8. Adding custom commands

In this lesson we’ll learn how to create our own Evennia Commands. If you are new to Python you’ll also learn some more basics about how to manipulate strings and get information out of Evennia.

A Command is something that handles the input from a user and causes a result to happen. An example is look, which examines your current location and tells how it looks like and what is in it.

Commands are not typeclassed

If you just came from the previous lesson, you might want to know that Commands and CommandSets are not typeclassed. That is, instances of them are not saved to the database. They are “just” normal Python classes.

In Evennia, a Command is a Python class. If you are unsure about what a class is, review the previous lessons! A Command inherits from evennia.Command or from one of the alternative command- classes, such as MuxCommand which is what most default commands use.

All Commands are in turn grouped in another class called a Command Set. Think of a Command Set as a bag holding many different commands. One CmdSet could for example hold all commands for combat, another for building etc. By default, Evennia groups all character-commands into one big cmdset.

Command-Sets are then associated with objects, for example with your Character. Doing so makes the commands in that cmdset available to the object. So, to summarize:

• Commands are classes

• A group of Commands is stored in a CmdSet

• CmdSets are stored on objects - this defines which commands are available to that object.

8.1. Creating a custom command

Open mygame/commands/command.py:

"""
(module docstring)
"""

from evennia import Command as BaseCommand
# from evennia import default_cmds

class Command(BaseCommand):
    """
    (class docstring)
    """
    pass

# (lots of commented-out stuff)
# ...

Ignoring the docstrings (which you can read if you want), this is the only really active code in the module.

We can see that we import Command from evennia and use the from ... import ... as ... form to rename it to BaseCommand. This is so we can let our child class also be named Command for reference. The class itself doesn’t do anything, it just has pass. So in the same way as Object in the previous lesson, this class is identical to its parent.

The commented out default_cmds gives us access to Evennia’s default commands for easy overriding. We’ll try that a little later.

We could modify this module directly, but to train imports we’ll work in a separate module. Open a new file mygame/commands/mycommands.py and add the following code:

from commands.command import Command

class CmdEcho(Command):
    key = "echo"

This is the simplest form of command you can imagine. It just gives itself a name, “echo”. This is what you will use to call this command later.

Next we need to put this in a CmdSet. It will be a one-command CmdSet for now! Change your file as such:

from commands.command import Command
from evennia import CmdSet

class CmdEcho(Command):
    key = "echo"


class MyCmdSet(CmdSet):

    def at_cmdset_creation(self):
        self.add(CmdEcho)

Our EchoCmdSet class must have an at_cmdset_creation method, named exactly like this - this is what Evennia will be looking for when setting up the cmdset later, so if you didn’t set it up, it will use the parent’s version, which is empty. Inside we add the command class to the cmdset by self.add(). If you wanted to add more commands to this CmdSet you could just add more lines of self.add after this.

Finally, let’s add this command to ourselves so we can try it out. In-game you can experiment with py again:

> py self.cmdset.add("commands.mycommands.MyCmdSet")

Now try

> echo
Command echo has no defined `func()` - showing on-command variables:
...
...

You should be getting a long list of outputs. The reason for this is that your echo function is not really “doing” anything yet and the default function is then to show all useful resources available to you when you use your Command. Let’s look at some of those listed:

Command echo has no defined `func()` - showing on-command variables:
obj (<class 'typeclasses.characters.Character'>): YourName
lockhandler (<class 'evennia.locks.lockhandler.LockHandler'>): cmd:all()
caller (<class 'typeclasses.characters.Character'>): YourName
cmdname (<class 'str'>): echo
raw_cmdname (<class 'str'>): echo
cmdstring (<class 'str'>): echo
args (<class 'str'>):
cmdset (<class 'evennia.commands.cmdset.CmdSet'>): @mail, about, access, accounts, addcom, alias, allcom, ban, batchcode, batchcommands, boot, cboot, ccreate,
    cdesc, cdestroy, cemit, channels, charcreate, chardelete, checklockstring, clientwidth, clock, cmdbare, cmdsets, color, copy, cpattr, create, cwho, delcom,
    desc, destroy, dig, dolphin, drop, echo, emit, examine, find, force, get, give, grapevine2chan, help, home, ic, inventory, irc2chan, ircstatus, link, lock,
    look, menutest, mudinfo, mvattr, name, nick, objects, ooc, open, option, page, password, perm, pose, public, py, quell, quit, reload, reset, rss2chan, say,
    script, scripts, server, service, sessions, set, setdesc, sethelp, sethome, shutdown, spawn, style, tag, tel, test2010, test2028, testrename, testtable,
    tickers, time, tunnel, typeclass, unban, unlink, up, up, userpassword, wall, whisper, who, wipe
session (<class 'evennia.server.serversession.ServerSession'>): Griatch(#1)@1:2:7:.:0:.:0:.:1
account (<class 'typeclasses.accounts.Account'>): Griatch(account 1)
raw_string (<class 'str'>): echo

