Fields and the __init__ Method
Initialize a struct's state.
Fields and the __init__ Method is a free Mojo Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Mojo Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Fields Start Empty
Declaring fields gives a struct its shape, but it does not fill them in. You still need to set each field's starting value. 🧱
Meet __init__
The special method __init__ runs when you create an instance. Its job is to set up the struct's fields with real values.
fn __init__(out self):
self.x = 0
self.y = 0The self Parameter
Inside __init__, self refers to the new instance being built. You assign to self.field to give that instance its data.
self.x = 0Why out self
Writing out self tells Mojo this method is constructing the value, so self starts uninitialized and you must fill every field.
fn __init__(out self):Taking Arguments
An initializer usually takes arguments so each instance can start with different data passed in by the caller.
fn __init__(out self, x: Int, y: Int):
self.x = x
self.y = ySet Every Field
You must assign every field in __init__. Mojo will not let a struct exist with some fields left uninitialized.
Creating an Instance
To build a value, call the struct like a function. Mojo runs __init__ and hands you back a ready instance.
var p = Point(3, 4)Reading a Field
Once built, reach a field with dot access. So p.x reads the x you stored when the instance was created.
print(p.x, p.y)Arguments Map to Fields
A common pattern is naming arguments like the fields and assigning each across. This keeps the initializer easy to read.
self.x = x
self.y = yDefault Starting Values
You can give fields sensible defaults by assigning constants in __init__ when no useful argument is provided by the caller.
fn __init__(out self):
self.x = 0
self.y = 0The Full Picture
Fields declare what data exists; __init__ decides its starting value. Together they turn a blueprint into a usable instance.
struct Point:
var x: Int
var y: Int
fn __init__(out self, x: Int, y: Int):
self.x = x
self.y = yQuick Check
Let us confirm how initialization works.
Recap
You used __init__ to set up fields when an instance is born. Next you will add methods so your type can act, not just hold data. ✨
Frequently asked questions
Is the “Fields and the __init__ Method” lesson free?
Yes — the full text of “Fields and the __init__ Method” is free to read here on the web, and the Mojo Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Mojo Academy course, upgrade to CoddyKit PRO.
What will I learn in “Fields and the __init__ Method”?
Initialize a struct's state. You practise Mojo Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Mojo Academy?
No prior experience is required. Mojo Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Fields and the __init__ Method” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Mojo Academy lesson?
Yes. Every Mojo Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Defining a struct
- Fields and the __init__ Method
- Adding Methods to a Struct
- Structs vs Python Classes