Python for Java Programmers > Class and static methods | Department of Computing

Python for Java Programmers

Chapter 8: Object-oriented programming

Class and static methods

face Josiah Wang

You might have used static methods in Java.

In Python, static methods are called class methods.

A class method is bound to the class and not to the class instance.

Therefore, a class method takes the class (cls) as its first argument, rather than the class instance (self) as in an instance method.

Thus, class methods do not have access to the object instance (via self), but only the class (via cls). Therefore, it does not have access to any instance attributes or methods.

You will also need to annotate the method with a @classmethod decorator (Line 9). This decorator informs Python that cls is a class, and not an instance!

class Person:
    count = 0

    def __init__(self, name, age):
        self.name = name
        self.age = age
        Person.count += 1

    @classmethod
    def get_population(cls):
        return f"Population: {cls.count}"

lecturer = Person("Josiah", 20)
print(Person.get_population())
print(lecturer.get_population())

You can access a class method with either the ClassName (Line 14) or an instance_name (Line 15).

When Line 14 is executed, Python will pass Person as the first parameter cls. Therefore, cls.count (Line 11) is actually Person.count.

When Line 15 is executed, the same will happen. Python will figure out that lecturer is an instance of Person. Since get_population() is annotated as a class method, Python will pass the class name Person as the first parameter instead of the lecturer instance.

(Python) static methods

Just to make things more confusing, Python also offers what it calls a static method. Note that this is slightly different from static methods in Java! A Python static method basically acts like a normal function, except that it is defined inside the scope of a class. Note that there is no compulsory first parameter self or cls here - it’s just a normal function! Like a class method, use a @staticmethod decorator to make a method static; this tells Python not to pass self or cls as the first argument in the method call.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @staticmethod
    def calculate_sum(num1, num2):
        return num1 + num2

total = Person.calculate_sum(5, 3)
print(total)