Python Coding challenge - Day 577| What is the output of the following Python Code?

 

Code Explanation:

1. Function Definition

def mutate(x, y=[]):

This defines a function mutate with:

A required parameter x

An optional parameter y, which defaults to an empty list [].

Important: In Python, default mutable arguments like lists are shared between function calls unless explicitly overridden.

2. Append x to y

    y.append(x)

Appends the value of x to the list y.

If y is not passed in when calling the function, it uses the default list, which persists between calls.

3. Reassign y to a New List

    y = []

This line reassigns the local variable y to a brand new empty list, but:

This does not affect the original list that y.append(x) modified.

The mutation (append) happened on the shared default list, but this reassignment is just local and temporary.

4. Return y

    return y

Returns the new empty list that was just created.

So, it always returns [], regardless of what was appended earlier.

Print Statements

First Call

print(mutate(1))  # y = [], then y.append(1), then y = [], return []

Appends 1 to the default list (y becomes [1])

Then reassigns y to a new list [] and returns it.

Output: []

Second Call

print(mutate(2))  # y still holds the earlier appended value: [1]

Now the default list y is already [1] from the previous call.

Appends 2, so default y becomes [1, 2]

Again, reassigns y = [] and returns it.

Output: []

Final Output

[]

[]

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