How to improve this "Flattening" function from a list

Question

At school we are studying recursion, but of course this is never obvious. We have to create a function that "Flatten" the list. I’ve seen something on the net, and I’ve solved mine in this way:

flattened_list = []

def flatten_list(ls=[]):
    global flattened_list

    for elem in ls:
        if not isinstance(elem, list):
            print("ADDING NO-LIST ELEMENT...")
            flattened_list.append(elem)
        else:
            print("RECURSION...")
            flatten_list(elem)

The problem is that the flattened_list is a global list, which has to be called exactly that, for the function to work. I can improve this aspect?

Answer 1

One option is to create a nested function:

def flatten_list(ls=[]):
    flattened_list = []

    def aux(ls):
        for elem in ls:
            if not isinstance(elem, list):
                print("ADDING NO-LIST ELEMENT...")
                flattened_list.append(elem)
            else:
                print("RECURSION...")
                aux(elem)

    aux(ls)

    return flattened_list

Functional example

---

Note however that there are more efficient and generic ways to implement this function.

Some suggestions:

• Soen: Flattening a Shallow list in Python
• Soen: What is the Fastest way to Flatten arbitrarily nested lists in Python?
• Soen: Flatten (an irregular) list of lists in Python

Answer 2

I already figured it out. Just pass the list as parameter and when I call the function in step recursive step step it smoothly and without modifying it, just?

flattened_list = []

def flatten_list(ls=[], flattened_list=[]):
    for elem in ls:
        if not isinstance(elem, list):
            print("ADDING NO-LIST ELEMENT...")
            flattened_list.append(elem)
        else: # elem is a list
            print("RECURSION...")
            flatten_list(elem, flattened_list) # passando a 'flattened_list' sem problemas
    return flattened_list
#

ls=[12, ["MANO,", "OLA", [12, "COMO", ["ESTAS", ["?", ["?"]]]]], 14, [20]]

# TESTES
print(flatten_list(ls, flattened_list))
print(flattened_list)
print(len(flattened_list))