Classes and objects

Classes let you name a concept, keep its data together, and attach the behavior that belongs with that data. If a tuple or named tuple starts to feel too anonymous, a class is usually the next step.

Use a class when a value should have:

• a clear name instead of a bundle of positions
• related pieces of data that travel together
• methods that operate on that data
• construction rules or invariants that matter to callers

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

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

actor main(env):
    circle = Circle(3.14)
    print(circle.diameter())
    env.exit(0)

What a class gives you

In the example above:

• radius is an attribute stored on each Circle
• diameter() is a method that uses that attribute
• self refers to the object the method is running on

Attributes are often introduced by assignments in __init__, but you can also declare them explicitly in the class body when the shape should be obvious up front.

class Person(object):
    name: str

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