What is the cost/ complexity of insert in list at some location?

Question

In Python, a list has list.insert(i, x) to "Insert an item at a given position.". In C++, there is a list as well. In C++, cost/complexity of inserting an element anywhere is O(1). Is it the same for a Python list? If not, can anything else be use to get O(1) insert time in Python?

Answer 1

list

The Average Case assumes parameters generated uniformly at random.

Internally, a list is represented as an array; the largest costs come from growing beyond the current allocation size (because everything must move), or from inserting or deleting somewhere near the beginning (because everything after that must move). If you need to add/remove at both ends, consider using a collections.deque instead.

So inserting an element at given position will always have the time complexity of O(n) as both insert method and slicing has time complexity of O(n) and O(k). Only append which inserts at the end of list have O(1) time complexity. From Python Wiki

Lists:
                               Complexity
Operation     | Example      | Class         | Notes
--------------+--------------+---------------+-------------------------------
Index         | l[i]         | O(1)          |
Store         | l[i] = 0     | O(1)          |
Length        | len(l)       | O(1)          |
Append        | l.append(5)  | O(1)          |
Clear         | l.clear()    | O(1)          | similar to l = []

Slice         | l[a:b]       | O(b-a)        | l[1:5]:O(l)/l[:]:O(len(l)-0)=O(N)
Extend        | l.extend(...)| O(len(...))   | depends only on len of extension
Construction  | list(...)    | len(...)      | depends on lenghth of argument

check ==, !=  | l1 == l2     | O(N)          |
Insert        | l[a:b] = ... | O(N)          |
Delete        | del l[i]     | O(N)          | 
Remove        | l.remove(...)| O(N)          | 
Containment   | x in/not in l| O(N)          | searches list
Copy          | l.copy()     | O(N)          | Same as l[:] which is O(N)
Pop           | l.pop(...)   | O(N)          |
Pop           | l.pop()      | O(1)          | same as l.pop(-1), popping at end
Extreme value | min(l)/max(l)| O(N)          |
Reverse       | l.reverse()  | O(N)          |
Iteration     | for v in l:  | O(N)          |

Sort          | l.sort()     | O(N Log N)    | key/reverse doesn't change this
Multiply      | k*l          | O(k N)        | 5*l is O(N): len(l)*l is O(N**2)

From here

Answer 2

The Python language doesn't specify the implementation of such operations, so different implementations may have different behavior. For CPython, the complexity of list.insert is O(n), as shown on this useful wiki page. I'm not aware of any list-like structure giving O(1) performance for inserting at an arbitrary index. (A dict gives O(1) insert performance in the average case, but is not ordered and does not enforce a contiguous sequence of indices.) The blist library provides an optimized list type that has an O(log n) insert.

Answer 3

No, it is not the same complexity. According to Python's official Time Complexity page¹, using list.insert always has O(n) (linear) complexity.

Also, a Python list is not exactly the same as a C++ list. In fact, a Python list is more comparable to a C++ std::vector if anything.

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_{¹Well, CPython's official page. I don't know about other implementations such as IronPython or Jython.}

Answer 4

Python is a language. Multiple implementations exist, and they may have different implementations for lists. So, without looking at the code of an actual implementation, you cannot know for sure how lists are implemented and how they behave under certain circumstances.

My bet would be that the references to the objects in a list are stored in contiguous memory (certainly not as a linked list...). If that is indeed so, then insertion using x.insert will cause all elements behind the inserted element to be moved. This may be done efficiently by the hardware, but the complexity would still be O(n).

For small lists the bisect operation may take more time than x.insert, even though the former is O(log n) while the latter is O(n). For long lists, however, I'd hazard a guess that x.insert is the bottleneck. In such cases you must consider using a different data structure.