Default Exit Errors¶
Evennia allows for exits to have any name. The command “kitchen” is a valid exit name as well as “jump out the window” or “north”. An exit actually consists of two parts: an Exit Object and an Exit Command stored on said exit object. The command has the same key and aliases as the object, which is why you can see the exit in the room and just write its name to traverse it.
If you try to enter the name of a non-existing exit, it is thus the same as trying a non-exising command; Evennia doesn’t care about the difference:
> jump out the window Command 'jump out the window' is not available. Type "help" for help.
Many games don’t need this type of freedom however. They define only the
cardinal directions as valid exit names (Evennia’s
also offers this functionality). In this case, the error starts to look
> west Command 'west' is not available. Maybe you meant "@set" or "@reset"?
Since we for our particular game know that west is an exit direction, it would be better if the error message just told us that we couldn’t go there.
Adding default error commands¶
To solve this you need to be aware of how to write and add new commands. What you need to do is to create new commands for all directions you want to support in your game. In this example all we’ll do is echo an error message, but you could certainly consider more advanced uses. You add these commands to the default command set. Here is an example of such a set of commands:
# for example in a file mygame/commands/movecommands.py from evennia import default_cmds class CmdExitError(default_cmds.MuxCommand): "Parent class for all exit-errors." locks = "cmd:all()" arg_regex = r"\s|$" auto_help = False def func(self): "returns the error" self.caller.msg("You cannot move %s." % self.key) class CmdExitErrorNorth(CmdExitError): key = "north" aliases = ["n"] class CmdExitErrorEast(CmdExitError): key = "east" aliases = ["e"] class CmdExitErrorSouth(CmdExitError): key = "south" aliases = ["s"] class CmdExitErrorWest(CmdExitError): key = "west" aliases = ["w"]
Make sure to add the directional commands (not their parent) to the
CharacterCmdSet class in
# in mygame/commands/default_cmdsets.py from commands import movecommands # [...] class CharacterCmdSet(default_cmds.CharacterCmdSet): # [...] def at_cmdset_creation(self): # [...] self.add(movecommands.CmdExitErrorNorth()) self.add(movecommands.CmdExitErrorEast()) self.add(movecommands.CmdExitErrorSouth()) self.add(movecommands.CmdExitErrorWest())
@reload these commands (assuming you don’t get any errors -
check your log) will be loaded. What happens henceforth is that if you
are in a room with an Exitobject (let’s say it’s “north”), the proper
Exit-command will overload your error command (also named “north”). But
if you enter an direction without having a matching exit for it, you
will fallback to your default error commands:
> east You cannot move east.
Further expansions by the exit system (including manipulating the way the Exit command itself is created) can be done by modifying the Exit typeclass directly.
So why didn’t we create a single error command above? Something like this:
class CmdExitError(default_cmds.MuxCommand): "Handles all exit-errors." key = "error_cmd" aliases = ["north", "n", "east", "e", "south", "s", "west", "w"] #[...]
The anwer is that this would not work and understanding why is important in order to not be confused when working with commands and command sets.
The reason it doesn’t work is because Evennia’s command system
compares commands both by
key and by
aliases. If either of
those match, the two commands are considered identical as far as
cmdset merging system is concerned.
So the above example would work fine as long as there were no Exits at
all in the room. But what happens when we enter a room with an exit
“north”? The Exit’s cmdset is merged onto the default one, and since
there is an alias match, the system determines our
be identical. It is thus overloaded by the Exit command (which also
correctly defaults to a higher priority). The result is that you can go
through the north exit normally but none of the error messages for the
other directions are available since the single error command was
completely overloaded by the single matching “north” exit-command.