def print_first_item(items):
        print items[0]

    This function runs in O(1) time (or "constant time") relative to its input. 

If this were C where the only types are fixed-size, that might be true. But in Python it seems like items[0] could display as 10 gigabytes of JSON.

And if stdout is piped to a process that isn't consuming its input, it might never terminate naturally.

Also, since Python has arbitrarily large integers, an expression like "index += 1" probably runs in O(log(index)) time, not O(1) time.