how to get the declaration of a list done during the unnesting recursively?
<p>I have been getting my head around to do the recursion <strong>without using any global variable</strong>. If am passing the lists the lists of lists ... and resultantly, i should have all the elements from the input in a single list. The problem is every time i call the function the list is declared from the start. </p>

<pre><code>    def unnest (alist):  
        List_ = list()

        for elements in alist: 
            if type(elements) == list: 
                unnest(elements)
            else: 
                List_.append(elements)
        return List_
    if __name__=='__main__':
        unnest([1,2,[3,4,5,[6,7,[8,9,[10,11,[12,[13,[14]]]]]]]])
</code></pre>

<p>You can use inner function which will do the recursion. This has several adventages:</p>

<ul>
<li>The function signature will not change </li>
<li>No use of global variables</li>
<li>You will not create a new list each recursion call (memory friendly)</li>
</ul>

<hr>

<pre><code>def flatten(old_list):    
    def flattenA(old_list,new_list):
        for item in old_list: 
            if isinstance(item,list):
                flattenA(item , new_list)
            else: 
                new_list.append(item)
        return new_list
    return flattenA(old_list,[])

if __name__=='__main__':
    print( flatten([1,2,[3,4,5,[6,7,[8,9,[10,11,[12,[13,[14]]]]]]]]) )
# prints [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
</code></pre>

python python-3.x recursion