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Methods that run on classes (static methods) in Python

• If you want to write a class or static method in Python, you can write an object (or dynamic) method and then call the classmethod function to make it into a method that can additionally run on the the class as a whole:

def counter(cls):
   # This is a 'dynamic' method
   return thing.count
# Which we'll wrap to make (in effect) static too.
ctr = classmethod(counter)


Note that the method takes an object parameter (which we have chosen to label "cls" rather than "self") but it hasn't been used in the code block.

• In more recent versions of Python, you can also use a decorator to achieve the same thing - in fact, it's just a change of the syntax and does the same thing underneath - a decorator is a wrapper:

@classmethod
def counter2(self):
   # "self" is the object or the class
   return thing.count


• You can also use a staticmethod decorator to define a static method - but in this case the static behavior replaces the object or dynamic behavior rather than being additional, and so you don't pass in an object reference:

@staticmethod
def counter():
   return thing.count


There are working examples of each of these in the source code which is [here], written on yesterday's public python class where I was teaching Python Programming.

(written 2010-03-25)

 
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