Why can't I subclass datetime.date?

Question

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)

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.

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

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

Thanks!

Answer 1

Here's the answer, and a possible solution (use a function or strptime instead of subclassing)

http://www.mail-archive.com/[email protected]/msg192783.html

Answer 2

You should probably use a factory function instead of creating a subclass:

def first_day_of_the_year(year):
  return datetime.date(year, 1, 1)

Answer 3

You're function isn't being bypassed; Python just never gets to the point where it would call it. Since datetime is implemented in C, it does its initialization in datetime.__new__ not datetime.__init__. (Note that this could change sometime.) You could presumably get around this by overriding __new__ instead of __init__. But as other people have suggested, the best way is probably not subclassing datetime at all.

Answer 4

You can wrap it and add extended functionality to your wrapper.

Here is an example:

class D2(object):
    def __init__(self, *args, **kwargs):
        self.date_object = datetime.date(*args, **kwargs)

    def __getattr__(self, name):
        return getattr(self.date_object, name)

And here is how it works:

>>> d = D2(2005, 10, 20)
>>> d.weekday()
3
>>> dir(d)
['__class__', '__delattr__', '__dict__', '__doc__', '__getattr__',
 '__getattribute__', '__hash__', '__init__', '__module__', '__new__',
 '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__',
 '__weakref__', 'date_object']
>>> d.strftime('%d.%m.%Y')
'20.10.2005'
>>>

Note that dir() doesn't list datetime.dates attributes.

Answer 5

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)

Answer 6

Please read the Python reference on Data model, especially about the __new__ special method.

Excerpt from that page (my italics):

__new__() is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation.

datetime.datetime is also an immutable type.

PS If you think that:

• an object implemented in C cannot be subclassed, or
• __init__ doesn't get called for C implemented objects, only __new__

then please try it:

>>> import array
>>> array
<module 'array' (built-in)>
>>> class A(array.array):
    def __init__(self, *args):
     super(array.array, self).__init__(*args)
     print "init is fine for objects implemented in C"

>>> a=A('c')
init is fine for objects implemented in C
>>>