When should I use @classmethod and when def method(self)?

Question

While integrating a Django app I have not used before, I found two different ways used to define functions in classes. The author seems to use them both very intentionally.

Answer 1

If you add decorator @classmethod, That means you are going to make that method as static method of java or C++. ( static method is a general term I guess ;) ) Python also has @staticmethod. and difference between classmethod and staticmethod is whether you can access to class or static variable using argument or classname itself.

class TestMethod(object):
    cls_var = 1
    @classmethod
    def class_method(cls):
        cls.cls_var += 1
        print cls.cls_var

    @staticmethod
    def static_method():
        TestMethod.cls_var += 1
        print TestMethod.cls_var
#call each method from class itself.
TestMethod.class_method()
TestMethod.static_method()

#construct instances
testMethodInst1 = TestMethod()    
testMethodInst2 = TestMethod()   

#call each method from instances
testMethodInst1.class_method()
testMethodInst2.static_method()

all those classes increase cls.cls_var by 1 and print it.

And every classes using same name on same scope or instances constructed with these class is going to share those methods. There's only one TestMethod.cls_var and also there's only one TestMethod.class_method() , TestMethod.static_method()

And important question. why these method would be needed.

classmethod or staticmethod is useful when you make that class as a factory or when you have to initialize your class only once. like open file once, and using feed method to read the file line by line.