Python implementation of quick sorting

#! /usr/bin/env python# -*- coding:utf-8 -*-import sysclass sort(object) :    @classmethod    def __bubble_sort(cls, data = [], start = 0, end = -1) :        if end < 0 :            end += len(data)        if end > len(data) :            end = len(data)        if end <= start :            return None        for i in range(start, end) :            for j in range(i + 1, end + 1) :                if data[i] > data[j] :                    data[i], data[j] = data[j], data[i]        return None    @classmethod    def __quick_sort(cls, data = [], start = 0, end = -1) :        def __mid__(data, a, b, c) :            if data[a] > data[b] :                if data[a] < data[c] :                    return a                elif data[b] > data[c] :                    return b                else :                    return c            else :                if data[b] < data[c] :                    return b                elif data[a] > data[c] :                    return a                else :                    return c        if (end - start + 1) < 3 :            return cls.__bubble_sort(data, start, end)        if end < 0 :            end += len(data)        if end > len(data) :            end = len(data)        if end <= start :            return None        pivot = __mid__(data, start, start + (end - start) / 2, end)        data[pivot], data[end] = data[end], data[pivot]        pivot = end        i = start        j = end - 1        while True :            if data[i] > data[pivot] :                data[i], data[j] = data[j], data[i]            else :                i += 1            if i > j :                break;            if data[j] < data[pivot] :                data[j], data[i] = data[i], data[j]            else :                j -= 1            if i > j :                break;        data[i], data[pivot] = data[pivot], data[i]        pivot = i        cls.__quick_sort(data, start, pivot - 1)        cls.__quick_sort(data, pivot + 1, end)    @classmethod    def do_sort(cls, data, start = 0, end = -1) :        cls.__quick_sort(data, start, end)