Explanation: the shortcuts based on
+ (including the implied use in
sum) are, of necessity,
O(T**2) when there are T sublists -- as the intermediate result list keeps getting longer, at each step a new intermediate result list object gets allocated, and all the items in the previous intermediate result must be copied over (as well as a few new ones added at the end). So, for simplicity and without actual loss of generality, say you have T sublists of k items each: the first k items are copied back and forth T-1 times, the second k items T-2 times, and so on; total number of copies is k times the sum of x for x from 1 to T excluded, i.e.,
k * (T**2)/2.
The list comprehension just generates one list, once, and copies each item over (from its original place of residence to the result list) also exactly once.
Make a function which append all the elements of list of list into the flatten list.
for i in lst_of_lst:
lst_of_lst = [1,2,[3,4,5,],[7,[8,]]]
flatten_lst = 
Original list: [1, 2, [3, 4, 5, ], [7, [8, ]]]
New list: [1, 2, 3, 4, 5, 6, 7, 8, 9]