Quick sort in Python (with tests)

qsort.py

class QSort:
    def __init__(self, A):
        self.A = A
    def partition(self, L, U):
        s = U
        p = L
        while s != p:
            if self.A[p] <= self.A[s]:
                p += 1
            else:
                self.swap(p,s)
                self.swap(p, s-1)
                s -= 1
        return s
    def _sort(self, L, U):
        if L >= U: return
        p = self.partition(L, U)
        self._sort(L, p-1)
        self._sort(p+1, U)
    def sort(self):
        self._sort(0, len(self.A)-1)
    def swap(self, a, b):
        self.A[a], self.A[b] = self.A[b], self.A[a]

def test_normal():
    A = [8,3,4,1]
    As = sorted(A)
    QSort(A).sort()
    assert A == As

def test_empty():
    A = []
    QSort(A).sort()
    assert A == []

def test_1():
    A = [1]
    QSort(A).sort()
    assert A == [1]