Arvind Singh wrote: > On 5/20/07, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: >> 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 > > The way, I get it: you are trying to *cache* the value of an > *expensive* calculation. You should worry about it if only if: > a. It is *really* expensive to calculate the value. > b. The value is read more often then it can change. > > Otherwise, just don't bother with it (i.e.use some_var = > obj.cal_value(*args) ). > > But if you really want to cache the value, maybe you should keep track > if the value is valid: > > class C(object): > def calc_attr(self, *args): > """Calculates value of "attr". > """ > self._attr = calculated_value > self._attr_valid = True > > def get_attr(self, *args): > """Use this to get values.""" > self._attr_valid or self.calc_attr(*args) > return self._attr > > def another_method(self, *args): > """Does some calculations which invalidate *cached* value of > "attr". > """ > # do something > self._attr_valid = False > >> (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 hope you know about all of it, but here it is, anyway: > > - In Python, *everything* is an object. > - Whenever Python finds a class definition, it creates a *class object*. > - A class objects acts as a *template* to create new objects called > *instances* (of that class). > - The template and the instance can both have *attributes* (to store > data). > - *Class attributes* can be accessed by both -- class as well as its > instances. > - *Instance attributes* can only be accesses by instances (because > class doesn't have to know about these). > - All the class attributes are shared among all the instances. If you > change an attribute of a class, all the instances of that class > (irrespective of when they were instantiated) will see the change. > > That gives us: > - *Instance methods* (or simply, "methods") can access: class > attributes, instance attributes, class methods, and instance methods. > (And are accessible via an instance only.) > - *Class methods* can ONLY access class attributes or other class > methods. (And are accessible via the class or its instance.) > - The first argument to instance methods is traditionally called > "self" (which is an *instance object*) and that of class methods is > called "cls" (which is a *class object*). > > > Design choices: > - The data which is to be shared by all the instances (and is mostly > immutable) should be kept as class attribute (to minimize memory > consumption). > - The methods which should produce same result for all instances (and > don't need to access instance attributes) should be declared as class > methods. > - Class attributes are also useful to *share state* among various > instances (so that they can co-operate). Such "sharing functionality" > is mostly implemented as class methods. > > > It's just whatever I could recollect and thought might be relevant. I > hope it helps. > > Arvind > > > PS: Defining something as "property" suggests (to the class users) > that it is inexpensive to access that value -- just a matter of style. > Thanks a lot for all kind replies!
I think I need a systematic learning of design patterns. I have found some tutorials about design pattern about python, but can somebody point me which is the best to start with ? Regrads, xiaojf -- http://mail.python.org/mailman/listinfo/python-list
