Classes and Objects

Acton supports the object-oriented paradigm, which means that it provides features to create classes and objects, which are instances of classes. Classes are a fundamental concept in an object-oriented world and they allow programmers to create their own data types with their own attributes and methods.

A class is defined using the class keyword followed by the name of the class. The convention is to use CamelCase for class names.

class Circle(object):
    def __init__(self, radius):
        self.radius = radius

    def diameter(self):
        return self.radius * 2

Attributes are variables that hold data for an object and methods are functions that operate on that data. In the Circle example above, radius is an attribute and diameter() is a method.

Attributes are typically inferred from assignments to attributes on self in __init__, as shown with radius above. They can also be explicitly declared with type annotations in the class body and you can mix freely between inferred and explicitly typed:

class Person(object):
    name: str  # Explicit declaration

    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age        # Inferred from assignment
        self.id = generate_id()  # Also inferred

Class methods must have self as the first argument, which refers to the object instance being operated on.

Creating an object

A Class is like a blueprint and an object is an instance of such a blueprint. To create an object, or "instantiate", we use the "blueprint" (class), like so:

circle = Circle(3.14)

Here we create the object circle from the class Circle, passing the parameter 3.14 which will be used to set the radius attribute of the object.

print(circle.diameter())

And here we print the diameter of the circle by calling the .diameter() method.