为什么不能对datetime.date进行子类化？ [英] Why can't I subclass datetime.date?

问题描述

为什么没有以下工作（Python 2.5.2）？

Why doesn't the following work (Python 2.5.2)?

>>> import datetime
>>> class D(datetime.date):
        def __init__(self, year):
            datetime.date.__init__(self, year, 1, 1)
>>> D(2008)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: function takes exactly 3 arguments (1 given)

我想创建一个类这就像 datetime.date ，但是使用不同的 __ init __ 函数。显然我的功能永远不会被调用。而是原来的 datetime.date .__ init __ 被调用并失败，因为预期有3个参数，而且我传递一个。

I wanted to create a class that was just like datetime.date, but with a different __init__ function. Apparently my function never gets called. Instead the original datetime.date.__init__ is called and fails because that expects 3 arguments and I am passing in one.

这里发生了什么？这是一个线索吗？

What's going on here? And is this a clue?

>>> datetime.date.__init__
<slot wrapper '__init__' of 'object' objects>

谢谢！

推荐答案

关于其他几个答案，这与C本身实施的日期无关。 __ init __ 方法不执行任何操作，因为它们是不可变的对象，因此构造函数（ __ new __ ）应该做所有的工作。你会看到类似于int，str等的相同行为。

Regarding several other answers, this doesn't have anything to do with dates being implemented in C per se. The __init__ method does nothing because they are immutable objects, therefore the constructor (__new__) should do all the work. You would see the same behavior subclassing int, str, etc.

>>> import datetime
>>> class D(datetime.date):
        def __new__(cls, year):
            return datetime.date.__new__(cls, year, 1, 1)


>>> D(2008)
D(2008, 1, 1)

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