Class methods
Class methods are methods that, like class variables, work on the class itself, and not on the class objects that are instantiated. You can say that they are methods bound to the class, not to the object.
When can this be useful?
There are several possibilities, here are the two most popular:
- we control access to class variables, e.g., to a class variable containing information about the number of created instances or the serial number given to the last produced object, or we modify the state of the class variables;
- we need to create a class instance in an alternative way, so the class method can be implemented by an alternative constructor.
Convention
- To be able to distinguish a class method from an instance method, the programmer signals it with the
@classmethoddecorator preceding the class method definition. - Additionally, the first parameter of the class method is
cls, which is used to refer to the class methods and class attributes.
As with self, cls was chosen arbitrarily (i.e., you can use a different name, but you must do it consistently).
You can find an example of a class method implementation in the editor.
The result of the code:
# of objects created: 0
# of objects created: 1
# of objects created: 2
output
Code
class Example:__internal_counter = 0
def __init__(self, value):
Example.__internal_counter +=1
@classmethod
def get_internal(cls):
return '# of objects created: {}'.format(cls.__internal_counter)
print(Example.get_internal())
example1 = Example(10)
print(Example.get_internal())
example2 = Example(99)
print(Example.get_internal())
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