Finding index in nested list

It always bothers me when someone sticks a for loop in the middle of a recursive function! Here's a different way to go about this problem:

def find_item(list_input, item, index=0):
    if list_input:
        head, *tail = list_input

        if head == item:
            return [index]

        if isinstance(head, list):
            if result := find_item(head, item):
                return [index] + result

        return find_item(tail, item, index + 1)
    
    return list_input

list_input  = [0, 5, [6, 8, [7, 3, 1]], 9, 10]

print(find_item(list_input, 7))

This solution doesn't require an explicit third argument. And returns an empty list upon failure to find the item, rather than None. Note the use of the new walrus operator :=

            if result := find_item(head, item):

If that's a problem, instead do:

            result = find_item(head, item)
            if result:

As @tzaman mentioned, you need to handle the return value of find_item recursive call. If the return value of the recursive call is a list, it will mean that searched item is found and we need to stop the recursion.

The following modification will return the earliest found index of the searched item. If no item is found, it will return None.

def find_item(list_input, item, current):
    if list_input == item:
        return current
    else:
        if isinstance(list_input, list):
            for j in range(len(list_input)):
                current.append(j)
                search_result = find_item(list_input[j], item, current)
                if isinstance(search_result, list):
                    return search_result
                del current[-1]

list_input  = [0, 5, [6, 8, [7, 3, 6]], 9, 10]
item = 7
print(find_item(list_input, item, []))

list_input  = [0, 5, [6, 8, [7, 3, 6]], 9, 10]
item = 9
print(find_item(list_input, item, []))

list_input  = [0, 5, [6, 8, [7, 3, 6]], [30, 4], 9, 10]
item = 4
print(find_item(list_input, item, []))

list_input  = [0, 5, [6, 8, [7, 3, 6]], [30, 4], 9, 10]
item = 400
print(find_item(list_input, item, []))

Output:

[2, 2, 0]
[3]
[3, 1]
None