--------------------------------------------------
echo - Command variables from evennia:
--------------------------------------------------
name of cmd (self.key): echo
cmd aliases (self.aliases): []
cmd locks (self.locks): cmd:all();
help category (self.help_category): General
object calling (self.caller): Griatch
object storing cmdset (self.obj): Griatch
command string given (self.cmdstring): echo
current cmdset (self.cmdset): ChannelCmdSet

These are all properties you can access with . on the Command instance, such as .key, .args and so on. Evennia makes these available to you and they will be different every time a command is run. The most important ones we will make use of now are:

• caller - this is ‘you’, the person calling the command.

• args - this is all arguments to the command. Now it’s empty, but if you tried echo foo bar you’d find that this would be " foo bar".

• obj - this is object on which this Command (and CmdSet) “sits”. So you, in this case.

The reason our command doesn’t do anything yet is because it’s missing a func method. This is what Evennia looks for to figure out what a Command actually does. Modify your CmdEcho class:

# ...

class CmdEcho(Command):
    """
    A simple echo command

    Usage:
        echo <something>

    """
    key = "echo"

    def func(self):
        self.caller.msg(f"Echo: '{self.args}'")

# ...

First we added a docstring. This is always a good thing to do in general, but for a Command class, it will also automatically become the in-game help entry! Next we add the func method. It has one active line where it makes use of some of those variables we found the Command offers to us. If you did the basic Python tutorial, you will recognize .msg - this will send a message to the object it is attached to us - in this case self.caller, that is, us. We grab self.args and includes that in the message.

Since we haven’t changed MyCmdSet, that will work as before. Reload and re-add this command to ourselves to try out the new version:

> reload
> py self.cmdset.add("commands.mycommands.MyCmdSet")
> echo
Echo: ''

Try to pass an argument:

> echo Woo Tang!
Echo: ' Woo Tang!'

Note that there is an extra space before Woo!. That is because self.args contains the everything after the command name, including spaces. Evennia will happily understand if you skip that space too:

> echoWoo Tang!
Echo: 'Woo Tang!'

There are ways to force Evennia to require an initial space, but right now we want to just ignore it since it looks a bit weird for our echo example. Tweak the code:

# ...

class CmdEcho(Command):
    """
    A simple echo command

    Usage:
        echo <something>

    """
    key = "echo"

    def func(self):
        self.caller.msg(f"Echo: '{self.args.strip()}'")

# ...

The only difference is that we called .strip() on self.args. This is a helper method available on all strings - it strips out all whitespace before and after the string. Now the Command-argument will no longer have any space in front of it.

> reload
> py self.cmdset.add("commands.mycommands.MyCmdSet")
> echo Woo Tang!
Echo: 'Woo Tang!'

Don’t forget to look at the help for the echo command:

> help echo

You will get the docstring you put in your Command-class.

8.1.1. Making our cmdset persistent

It’s getting a little annoying to have to re-add our cmdset every time we reload, right? It’s simple enough to make echo a persistent change though:

> py self.cmdset.add("commands.mycommands.MyCmdSet", persistent=True)

Now you can reload as much as you want and your code changes will be available directly without needing to re-add the MyCmdSet again. To remove the cmdset again, do

> py self.cmdset.remove("commands.mycommands.MyCmdSet")

But for now, keep it around, we’ll expand it with some more examples.

8.1.2. Figuring out who to hit

Let’s try something a little more exciting than just echo. Let’s make a hit command, for punching someone in the face! This is how we want it to work:

> hit <target>
You hit <target> with full force!

Not only that, we want the to see

You got hit by <hitter> with full force!

Here, <hitter> would be the one using the hit command and <target> is the one doing the punching.

Still in mygame/commands/mycommands.py, add a new class, between CmdEcho and MyCmdSet.

# ...

class CmdHit(Command):
    """
    Hit a target.

    Usage:
      hit <target>

    """
    key = "hit"

    def func(self):
        args = self.args.strip()
        if not args:
            self.caller.msg("Who do you want to hit?")
            return
        target = self.caller.search(args)
        if not target:
            return
        self.caller.msg(f"You hit {target.key} with full force!")
        target.msg(f"You got hit by {self.caller.key} with full force!")
# ...

A lot of things to dissect here:

• Line 3: The normal class header. We inherit from Command which we imported at the top of this file.

• Lines 4-10: The docstring and help-entry for the command. You could expand on this as much as you wanted.

• Line 11: We want to write hit to use this command.

• Line 14: We strip the whitespace from the argument like before. Since we don’t want to have to do self.args.strip() over and over, we store the stripped version in a local variable args. Note that we don’t modify self.args by doing this, self.args will still have the whitespace and is not the same as args in this example.

if-statements

The full form of the if statement is

if condition: … elif othercondition: … else: …

There can be any number of elifs to mark when different branches of the code should run. If the else condition is given, it will run if none of the other conditions was truthy. In Python the if..elif..else structure also serves the same function as case in some other languages.

• Line 15 has our first conditional, an if statement. This is written on the form if <condition>: and only if that condition is ‘truthy’ will the indented code block under the if statement run. To learn what is truthy in Python it’s usually easier to learn what is “falsy”:

  • False - this is a reserved boolean word in Python. The opposite is True.

  • None - another reserved word. This represents nothing, a null-result or value.

  • 0 or 0.0

  • The empty string "" or '' or """""" or ''''''

  • Empty iterables we haven’t seen yet, like empty lists [], empty tuples () and empty dicts {}.

  • Everything else is “truthy”.

  Line 16’s condition is not args. The not inverses the result, so if args is the empty string (falsy), the whole conditional becomes truthy. Let’s continue in the code:

• Lines 16-17: This code will only run if the if statement is truthy, in this case if args is the empty string.

• Line 17: return is a reserved Python word that exits func immediately.

• Line 18: We use self.caller.search to look for the target in the current location.

• Lines 19-20: A feature of .search is that it will already inform self.caller if it couldn’t find the target. In that case, target will be None and we should just directly return.

• Lines 21-22: At this point we have a suitable target and can send our punching strings to each.

Finally we must also add this to a CmdSet. Let’s add it to MyCmdSet which we made persistent earlier.

# ...

class MyCmdSet(CmdSet):

    def at_cmdset_creation(self):
        self.add(CmdEcho)
        self.add(CmdHit)

Errors in your code

With longer code snippets to try, it gets more and more likely you’ll make an error and get a traceback when you reload. This will either appear directly in-game or in your log (view it with evennia -l in a terminal). Don’t panic; tracebacks are your friends - they are to be read bottom-up and usually describe exactly where your problem is. Refer to The Python intro <Python-basic-introduction.html>_ for more hints. If you get stuck, reach out to the Evennia community for help.

Next we reload to let Evennia know of these code changes and try it out:

> reload
hit
Who do you want to hit?
hit me
You hit YourName with full force!
You got hit by YourName with full force!

Lacking a target, we hit ourselves. If you have one of the dragons still around from the previous lesson you could try to hit it (if you dare):

hit smaug
You hit Smaug with full force!

You won’t see the second string. Only Smaug sees that (and is not amused).

8.2. Summary

In this lesson we learned how to create our own Command, add it to a CmdSet and then to ourselves. We also upset a dragon.

In the next lesson we’ll learn how to hit Smaug with different weapons. We’ll also get into how we replace and extend Evennia’s default Commands.