Debug logging in Factorio mod

Question

I'm writing a Factorio mod, and I'm trying to figure out why it doesn't work. Can I do some kind of error / debug logging somewhere? Both manual (printing variables) and simple error reporting (accessing a property that doesn't exist) would be great. I know about the player.print method, but I can only get it to print string literals, not objects:

script.on_event(defines.events.on_player_created, function(event)
    pcall(function()
        local player = game.get_player(event.player_index)
        player.print("Hi player") -- works
        player.print(player) -- does not work. Any error message would be nice
    end)
end)

I've also tried the log file in the application folder, and the various debug modes (F5/F6 in game), but so far haven't found anything useful.

Answer 1

According to this forum post

You can launch the game from console, so standard print function would send its output there. You can redirect that output to files.

There is serpent pretty printer, which can print lua tables - syntax is print(serpent.block(arg)).

Answer 2

You can use the error() function which sends to factorio-current.log

For example:

error("Hi player")

or

error(serpent_block(table_name))

Answer 3

There is a write_file method in LuaGameScript.

game.write_file("mylog.log", serpent_block(table_name)) -- normal write
game.write_file("mylog.log", serpent_block(table_name), true) -- appending

(http://lua-api.factorio.com/0.15.23/LuaGameScript.html#LuaGameScript.write_file)

The file will end up in a folder called script-output under %APPDATA%/Factorio, next to your scenarios folder.