How to manipulate default argument value in python?

July 21, 2022

Consider the code below. It involves application of a very interesting python concept of using default argument during recursion[reversing a list]
A = [1,2,3,4]

def recur(A,Q=[]): 
    if len(A)==1: 
        Q.append(A[0])
        return 
    recur(A[1:],Q)
    Q.append(A[0])
    return Q
print(recur(A)) # 4 3 2 1

Issue is, when I call this function again, Q will not be empty. Is there a way I can set Q=[] when this recursive call finishes ?

NOTE: my intent is not to reverse a list, it’s just for demonstration. Thanks

>Solution :

When you first run the script, you’re initializing and making Q immutable, reguardless of calling the function or future input. You’ll need to recreate Q as a new list every time within the function.

Try this:

def recur(A,Q=None):
    if Q == None:
        Q = []
    if len(A)==1: 
        Q.append(A[0])
        return 
    recur(A[1:],Q)
    Q.append(A[0])
    return Q

print(recur([4, 3, 2, 1])) # 1 2 3 4
print(recur([1, 2, 3, 4])) # 4 3 2 1