questions about programming styles

[fdu.xiaojf at gmail.com]

Hi all, I'm not skilled at programming, so sorry for my ignorance.
My questions:

(1)
which is the better way to calculate the value of attributes of a class ?
for example:

 (A)
    def cal_attr(self, args):
        #do some calculations
        self.attr = calculated_value
and then if the vlue of attribute is needed,
    self.cal_attr(args)
    some_var = self.attr
or I can define cal_attr() as follows:
 (B)
    def cal_attr(self, args):
        #do some calculations
        return calculated_value
and then, if the value of attribute is needed,
    self.attr = self.cal_attr(args)
    some_var = self.attr

(2)
when to use class methods and when to use functions ?

In my opinion, both of class methods and functions have advantages and
disadvantages. I have to pass many arguments to a function, which is
annoying. When using class methods, the arguments can be stored as
attributes of the class, which is convenient for later use. But I have
to create an object in advance.


I have googled the web, but haven't found too much specific answers.
Can somebody kindly answer my questions or point me to the resources
available on the web ?

Thanks a lot.