Subclassing standard class: how to write my own __init__?

[kpalamartchouk]

Dear All,

I am trying to create a class that would extend functionality of
datetime.date by implementing some functions I need, for example an
optional initialisation by (year, day_of_year) instead of (year,
month, day). I would like the class constructor to behave in the
datetime's default way if there are no keyword arguments and do my own
stuff if there are some.

Here is the minimal example:

=========================================
from datetime import date, timedelta

class MyDateTime(date):
def __init__(self, *arg, **kw):
if len(kw):
year = kw['year']
doy = kw['doy']
my_date = date(year=year, month=1, day=1) + timedelta
(days=doy-1)
date.__init__(self, year = my_date.year, month =
my_date.month, day = my_date.day)
else:
date.__init__(self, *arg)

date1 = MyDateTime(2008, 12, 19)
date2 = MyDateTime(year=2008, doy=354)
=========================================

It works fine when I don't supply any keyword arguments. But if I do,
TypeError is happening:

TypeError: function takes exactly 3 arguments (1 given)

Could you help me to understand where I am wrong?

Thanks

 

[Bruno Desthuilliers]

(e-mail address removed) a écrit :

Dear All,

I am trying to create a class that would extend functionality of
datetime.date by implementing some functions I need, for example an
optional initialisation by (year, day_of_year) instead of (year,
month, day).

If that's all you want, then you don't need a subclass - just a plain
function returning a correctly constructed date object.

I would like the class constructor to behave in the
datetime's default way if there are no keyword arguments and do my own
stuff if there are some.

Here is the minimal example:

=========================================
from datetime import date, timedelta

class MyDateTime(date):

Naming it MyDateTime when it's a date is a bit misleading

def __init__(self, *arg, **kw):
if len(kw):

if kw: # empty dict eval to False

year = kw['year']
doy = kw['doy']

Will raise a KeyError. Either don't use **kw, or catch the KeyError and
raise a TypeError instead (which is what's expected when calling a
function with wrong arguments)

my_date = date(year=year, month=1, day=1) + timedelta
(days=doy-1)
date.__init__(self, year = my_date.year, month =
my_date.month, day = my_date.day)
else:
date.__init__(self, *arg)

date1 = MyDateTime(2008, 12, 19)
date2 = MyDateTime(year=2008, doy=354)
=========================================

It works fine when I don't supply any keyword arguments. But if I do,
TypeError is happening:

TypeError: function takes exactly 3 arguments (1 given)
>
Could you help me to understand where I am wrong?

Some classes implements the __new__ method too (the 'proper'
constructor), and date is one of them.

class MyDate(date):
def __new__(cls, year, month=None, day=None, doy=None):
if doy:
assert month is None and day is None
d = date(year,1, 1) + timedelta(days=doy-1)
month = d.month
day = d.day
return date.__new__(cls, year, month, day)

date1 = MyDate(2008, 12, 19)
date2 = MyDate(2008, doy=365)

HTH