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

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)