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Butch Wesley edited this page Aug 15, 2022 · 20 revisions

The double method works similarly to load. It will return a loaded class or scene that has empty implementations for all the methods defined in the script you pass it. It will also include empty implementations for any methods in user-defined super classes. It does not include implementations for any of the Godot Built-in super classes such as Node2D or WindowDialog (unless you have overloaded them in your script or in one of the user-defined super classes your script inherits from). Actually, you can if you use the FULL Doubling strategy described at the bottom of this page.

Characteristics of a Double

  • You can double Scripts, Inner Classes, and Packed Scenes. Once you have a double, you can then call new or instance on it to create instances of a doubled object.
  • The double inherits (extends) the base object.
  • Doubles retain all properties/variables and their default values from the base object.
  • _init is always called on the base object by Godot. You cannot stub _init "to do nothing" (you can stub parameters). In most cases this is not an issue since all the methods of a double do nothing. If it causes an issue you can check a parameter and then take no action in the base object's _init based on the value.
  • If your _init method has required parameters you must stub default values before trying to double the object.
  • All parameters in all functions in the double are defaulted to null. Even if they did not have a default value in the base object. You can stub default parameter values for any method, including _init.
  • All methods in a doubled object will return null. You can stub a return value for all calls to a function, or calls with specific parameter values. You can also stub a method to_call_super causing it to retain its original functionality.
  • Inner Classes in the source script will retain all of their functionality. They are not doubled in any way. You can create doubles of specific Inner Classes but the Inner Classes in a doubled script are not altered.
  • You can create Partial Doubles which will act exactly like its source object but you can spy on and stub any method. It's like the inverse of a double.
  • All instances of Doubles and Partial Dobules are freed after a test finishes. This means you do not have to free them manually and you should not use them in before_all.
  • You cannot double static methods. See "Doubling Static Methods" below.

Doubling a Script

To double a script just give it a path or an already loaded script.

const MY_SCRIPT_PATH = 'res://'
var MyScript = load(MY_SCRIPT_PATH)

# Load the doubled object.
var DoubledMyScript = double(MY_SCRIPT_PATH)
# or
var DoubledMyScript = double(MyScript)

# Create an instance of a doubled object
var doubled_script =
# or
var doubled_script = double(MyScript).new()

Doubling an Inner Class

When doubling an Inner Class you have to specify the path/script-object where the Inner Class is and then pass a / delimited list of Inner Classes that represents the hierarchy of the Inner Classes.

# -----------------------------------------------
# Given this as res://sripts/
# -----------------------------------------------
class InnerA:
  var something = null

  class InnerA2:
    var something_else = null

class InnerB:
  var thing = null

# -----------------------------------------------
# You would double the various Inner Classes like this:
# -----------------------------------------------
const SCRIPT_WITH_INNERS_PATH = 'res://'
var ScriptWithInners = load(SCRIPT_WITH_INNERS_PATH)

# Load the doubled objects
var DoubledInnerB = double(SCRIPT_WITH_INNERS_PATH, 'InnerB')
# or
var DoubledInnerA2 = double(ScriptWithInners, 'InnerA/InnerA2')

# Create an instance of a doubled inner class
var doubled_inner_a2 =
# or
var doubled_inner_b = double(SCRIPT_WITH_INNERS, 'InnerB').new()

When doubling Inners and passing a loaded class, you have to pass the class that contains the Inner Class(es). Given the example above, you CANNOT do double(ScriptWithInners.InnerB). You MUST always pass the path to the inner classes.

Doubling a Scene

A doubled version of your scene is created along with a double of its script. The doubled scene is altered to load the doubled script instead of the original. A reference to the newly doubled scene is returned. You can call instance on the returned reference.

const MY_SCENE_PATH = 'res://my_scene.tscn'
var MyScene = load(MY_SCENE_PATH)

# Load up the doubled objects
var DoubledScene = double(MY_SCENE_PATH)
# or
var DoubledScene = double(MyScene)

# Create an instance
var doubled_scene = DoubledScene.instance()
# or
var doubled_scene = double(MY_SCENE_PATH).instance()

Doubling Static Methods

Currently you cannot double static methods. In fact if you try to double a class with a static method then you will get an error that looks similar to:

Parser Error: Function signature doesn't match the parent. Parent signature is: 'Variant foo()'.

As of now, GUT does not have the ability to detect static methods in the code. As a workaround there is the ignore_method_when_doubling method. This method takes in a variant as the first parameter and a method name as the second. The first parameter can be a path to a script, a path to a scene, a loaded script, or a loaded scene.

