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3.6 - Class Methods and Static Methods

3.6.1 - Introduction to Class Methods​

Class methods in Python are methods that are bound to the class rather than its objects. They can access and modify the class state across all instances of the class. Class methods are defined using the @classmethod decorator.

Example 1: Basic Class Method

class MyClass:
    counter = 0

    def __init__(self):
        MyClass.counter += 1

    @classmethod
    def instances_created(cls):
        return f"Total instances created: {cls.counter}"

# Create instances
obj1 = MyClass()
obj2 = MyClass()
print(MyClass.instances_created())  # Output: Total instances created: 2

Example 2: Using Class Methods for Factory Functions

class Date:
    def __init__(self, day=0, month=0, year=0):
        self.day = day
        self.month = month
        self.year = year

    @classmethod
    def from_string(cls, date_as_string):
        day, month, year = map(int, date_as_string.split('-'))
        return cls(day, month, year)

date = Date.from_string('11-09-2012')
print(date.day, date.month, date.year)  # Output: 11 9 2012

3.6.2 - Introduction to Static Methods​

Static methods in Python are methods that belong to a class but don't need a class or instance reference. They are defined using the @staticmethod decorator and are used for utility functions that perform a task in isolation.

Example 1: Basic Static Method

class MathUtils:

    @staticmethod
    def add(x, y):
        return x + y

print(MathUtils.add(5, 7))  # Output: 12

Example 2: Static Methods with No Class State

class TemperatureConverter:

    @staticmethod
    def celsius_to_fahrenheit(celsius):
        return (celsius * 9/5) + 32

    @staticmethod
    def fahrenheit_to_celsius(fahrenheit):
        return (fahrenheit - 32) * 5/9

print(TemperatureConverter.celsius_to_fahrenheit(0))  # Output: 32.0
print(TemperatureConverter.fahrenheit_to_celsius(32))  # Output: 0.0

3.6.3 - Differences and Use Cases​

Understanding the differences and appropriate use cases for class methods and static methods is crucial:

  • Class Methods: Use class methods when the method requires access to the class itself or needs to modify class state. They are often used for factory methods that create class instances in different ways.

  • Static Methods: Use static methods when the functionality you are implementing is related to the class's purpose but does not require class or instance data. They are useful for utility or helper functions.

3.6.4 - Combining Class and Static Methods​

In practice, a class can have both class methods and static methods, each serving its specific purpose.

Example: Combined Use of Class and Static Methods

class Circle:
    pi = 3.14159

    def __init__(self, radius):
        self.radius = radius

    @classmethod
    def from_diameter(cls, diameter):
        radius = diameter / 2
        return cls(radius)

    @staticmethod
    def area(radius):
        return Circle.pi * radius ** 2

circle1 = Circle(5)
circle2 = Circle.from_diameter(10)
print(circle1.area(circle1.radius))  # Output: 78.53975
print(circle2.area(circle2.radius))  # Output: 78.53975