method that can be called from a class and also from an instance

Steven D'Aprano steve+comp.lang.python at pearwood.info
Fri Nov 23 05:32:31 CET 2012


On Thu, 22 Nov 2012 16:51:27 +0100, Peter Otten wrote:

> Marc Aymerich wrote:
> 
>> Hi,
>> 
>> I want to create a method within a class that is able to accept either
>> a class or an instance.
[...]
> Why would you overload a method that way?


The use-case I have is that I have a number of classes with default 
state. Most instances don't override any of the state, so the instances 
don't add anything except an extra conceptual layer:

instance = MyClass()  # notice that there are no arguments passed
instance.method(args)

Since the instances don't have any state except for that already held by 
the class, they are redundant and pointless. Just knowing the class is 
enough to specify the behaviour. If I used class methods, I could do this:

MyClass.method(args)


But here's the thing -- sometimes I *do* have instances that override the 
default state:

instance = MyClass(x, y, z)
instance.method(args)

Now if method is a class method, my per-instance state is ignored. So I 
want a method that can be called from the class, and see the default 
state, or from the instance, and see the per-instance state. Neither 
classmethod, staticmethod nor ordinary instance methods do the job, but 
my custom dualmethod does.

http://code.activestate.com/recipes/577030/



-- 
Steven


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