Python - Set class property to depend on values of

Sorry if this already exists somewhere in the question archives, but I'm not sure how to ask it and searching didn't lead to any great revelations.

In Python (2.6.x) I have created a class

class timetuple(object):
    def __init__(self):
        self.weekday = 6
        self.month   = 1
        self.day     = 1
        self.year    = 2011
        self.hour    = 0
        self.min     = 0
        self.sec     = 0
    def jd(self):
        self.jd = julian_date(self)

def julian_date(obj):
    (Code to calculate a Julian Date snipped)

start = timetuple()
start.day   = 23
start.month = 2
start.year  = 2011
start.hour  = 13
start.min   = 30
start.sec   = 0

print start.__dict__
start.jd()
print start.__dict__
print start.jd

Which returns

{'hour': 13, 'min': 30, 'month': 2, 'sec': 0, 'weekday': 6, 'year': 2011, 'date': 23, 'day': 1}
{'hour': 13, 'min': 30, 'month': 14, 'jd': 2455594.0625, 'sec': 0, 'weekday': 6, 'year': 2010, 'date': 23, 'day': 1}
2455594.0625

So the .jd property (or do I call this a function or a method? I'm unsure of the lingo here honestly) doesn't exist before the start.jd() call. Is there a way I can somehow rewrite this to make it always exist based on the current values in the timetuple class, or have it update itself when the .jd property is called?

I know I can do it the long way by just making a .jd property in the init(self) section and then do something like

start = timetuple()
start.jd = julian_date(start)

but I'd like to know how to set up my classes better honestly :)

You want to actually define a property, as opposed to a variable:

class A(object):

    def __init__(self):
        self.a = 1
        self.b = 1

    @property
    def a_plus_b(self):
        return self.a + self.b

foo = A()
print foo.a_plus_b # prints "2"
foo.a = 3
print foo.a_plus_b # prints "4"
foo.b = 4
print foo.a_plus_b # prints "7"