what's the difference between property and class method?

Question

What's the difference between a class property and a class method? As I understand it, property is calculated when an object is created. And method makes calculations when I call it.

Is there any other difference than that?

For example, I have a property in my class Product():

@property
    def total_ammount_in_store(self):
        consignments = self.product.package.consignments
        total_ammount = 0
        for consignment in consignments:
            total_ammount += consignment.package_ammount

When I render some page, I pass some products. For example: {'products':Product.objects.filter(expiration_data < datetime.now())

I don't need to calculate total_ammount_in_store every time I get an instance of Product. What if I just need to calculate it when I call it in a template: {{product.total_ammount_in_store}}? Is it possible?

Is method also calculated when the object is created?

Answer 1

The property is called each time you access product.total_ammount_in_store, not at the time when the product is created.

Therefore including {{ product.total_ammount_in_store }} in your template will do the right thing.

By using the property decorator, you can access product.total_ammount_in_store instead of product.total_ammount_in_store() if it was an instance method. In the Django template language, this difference is not so apparent, because Django will call the method automatically in the template.

Don't confuse an instance method with a class method, which is quite different. A class method belongs to your class Product, not an individual instance product. You don't have access to instance variables e.g. self.package when you call a class method.