Phase 0 · ToolsPython core

Classes and objects

About 30 minLooking ahead: some prerequisites aren't done yet

Why this matters

Most libraries you'll use in AI work hand you objects. The Anthropic SDK gives you a client object with a messages.create method. The reply it returns is an object with .content and .usage. A NumPy array is an object with .shape and .mean(). Once you know how classes work, those dots stop being magic. You can read them, and bundle your own data and logic the same way.

Define a class with __init__ and create objects from it
Store data on an object as attributes and act on it with methods
Explain what self refers to
Read a dataclass, and judge when writing a class is worth it

Data and the functions that use it

You've stored a campaign as a dictionary and written functions like roas(revenue, spend) that work on that data. The data and the functions belong together, but nothing in the code says so.

A class bundles them. Think of a cookie cutter: one shape, used to make many cookies, each with its own icing. The class is the cutter. It describes what data each thing holds (its attributes) and what it can do (its methods). An object, also called an instance, is one cookie: one particular thing made from the class.

class Campaign:
    def __init__(self, name, spend, revenue):
        self.name = name
        self.spend = spend
        self.revenue = revenue

    def roas(self):
        return self.revenue / self.spend

spring = Campaign("Spring Sale", 3120.5, 15840)
video = Campaign("Brand Video", 6400, 2870)

print(spring.name, round(spring.roas(), 2))
print(video.name, round(video.roas(), 2))

The pieces:

class Campaign: starts the blueprint. Class names use CapWords (EmailCampaign), unlike the snake_case of variables and functions.
__init__ (two underscores on each side) runs automatically when you create an object. Its job is to set the starting attributes.
self.name = name stores the value as an attribute on this particular object.
roas is a method: a function defined inside the class. You call it with a dot, spring.roas().
Campaign("Spring Sale", 3120.5, 15840) creates a new object and runs __init__ with those arguments.

spring and video share the same code but hold different data. Each object keeps its own attributes.

What self is

Every method's first parameter is self: the object the method was called on. When you write spring.roas(), Python runs Campaign.roas(spring) behind the scenes. So inside the method, self is spring, and self.revenue is spring's revenue.

class Campaign:
    def __init__(self, name, spend, revenue):
        self.name = name
        self.spend = spend
        self.revenue = revenue

    def roas(self):
        return self.revenue / self.spend

spring = Campaign("Spring Sale", 3120.5, 15840)
print(spring.roas())
print(Campaign.roas(spring))

Both lines do the same thing. You always write self as the first parameter in the definition. You never pass it yourself: Python fills it in from whatever is before the dot.