Where does print() go?

[ac-marshy]

Sorry if this is an obvious question.

If I'm using do_string on a LuaState, and there's a print statement in there, where does the printed text go?

Here is a very basic script I am running on an otherwise empty scene.

func _ready():
	var lua:LuaState = LuaState.new()
	lua.open_libraries(LuaState.LUA_BASE)
	
	var chunk = "print('Hello World!')" 	# Lua's print
	print(lua.do_string(chunk))		# Godot's print
	print(lua.do_string("return 'Goodbye!'"))

On running the scene, Godot's output console displays:

<null>
Goodbye!

Where is Lua's print going?

[gilzoide]

Hey @ac-marshy, thanks for the question.

Ok, so by default, Lua's print function prints the contents to the process's (in this case the Godot editor or game) standard output. If you open them from a terminal (or cmd or powershell in Windows), you should see the contents printed in the terminal. When you play a game from the Godot editor, if you can't see the output, its likely that the game's standard output is not connected to the editor (and this is in Godot's side). We need Godot's print functions to get stuff into the Output tab.

When you open only the LUA_BASE library, you only have Lua's print. When you open GODOT_UTILITY_FUNCTIONS, on the other hand, you get Godot's print function and friends, and calling them will output contents to the editor's Output tab. If you don't want to open GODOT_UTILITY_FUNCTIONS and have access to all utility functions, but only wants to get the print from Godot, you can manually bind it like so:

# Using printt here because it matches Lua's `print` behavior of adding "\t" between arguments
lua.globals["print"] = printt

[ac-marshy]

Thanks for taking the time to answer. By the by, this project is awesome and I hope you are able to continue working on it. :)

Unfortunately, this step didn't quite work on its own. I updated my code snippet above like so:

func _ready():
	var lua:LuaState = LuaState.new()
	lua.open_libraries(LuaState.LUA_BASE)
	lua.globals["print"] = printt # Manually bind Lua's print to Godot's printt
	var chunk = "print('Hello World!')" 	# Lua's print
	print(lua.do_string(chunk))				# Godot's print
	print(lua.do_string("return 'Goodbye!'"))

The output was thus:

[string "print('Hello World!')"]:1: attempt to call a sol.godot::Variant value (global 'print')
stack traceback:
	[string "print('Hello World!')"]:1: in main chunk
Goodbye!

Because the error mentions Variant, I tried adding LuaState.GODOT_VARIANT to the open_libraries call...

lua.open_libraries(LuaState.LUA_BASE + LuaState.GODOT_VARIANT)

... and that seemed to make it work. Output was thus:

Hello World!
<null>
Goodbye!

I understand that the <null> is the final result of executing the chunk, of course.

My goal is to create LuaStates on Godot objects and only expose a few functions per object to Lua. For example, a panel that a player can script in and see text output. This uses a Godot CodeEdit node for the program and a RichTextLabel for the output.

I don't think exposing GODOT_VARIANT will cause any problems for my sandboxing needs. What do you think?

Thanks again for your answer!

EDIT: As I read up on sandboxing Lua, I thought I'd share some links in case others find them useful.
http://lua-users.org/wiki/SandBoxes

[gilzoide]

Hey @ac-marshy. I'm so so sorry, support for callables without opening GODOT_VARIANT library was implemented in #64 and merged to main, but it wasn't released yet, I totally forgot this detail 😅 You can manually download a build of it here and replace it in your project if you want.

My goal is to create LuaStates on Godot objects and only expose a few functions per object to Lua. For example, a panel that a player can script in and see text output. This uses a Godot CodeEdit node for the program and a RichTextLabel for the output.

Nice! I guess this is a mechanic many people will choose to use. We already have issue #61 asking for Lua code highlight in CodeEdit, it will be much nicer for users to write scripts in built projects after that =D

I don't think exposing GODOT_VARIANT will cause any problems for my sandboxing needs. What do you think?

I'm not 100% sure what can be done with it alone. Since you're not opening GODOT_SINGLETONS, I think it may be safe enough, but maybe only if you don't expose any Object variants to Lua or something like that. But I may be wrong. Again, not 100% sure about it 🤔