[Tutor] what is @classmethod and @staticmethod ??

[Alan Gauld]

"maser" <maseriyer at yahoo.com> wrote 

> Thanks, Andreas. Why do we need to use classmethod/
> staticmethod and where do we need to use them ?

Although there are minor technical details where they differ 
static methods and class methods are nearly identical. The 
names come from two programming traditions with different 
names for the same thing. Smalltalk and Lisp etc use class 
method. Delphi, C++ and Javba use static method.

In terms of usage they are both the same and have two basic 
purposes:
1) As per my earlier response and example to do something 
to the entire class of objects, either select one instance using 
some kind of search mechanism - database queries are often 
coded in class methods, or a count of all instances (perhaps 
with filters which is similar to a database sel;ect) or it can 
be conversions of one  type/class to another. And this can 
in turn be a kind of factory method or pseudo constructor.

2) class methods can also be used to provide generic 
collections of functions, perhaps with some shared 
data/variables. Booch called these "Class Utilities" in his 
book. They could for example provide a pseudo object 
interface to legacy code or provide namespace protection 
for a commonly used data name.

The second form can usually be provided with functions 
in a module in Python but thats not the case in some 
other languages.

As a practical example of the former consider a client/server 
application where the client sends requests to the server 
specifying the class and instance and method of the target 
object. The interprocess message handler can delegate the 
message to a class method to look up the instance and 
in turn call the actual method of the returned instance. 
This is much lighter weight than using a full blown Object 
Request Broker but provides a simar level of decoupling
and ease of use in the client code. I've built several 
systems using this pattern and the RPC code hardly 
changes.

Another example is where you need to produce highly 
dynamic object systems with lots of introspection and
varying object structures. This is rarely needed in Python 
because the built in introspection is so good but in languages 
like C++ it may be the easiest way of creating new "classes"
(not objects) in real time (although in truth these "classes" are 
restricted to new data members and accessor methods)
One practical case using this was a hospital diagniosis 
system where doctors needed to be able to introduce 
new subclasses of disease/diagnosis and have these 
classes merge seamlessly with the base hard coded 
disease classes. Static methods were used to maintain 
dictionaries of dictionaries representing the sub classes 
and their features. These could then be passed to the 
instances as required and the instances performed a 
similar kind of attribute access to that used by Python.

Its probably worth saying that these kinds of uses are 
powerful when needed but they are not by any means 
needed in every program. Although once you use them 
you tend to find more uses for them than you previously 
expected! :-)

HTH,

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld