Return custom errors on missing Exits

> north
Ouch! You bump into a wall!
> out 
 But you are already outside ...? 

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 exit-object, which is why you can see the exit in the room and just write its name to traverse it.

So if you try to enter the name of a non-existing exit, Evennia treats is the same way as if you were trying to use a non-existing command:

 > 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. They define only the cardinal directions as valid exit names ( Evennia’s tunnel command also offers this functionality). In this case, the error starts to look less logical:

 > 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.

 > west 
 You cannot move west.

The way to do this is to give Evennia an alternative Command to use when no Exit-Command is found in the room. See Adding Commands for more info about the process of adding new Commands to Evennia.

In this example we will just echo an error message, but you could do everything (maybe you lose health if you bump into a wall?)

# for example in a file mygame/commands/

from evennia import default_cmds, CmdSet

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 error based on key"""
        self.caller.msg(f"You cannot move {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"]

# you could add each command on its own to the default cmdset,
# but putting them all in a cmdset here allows you to
# just add this and makes it easier to expand with more 
# exit-errors in the future

class MovementFailCmdSet(CmdSet):
    def at_cmdset_creation(self): 

We pack our commands in a new little cmdset; if we add this to our CharacterCmdSet, we can just add more errors to MovementFailCmdSet later without having to change code in two places.

# in mygame/commands/

from commands import movecommands

# [...]
class CharacterCmdSet(default_cmds.CharacterCmdSet):
    # [...]
    def at_cmdset_creation(self):
        # [...]
        # this adds all the commands at once

reload the server. 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 a direction without having a matching exit for it, you will fall back 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.

Why not a single command?

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"]

This would not work the way we want. Understanding why is important.

Evennia’s command system compares commands by key and/or aliases. If any key or alias match, the two commands are considered identical. When the cmdsets merge, priority will then decide which of these ‘identical’ commandss replace which.

So the above example would work fine as long as there were no Exits at all in the room. But when we enter a room with an exit “north”, its Exit-command (which has a higher priority) will override the single CmdExitError with its alias ‘north’. So the CmdExitError will be gone and while “north” will work, we’ll again get the normal “Command not recognized” error for the other directions.