within the function's scope. It does not affect my_list outside the function because the reassignment creates a new list that is local to the function.

Thus, the final output is [1, 2, 3, 4].