python - 如何在 Python 3 中扩展 datetime.date？

Question

我需要一个类来扩展 datetime.date 添加与我的应用程序相关的逻辑。它曾经在 Python 2 中运行良好，但我不明白如何在 Python 3 中编写构造函数。我认为在 Python 3 中编写派生构造函数的标准方法是这样的:

from datetime import date

class my_date(date):
    def __init__(self, year, month, day):
        super().__init__(year, month, day)

date(2020, 4, 18)
my_date(2020, 4, 18)

但是，这段代码失败了:

Traceback (most recent call last):
  File "/tmp/test.py", line 8, in <module>
    my_date(2020, 4, 18)
  File "/tmp/test.py", line 5, in __init__
    super().__init__(year, month, day)
TypeError: object.__init__() takes exactly one argument (the instance to initialize)

我应该怎么做才能正确调用 super 构造函数？

特别是，datetime.date 明确接受用三个数字进行初始化。为什么错误消息说它需要一个参数？为什么问题出在 object.__init__() 而不是 datetime.date.__init__()？

Answer 1

来自评论的回答:

Jens

Looking at the Python source of datetime.date you’ll notice that it doesn’t have an __init__() method, only a __new__() constructor. That would explain the above error. Also note that year, month, day are decorated (i.e. read only) properties, not instance values.

accdias

Replace super().__init__(year, month, day) with pass

from datetime import date

class my_date(date):
        pass

d1 = date(2020, 4, 18)
d2 = my_date(2020, 4, 18)

print(d1,d2) 

输出:

2020-04-18 2020-04-18

无论您需要做什么，您都可以考虑使用组合而不是继承:

class MyDateClass:
    def __init__(self, a_date):
        self.my_date = a_date

    # add members/methods that do stuff wiht a_date

而不是继承日期。

见

• Inheritance vs. Aggregation
• What is the difference between inheritance and composition?