Static typing in GDScript

In this guide, you will learn:

  • How to use types in GDScript

  • That static types can help you avoid bugs

Where and how you use this new language feature is entirely up to you: you can use it only in some sensitive GDScript files, use it everywhere, or write code like you always did!

Static types can be used on variables, constants, functions, parameters, and return types.

A brief look at static typing

With typed GDScript, Godot can detect even more errors as you write code! It gives you and your teammates more information as you're working, as the arguments' types show up when you call a method.

Imagine you're programming an inventory system. You code an Item node, then an Inventory. To add items to the inventory, the people who work with your code should always pass an Item to the Inventory.add method. With types, you can enforce this:

# In 'item.gd'.
class_name Item
# In 'inventory.gd'.
class_name Inventory


func add(reference: Item, amount: int = 1):
    var item = find_item(reference)
    if not item:
        item = _instance_item_from_db(reference)

    item.amount += amount

Another significant advantage of typed GDScript is the new warning system. From version 3.1, Godot gives you warnings about your code as you write it: the engine identifies sections of your code that may lead to issues at runtime, but lets you decide whether or not you want to leave the code as it is. More on that in a moment.

Static types also give you better code completion options. Below, you can see the difference between a dynamic and a static typed completion options for a class called PlayerController.

You've probably stored a node in a variable before, and typed a dot to be left with no autocomplete suggestions:

code completion options for dynamic

This is due to dynamic code. Godot cannot know what node or value type you're passing to the function. If you write the type explicitly however, you will get all public methods and variables from the node:

code completion options for typed

In the future, typed GDScript will also increase code performance: Just-In-Time compilation and other compiler improvements are already on the roadmap!

Overall, typed programming gives you a more structured experience. It helps prevent errors and improves the self-documenting aspect of your scripts. This is especially helpful when you're working in a team or on a long-term project: studies have shown that developers spend most of their time reading other people's code, or scripts they wrote in the past and forgot about. The clearer and the more structured the code, the faster it is to understand, the faster you can move forward.

How to use static typing

To define the type of a variable or a constant, write a colon after the variable's name, followed by its type. E.g. var health: int. This forces the variable's type to always stay the same:

var damage: float = 10.5
const MOVE_SPEED: float = 50.0

Godot will try to infer types if you write a colon, but you omit the type:

var life_points := 4
var damage := 10.5
var motion := Vector2()

Currently you can use three types of… types:

  1. Built-in

  2. Core classes and nodes (Object, Node, Area2D, Camera2D, etc.)

  3. Your own custom classes. Look at the new class_name feature to register types in the editor.

Note

You don't need to write type hints for constants, as Godot sets it automatically from the assigned value. But you can still do so to make the intent of your code clearer.

Custom variable types

You can use any class, including your custom classes, as types. There are two ways to use them in scripts. The first method is to preload the script you want to use as a type in a constant:

const Rifle = preload("res://player/weapons/rifle.gd")
var my_rifle: Rifle

The second method is to use the class_name keyword when you create. For the example above, your rifle.gd would look like this:

class_name Rifle
extends Node2D

If you use class_name, Godot registers the Rifle type globally in the editor, and you can use it anywhere, without having to preload it into a constant:

var my_rifle: Rifle

Variable casting

Type casting is a key concept in typed languages. Casting is the conversion of a value from one type to another.

Imagine an Enemy in your game, that extends Area2D. You want it to collide with the Player, a CharacterBody2D with a script called PlayerController attached to it. You use the on_body_entered signal to detect the collision. With typed code, the body you detect is going to be a generic PhysicsBody2D, and not your PlayerController on the _on_body_entered callback.

You can check if this PhysicsBody2D is your Player with the as casting keyword, and using the colon : again to force the variable to use this type. This forces the variable to stick to the PlayerController type:

func _on_body_entered(body: PhysicsBody2D) -> void:
    var player := body as PlayerController
    if not player:
        return

    player.damage()

As we're dealing with a custom type, if the body doesn't extend PlayerController, the playervariable will be set to null. We can use this to check if the body is the player or not. We will also get full autocompletion on the player variable thanks to that cast.

Note

If you try to cast with a built-in type and it fails, Godot will throw an error.

Safe lines

You can also use casting to ensure safe lines. Safe lines are a new tool in Godot 3.1 to tell you when ambiguous lines of code are type-safe. As you can mix and match typed and dynamic code, at times, Godot doesn't have enough information to know if an instruction will trigger an error or not at runtime.

This happens when you get a child node. Let's take a timer for example: with dynamic code, you can get the node with $Timer. GDScript supports duck-typing, so even if your timer is of type Timer, it is also a Node and an Object, two classes it extends. With dynamic GDScript, you also don't care about the node's type as long as it has the methods you need to call.

You can use casting to tell Godot the type you expect when you get a node: ($Timer as Timer), ($Player as CharacterBody2D), etc. Godot will ensure the type works and if so, the line number will turn green at the left of the script editor.

Unsafe vs Safe Line

Unsafe line (line 7) vs Safe Lines (line 6 and 8)

Note

You can turn off safe lines or change their color in the editor settings.

Define the return type of a function with the arrow ->

To define the return type of a function, write a dash and a right angle bracket -> after its declaration, followed by the return type:

func _process