Time complexity for running len(array) in python?

Question

How does Python find the length of the array? Is it stored somewhere in an internal data-structure or does it have to iterate through the entire thing to find the length?

I ask because I'm using it for a binary search which should run in O(log(n)), but this obviously doesn't make sense if I have to iterate through the whole thing just to find the length.

Answer 1

Lists can store their length as part of their structure. Since it only needs to be stored in one place, it can add a maximum of O(1) to all computations and therefore not be too much overhead.

Getting the length is therefore O(1) because it's just the lookup of a field.

For more information, see the docs .