Appending an item to the array

2026-03-17

`list.append()` updates a list in place, while `a = a + [x]` creates a new list and costs O(K) time

append vs + [item]

These two lines look similar, but they behave differently.

Option A:

a.append("a")

Option B:

a = a + ["a"]

What is the difference?

Option A updates the existing list in place.

Option B creates a new list, copies all elements from a, and then adds the new item.

If the length of a is K, then:

  • a.append("a") is O(1) amortized
  • a = a + ["a"] is O(K)

So even though both forms add one item, Option B is more expensive.

The same issue in a dictionary of lists

This pattern has the same problem:

dic = {}
dic[key] = dic.get(key, []) + ["a"]

dic.get(key, []) + ["a"] creates a new list every time.

Better approach: defaultdict

If you want to accumulate values efficiently, keep the existing list and append to it:

from collections import defaultdict

hashmap = defaultdict(list)
hashmap[key].append(w)

This avoids repeated list copying and keeps the code simple.