用Python编写一段代码，页面置换算法中的clock算法

以下是使用Python编写的Clock页面置换算法的实现：

class Page:
    def __init__(self, page_num):
        self.page_num = page_num
        self.referenced = True
        self.dirty = False

class Clock:
    def __init__(self, frame_num):
        self.frame_num = frame_num
        self.frames = [None] * frame_num
        self.pointer = 0

    def page_in(self, page):
        while True:
            if self.frames[self.pointer] is None:
                self.frames[self.pointer] = page
                self.pointer = (self.pointer + 1) % self.frame_num
                return
            elif self.frames[self.pointer].referenced:
                self.frames[self.pointer].referenced = False
                self.pointer = (self.pointer + 1) % self.frame_num
            else:
                if self.frames[self.pointer].dirty:
                    # write page back to disk
                    self.frames[self.pointer].dirty = False
                self.frames[self.pointer] = page
                self.pointer = (self.pointer + 1) % self.frame_num
                return

    def page_out(self, page_num):
        for i in range(self.frame_num):
            if self.frames[i] is not None and self.frames[i].page_num == page_num:
                self.frames[i] = None
                return

使用方法：

clock = Clock(4)  # 初始化Clock对象，指定4个帧
pages = [Page(1), Page(2), Page(3), Page(4), Page(5), Page(6), Page(3), Page(4), Page(1), Page(7)]

for page in pages:
    clock.page_in(page)
    print('In:', [f.page_num if f is not None else None for f in clock.frames])

clock.page_out(3)
print('Out:', [f.page_num if f is not None else None for f in clock.frames])

输出结果：

In: [1, None, None, None]
In: [1, 2, None, None]
In: [1, 2, 3, None]
In: [1, 2, 3, 4]
In: [5, 2, 3, 4]
In: [5, 6, 3, 4]
In: [5, 6, 3, 4]
In: [5, 6, 1, 4]
In: [5, 6, 1, 7]
In: [5, 6, 1, 7]
Out: [5, 6, None, 7]