Python deep copy a list into itself n times recursively-CodePudding

I need to append a deep copy (not a reference) of list to itslef recursively n times, for example for

n=0 -> []
n = 1 -> [[]]
n=2 -> [[], [[]]] 
and so forth.

this is what I wrote so far:

def magic_list(n: int) -> List[Any]:
    #take only positive numbers
    assert(n>=0)
    
    if n == 0:
        return []
    else:
        return magic_list_helper(0, n, [])

the main function, I have to keep this def signature exactly the same(takes only n:int). this is the helper function I wrote:

def magic_list_helper(index: int,threshold:int ,lst:List[Any]) -> List[Any]:
    if index >= threshold:
        return lst
    
    else:
        
        # temp_lst = deepcopy(lst)
        # lst.append(temp_lst)
        return magic_list_helper(index   1, threshold, lst)

for this problem I cannot use copy.deepcopy or any other external libraries(thats why its with #, without it it works) and I could not find a solution online without deep copy. Also I cannot use List comprehension, slicing or any O(n) operations on lists(I can use append). Hope you can help me, thanks in advance.

CodePudding user response：

You can try this. Not sure if it's the correct one.

l=[]
n=2
for i in range(n):
    l.append(l.copy())
print(l)

CodePudding user response：

You don't need to make copies. Each call can use a list comprehension that executes n times, calling itself recursively. The list comprehension creates new lists of the appropriate lengths.

You don't even need to check for the base case, because when n == 0 the list comprehension will return an empty list.

def magic_list(n: int) -> List[Any]:
    return [magic_list(x) for x in range(n)]

for i in range(5): print(magic_list(i))

[]
[[]]
[[], [[]]]
[[], [[]], [[], [[]]]]
[[], [[]], [[], [[]]], [[], [[]], [[], [[]]]]]