How is Python's List Implemented?

Question

Is it a linked list, an array? I searched around and only found people guessing. My C knowledge isn\'t good enough to look at the source code.

Answer 1

As others have stated above, the lists (when appreciably large) are implemented by allocating a fixed amount of space, and, if that space should fill, allocating a larger amount of space and copying over the elements.

To understand why the method is O(1) amortized, without loss of generality, assume we have inserted a = 2^n elements, and we now have to double our table to 2^(n+1) size. That means we're currently doing 2^(n+1) operations. Last copy, we did 2^n operations. Before that we did 2^(n-1)... all the way down to 8,4,2,1. Now, if we add these up, we get 1 + 2 + 4 + 8 + ... + 2^(n+1) = 2^(n+2) - 1 < 4*2^n = O(2^n) = O(a) total insertions (i.e. O(1) amortized time). Also, it should be noted that if the table allows deletions the table shrinking has to be done at a different factor (e.g 3x)