Label.text.erase() - Is this a bug?

DaveTheCoder

In the following script for a Label, if the property Use Temp Variable is true, the last character of the Label's text is removed every two seconds. If Use Temp Variable is false, no characters are removed.

Is this a bug? (Godot 3.5.1)

extends Label

export var use_temp_variable: bool = true


func _ready() -> void:
	text = "This is a text string."
	var timer: Timer = Timer.new()
	add_child(timer)
	timer.start(2.0)
	# warning-ignore:return_value_discarded
	timer.connect("timeout", self, "_on_Timer_timeout")


func _on_Timer_timeout() -> void:
	print_debug("text=", text)
	if use_temp_variable:
		var s: String = text
		s.erase(s.length() - 1, 1)
		text = s
	else:
		text.erase(text.length() - 1, 1)

This example is self-contained. To try it, create a Label and attach the above script to it. Run the project and toggle the Use Temp Variable property in the Inspector. Observe the Label's content in the viewport or the output from the print_debug() statement.

AnidemDex

Strings are passed by value, not by reference.
getting text is just another string that contains the same string as your label's text, but modifying it doesn't affect the original string.

What you need to do is to get the text and store it somewhere, modify it and then re-assign text to be that new string; just as you did on:

if use_temp_variable:
	var s: String = text
	s.erase(s.length() - 1, 1)
	text = s

DaveTheCoder

I had considered the pass-by-value/reference issue, but was not sure how it applied here, since the string is not a parameter, but an object whose method is called.

Is Label.text.erase() implemented under-the-hood as a function call that involves passing text as a parameter?

cybereality

When you call Label.text, it's not a variable, it's a property that (under the hood) is really a function.

https://docs.godotengine.org/en/stable/classes/class_label.html#class-label-property-text

It's the same as if you make your own variable with a getter and setter. It is essentially returning a copy of the string, not a reference to the variable.

DaveTheCoder

So to break it down:

text.erase(text.length() - 1, 1)

is equivalent to:

get_text().erase(get_text().length() - 1, 1)

get_text() returns a copy of the text property, and erase() is performed on the copy.

cybereality

Yes, which does nothing except modify an anonymous temporary value.

DaveTheCoder

This still might be considered a bug, or at least a feature that needs enhancing, since erase() is the only String method that modifies the string and returns no result. That suggests (incorrectly) that it does an in-place modification.

But I see that the method has been removed in Godot 4, so it's probably not worthwhile submitting an issue. The left() method is similar and could be used instead.

Kojack

DaveTheCoder That suggests (incorrectly) that it does an in-place modification.
Technically it did do an in-place modification. But it was to a temporary variable hidden behind accessing the text property.

Unity has a similar thing, the position Vector3 in a transform isn't a variable, its a property. So you can't do things like: transform.position.y = 0; because position is really a function that returns a copy of the value. So it needs something more like:
Vector3 pos = transform.position;
pos.y=0;
transform.position = pos;

DaveTheCoder

For reference, here are related issues and a PR:
https://github.com/godotengine/godot/issues/37918
https://github.com/godotengine/godot/issues/43311
https://github.com/godotengine/godot/pull/54869

cybereality

That was the biggest mistake Unity ever made. Worse than hiring Riccitiello.