雙向轉換

描述符物件可以允許相關的物件屬性自動響應更改。

假設我們想要建模具有給定頻率（赫茲）和週期（以秒為單位）的振盪器。當我們更新頻率時，我們希望更新週期，當我們更新週期時，我們希望頻率更新：

 >>> oscillator = Oscillator(freq=100.0)  # Set frequency to 100.0 Hz
>>> oscillator.period  # Period is 1 / frequency, i.e. 0.01 seconds
0.01
>>> oscillator.period = 0.02  # Set period to 0.02 seconds
>>> oscillator.freq # The frequency is automatically adjusted
50.0
>>> oscillator.freq = 200.0  # Set the frequency to 200.0 Hz
>>> oscillator.period  # The period is automatically adjusted
0.005

我們選擇其中一個值（頻率，以赫茲為單位）作為錨點，即可以在沒有轉換的情況下設定的值，併為其編寫描述符類：

class Hertz(object):
    def __get__(self, instance, owner):
        return self.value

    def __set__(self, instance, value):
        self.value = float(value)

其他值（週期，以秒為單位）是根據錨定義的。我們編寫了一個描述符類來完成轉換：

class Second(object):
    def __get__(self, instance, owner):
        # When reading period, convert from frequency
        return 1 / instance.freq
    
    def __set__(self, instance, value):
        # When setting period, update the frequency
        instance.freq = 1 / float(value)

現在我們可以寫出振盪器類了：

class Oscillator(object):
    period = Second()  # Set the other value as a class attribute

    def __init__(self, freq):
        self.freq = Hertz()  # Set the anchor value as an instance attribute
        self.freq = freq  # Assign the passed value - self.period will be adjusted