Why += on strings is slow in a loop

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Accepted Answer

Because strings are immutable, s += chunk is exactly s = s + chunk - a new string is allocated and every character from both sides is copied into it. After 1000 iterations, the 1000th concatenation copies ~999 characters just to add one more. Total work is O(n^2). Using a list and ''.join() at the end avoids all intermediate copies - each character is written exactly once.