我可以更改 python 子类中的基类值吗？

Question

当子类化基本类型（例如 float）时，是否可以“重新计算”或“重新分配”原始值？如果我有以下类定义，

import collections

class MovingAverage(float):
    def __new__(self, initial_value, window):
        self.d = collections.deque([initial_value], window)
        return float.__new__(self, initial_value)
    def add(self, new_value):
        self.d.append(new_value)
        print sum(self.d) / len(self.d)
        # here, I want to _set_ the value of MovingAverage to
        #     sum(self.d) / len(self.d)

当我从

>>> ma = MovingAverage(10, 3)
>>> ma
10.0

but

>>> ma.add(3)
6.5
>>> ma
10.0

开始时，我尝试过的另一个类定义是这样的：

import collections

class MovingAverage(float):
    def __new__(self, iterable, window):
        self.d = collections.deque(iterable, window)
        initial_value = sum(iterable) / len(iterable)
        return float.__new__(self, initial_value)
    def add(self, new_value):
        self.d.append(new_value)
        return MovingAverage(self.d, self.d.maxlen)

这次，当我从

>>> ma = MovingAverage([10], 3)
>>> ma
10.0

and

>>> ma.add(3)
6.5
>>> ma
10.0
>>> ma = ma.add(3)
>>> ma
5.333333333333333

开始时，我认为（我还没有测试发现）这使得这显着慢点。那么，可以吗？我可以以某种方式设置它，以便从 ma 返回的值就是我正在寻找的值吗？或者我是否需要定义一个 value 属性，将基类更改为 object，并放弃我有机会控制该类的返回值的假象？

Answer 1

不可以。由于这些类型是不可变的，因此您应该使用封装，而不是继承。

Answer 2

我不认为上面的 Ignacios 答案需要任何改进，但由于我有这门课，我只是认为我应该分享。它避免了多次进行大和运算，还避免了使用（更）幼稚的算法时可能出现的舍入错误：

class MovingAverage:
    def __init__(self):
        self.sum = None
        self.num = 0

    def add(self, val):
        if self.sum is None:
            self.sum = val
        else:
            self.sum += val
        self.num += 1
        return self.val

    @property
    def val(self):
        if self.sum is None:
            return None
        else:
            return self.sum/self.num

if __name__ == "__main__":
    print("Running small test suite")
    fail = False

    m = MovingAverage()
    try:
        assert m.val is None, "A new MovingAverage should be None"
        assert m.add(10) == 10, "New average should be returned"
        assert m.val == 10, "The average should update for each add"
        assert m.add(20) == 15, "New average should be returned"
        assert m.val == 15, "The average should update for each add"
        assert m.add(0) == 10, "Adding zero should work"
        assert m.add(-10) == 5, "Adding a negative number should work"
        assert m.add(-1) == 19/5, "Result should be able to be a fraction"
    except AssertionError as e:
        print("Fail: %s" % e.args[0])
        fail = True

    if not fail: print("Pass")