The built-in classmethod function, is a decorator function that allows a method to be bound to a class rather than an instance. It is used to define a method that can be invoked directly without necessarily creating an instance of the class. This allows you to access and modify class-specific attributes and perform operations that are relevant to the entire class.
Syntax:
@classmethod
def func(cls, arguments):
#body
or
def func(cls, arguments):
#body
TargetClass.some_method = classmethod(func)Instance methods are meant to be used by objects and not by the class itself. By convention, instance methods have self as the first parameter, which refers to the instance calling the method while class methods have cls as the first parameter.
class Myclass():
def __init__(self):
self.name = 'class instance'
def example_method(self):
print('Hello, World!')
myInstance= Myclass()
myInstance.example_method()
Myclass.example_method()As shown above, calling the method with the class itself raises an error. By decorating the method with the @classmethod decorator, it effectively becomes accessible from both the class and its objects/instances, as shown below.
class Myclass:
def __init__(self):
self.name = 'class instance'
@classmethod
def example_method(cls):
print('Hello, World!')
Myclass.example_method()
print('----------------------')
myInstance = Myclass()
myInstance.example_method()
more examples are shown below
class School:
instances = 0 #keep count of how many instances are initialised
def __init__(self):
self.name = 'an instance'
School.instances += 1
def countInstances(cls):
return cls.instances
School.count_instances = classmethod(countInstances)
S1 = School()
S2 = School()
S3 = School()
S4 = School()
print(School.count_instances())4
class Library:
books = []
authors = []
def __init__(self,name = None):
self.name = name
def add_book(self, title, author):
self.books.append(title)
self.authors.append(author)
@classmethod
def get_authors(cls):
return cls.authors
@classmethod
def get_books(cls):
return cls.books
@classmethod
def get_all(cls):
return list(zip(cls.books, cls.authors))
myLibrary = Library("John's Library")
b1 = myLibrary.add_book('Book Thief', 'Markus Zusak')
b2 = myLibrary.add_book('Tell me your dreams', 'Sydney Sheldon')
b3 = myLibrary.add_book('Maybe in another life.', 'Taylor Jenkins')
print(Library.get_authors())
print(Library.get_books())
print(Library.get_all())We also have static methods ,which unlike class methods, have no access to the class state and cannot modify it. Static methods are defined using the builtin @staticmethod decorator/function.