Calling this method will prevent GUT from trying to make a stubbed out version of the method in the generated double allowing you to successfully double your classes that contain static methods.

These ignored methods are cleared after each test is ran to avoid any unexpected results in your tests, so you may want to add this call to your before_each.

There's more info and examples on this method on the Methods page.

Methods with varargs and NativeScript parameter mismatches

Some methods provided by Godot can contain and endless list of parameters (varargs). Trying to call one of these methods on a double can result in runtime errors due to a paramter mismatch. See the sections in Stubbing that address parameters.

Sometimes NativeScript parameters aren't found in doubles (maybe always). This helps there too.

For even more reading see issues #252 and #246.

Doubled method default parameters

GUT stubs all parameters in doubled user methods to be null. This is because Godot only provides defaults for built-in methods. When using partial-doubles or stubbing a method to_call_super null can get passed around when you wouldn't expect it causing errors such as Invalid operands 'int' and 'Nil'. See the "Stubbing Method Paramter Defaults" in Stubbing for a way to stub method default values.

Doubling Built-Ins

You can double built-in objects that are not inherited by a script such as a Node2D or a Raycast2D. These doubles are always created using the Doubling Strategy of "FULL" (see below) since there are not any overloaded methods.

For example you can double or partial_double like this:

var doubled_node2d = double(Node2D).new()
stub(doubled_node2d, 'get_position').to_return(-1)

var partial_doubled_raycast = partial_double(Raycast2D).new()

What Do I Do With My Double?

Doubles are useful when you want to test an object that requires another object but you don't want to be deal with the overhead of other object's implementation.

Sometimes an empty implementation isn't enough. Sometimes you need specific values or things to be returned from one of your doubled methods. For this we have Stubbing. Stubbing allows you to specify return values in various different scenarios such as "returning a 9 whenever a method is called" or "returning 57 when a method is called with the parameters 'a' and 24".

You can also spy on your double. Once you have a double there are special asserts that can be used with it to verify that a method in the doubled object was called. You can assert very specific calls too, such as with a specific list of parameters, or make sure it was called a certain number of times.

The Fine Details

Example of Script vs Built-in doubling

Only methods that have been defined in the script you pass in OR methods that have been defined in parent scripts get the empty implementation in the double. Methods defined in any built-in Godot class do not get overridden unless the script (or parent script(s)) have them implemented.

This is in the process of being changed but it is not fully implemented yet. See the section on Double Strategies.

For example, given the following script at location res://

extends Node2D

var _value = 1
func set_position(pos):
  print('setting position')

func get_value():
  return _value

func set_value(value):
  _value = value

And, in a test, you double this class:

  var Doubled = double('res://')
  var doubled =


  • You can stub get_value, set_value, set_position.
  • You cannot stub get_position since this class does not implement it. This will result in a runtime error.
  • You can use assert_method(doubled, 'get_value')
  • You should not use assert_method(doubled, 'get_position'). You can but it will always return 0.

Doubling Strategy

Remember all that stuff I said earlier about not being able to double Godot Built-Ins? Forget about it...or forget half of it, maybe 45% of it.

You can spy on and stub most of the Built-Ins in Godot if you enable the FULL Doubling Strategy. I've enabled this feature in my own game and it didn't crash (I currently have 75 test scripts and 3633 asserts). As reassuring as that was I'm still not sure that it won't blow up for someone so it is off by default.

The following methods cannot be spied on due to implementation details with either Gut or GDScript. There might be more.

has_method      _draw
get_script      _physics_process
get             _input
_notification   _unhandled_input
get_path        _unhandled_key_input
_enter_tree     _get
_exit_tree      emit_signal
_process        _set

If you've defined one of these methods in your class then you can double/spy on them just as you normally would.

Setting the Doubling Strategy

You can set the default strategy from the command line, .gutconfig, or by calling set_double_strategy on your Gut instance.

You can also override the default strategy at the Test Script level or for a specific call to double. When set at the script level, it will reset at the end of the script or Inner Test Class. When passed to double it will only take effect for that one double.


Valid values are partial(default) or full


Command Line

Use the -gdouble_strategy option with the values partial or full


Script Level


When Calling double

Just add another parameter to your call to double using the DOUBLE_STRATEGY enum.

double('res://', 'InnerA', DOUBLE_STRATEGY.FULL)
double('res://my_scene.tscn', DOUBLE_STRATEGY.PARTIAL)

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