Why can't I subclass datetime.date?

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)

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
>>> 

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