What's the time complexity of constructing a Counter object from an iterable in Python?

Question

cnt = Counter ("Hello")
cnt = Counter ([1,2,3,3,3])

I think the time complexities for the code above are O(N), where N is the length of each collection.

I didn't find the exact time complexity of Counter() object here . I really appreciate it if someone can confirm my guess.

Answer 1

As the source code and documentation show, Counter is just a subclass of dict , optimized for counting objects. Constructing it from an iterable has the time complexity of O(n) , since it iterates all items and updates counters for each item in O(1) constant time.

The time complexity of other dict operations will also hold for Counter .