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!
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:
__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 >>>
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