Question

I was writing a python function that looked something like this

def foo(some_list):
   for i in range(0, len(some_list)):
       bar(some_list[i], i)

so that it was called with

x = [0, 1, 2, 3, ... ]
foo(x)

I had assumed that index access of lists was O(1), but was surprised to find that for large lists this was significantly slower than I expected.

My question, then, is how are python lists are implemented, and what is the runtime complexity of the following

• Indexing: list[x]
• Popping from the end: list.pop()
• Popping from the beginning: list.pop(0)
• Extending the list: list.append(x)

For extra credit, splicing or arbitrary pops.

Answer 1

there is a very detailed table on python wiki which answers your question.

However, in your particular example you should use enumerate to get an index of an iterable within a loop. like so:

for i, item in enumerate(some_seq):
    bar(item, i)