Solving the complexity of bubblesort

bubblesort.py

class BubbleSort:
    def __init__(self, items):
        self.items = items
        self.operations = 0

    def sort(self):
        while not self.sorted():
            for i in range(1, len(self.items)):
                self.operations += 1
                a, b = self.items[i - 1:i + 1]
                if a > b:
                    self.items[i - 1] = b
                    self.items[i] = a
        return self.items

    def sorted(self):
        last = self.items[0]
        for item in self.items[1:]:
            if item < last:
                return False
            last = item
        return True


def count_worst_complexity(case):
    items = list(range(case - 1, -1, -1))
    sort = BubbleSort(items)
    sort.sort()
    return sort.operations


def main():
    for n in range(2, 21):
        print(n, count_worst_complexity(n))


if __name__ == '__main__':
    main()