13. Building a chair you can sit on

In this lesson we will make use of what we have learned to create a new game object: a chair you can sit on.

Our goals are:

• We want a new ‘sittable’ object, a Chair in particular.

• We want to be able to use a command to sit in the chair.

• Once we are sitting in the chair it should affect us somehow. To demonstrate this store the current chair in an attribute is_sitting. Other systems could check this to affect us in different ways.

• A character should be able to stand up and move away from the chair.

• When you sit down you should not be able to walk to another room without first standing up.

13.1. Make us not able to move while sitting

When you are sitting in a chair you can’t just walk off without first standing up. This requires a change to our Character typeclass. Open mygame/typeclasses/characters.py:

# in mygame/typeclasses/characters.py

# ...

class Character(DefaultCharacter):
    # ...

    def at_pre_move(self, destination, **kwargs):
       """
       Called by self.move_to when trying to move somewhere. If this returns
       False, the move is immediately cancelled.
       """
       if self.db.is_sitting:
           self.msg("You need to stand up first.")
           return False
       return True

When moving somewhere, character.move_to is called. This in turn will call character.at_pre_move. If this returns False, the move is aborted.

Here we look for an Attribute is_sitting (which we will assign below) to determine if we are stuck on the chair or not.

13.2. Making the Chair itself

Next we need the Chair itself, or rather a whole family of “things you can sit on” that we will call sittables. We can’t just use a default Object since we want a sittable to contain some custom code. We need a new, custom Typeclass. Create a new module mygame/typeclasses/sittables.py with the following content:

# in mygame/typeclasses/sittables.py

from typeclasses.objects import Object

class Sittable(Object):

    def do_sit(self, sitter):
        """
        Called when trying to sit on/in this object.

        Args:
            sitter (Object): The one trying to sit down.

        """
        current = self.db.sitter
        if current:
            if current == sitter:
                sitter.msg(f"You are already sitting on {self.key}.")
            else:
                sitter.msg(f"You can't sit on {self.key} "
                        f"- {current.key} is already sitting there!")
            return
        self.db.sitter = sitter
        sitter.db.is_sitting = self
        sitter.msg(f"You sit on {self.key}")

This handles the logic of someone sitting down on the chair.

• Line 3: We inherit from the empty Object class in mygame/typeclasses/objects.py. This means we can theoretically modify that in the future and have those changes affect sittables too.

• Line 7: The do_sit method expects to be called with the argument sitter, which is to be an Object (most likely a Character). This is the one wanting to sit down.

• Line 15: Note that, if the Attribute sitter is not defined on the chair (because this is the first time someone sits in it), this will simply return None, which is fine.

• Lines 16-22 We check if someone is already sitting on the chair and returns appropriate error messages depending on if it’s you or someone else. We use return to abort the sit-action.

• Line 23: If we get to this point, sitter gets to, well, sit down. We store them in the sitter Attribute on the chair.

• Line 24: self.obj is the chair this command is attachd to. We store that in the is_sitting Attribute on the sitter itself.

• Line 25: Finally we tell the sitter that they could sit down.

Let’s continue:

# add this right after the `do_sit method` in the same class 

    def do_stand(self, stander):
        """
        Called when trying to stand from this object.

        Args:
            stander (Object): The one trying to stand up.

        """
        current = self.db.sitter
        if not stander == current:
            stander.msg(f"You are not sitting on {self.key}.")
        else:
            self.db.sitter = None
            del stander.db.is_sitting
            stander.msg(f"You stand up from {self.key}.")

This is the inverse of sitting down; we need to do some cleanup.

• Line 12: If we are not sitting on the chair, it makes no sense to stand up from it.

• Line 15: If we get here, we could stand up. We make sure to un-set the sitter Attribute so someone else could use the chair later.

• Line 16: The character is no longer sitting, so we delete their is_sitting Attribute. We could also have done stander.db.is_sitting = None here, but deleting the Attribute feels cleaner.

• Line 17: Finally, we inform them that they stood up successfully.

One could imagine that one could have the future sit command (which we haven’t created yet) check if someone is already sitting in the chair instead. This would work too, but letting the Sittable class handle the logic around who can sit on it makes sense.

We let the typeclass handle the logic, and also let it do all the return messaging. This makes it easy to churn out a bunch of chairs for people to sit on.

13.2.1. Sitting on or in?

It’s fine to sit ‘on’ a chair. But what if our Sittable is an armchair?

> py evennia.create_object("typeclasses.sittables.Sittable", key="armchair", location=here)
> py self.search("armchair").do_sit(me)
You sit on armchair.

This is not grammatically correct, you actually sit “in” an armchair rather than “on” it. It’s also possible to both sit ‘in’ or ‘on’ a chair depending on the type of chair (English is weird). We want to be able to control this.

We could make a child class of Sittable named SittableIn that makes this change, but that feels excessive. Instead we will modify what we have:

# in mygame/typeclasses/sittables.py

from typeclasses.objects import Object

class Sittable(Object):

    def do_sit(self, sitter):
        """
        Called when trying to sit on/in this object.

        Args:
            sitter (Object): The one trying to sit down.

        """
        adjective = self.db.adjective or "on"
        current = self.db.sitter
        if current:
            if current == sitter:
                sitter.msg(f"You are already sitting {adjective} {self.key}.")
            else:
                sitter.msg(
                    f"You can't sit {adjective} {self.key} "
                    f"- {current.key} is already sitting there!")
            return
        self.db.sitter = sitter
        sitter.db.is_sitting = self
        sitter.msg(f"You sit {adjective} {self.key}")

    def do_stand(self, stander):
        """
        Called when trying to stand from this object.

        Args:
            stander (Object): The one trying to stand up.

        """
        current = self.db.sitter
        if not stander == current:
            stander.msg(f"You are not sitting {self.db.adjective} {self.key}.")
        else:
            self.db.sitter = None
            del stander.db.is_sitting
            stander.msg(f"You stand up from {self.key}.")

• Line 15: We grab the adjective Attribute. Using self.db.adjective or "on" here means that if the Attribute is not set (is None/falsy) the default “on” string will be assumed.

• Lines 19,22,27,39, and 43: We use this adjective to modify the return text we see.

reload the server. An advantage of using Attributes like this is that they can be modified on the fly, in-game. Let’s look at how a builder could use this with normal building commands (no need for py):

> set armchair/adjective = in 

Since we haven’t added the sit command yet, we must still use py to test:

> py self.search("armchair").do_sit(me)
You sit in armchair.

13.2.2. Extra credits

What if we want some more dramatic flair when you sit down in certain chairs?

You sit down and a whoopie cushion makes a loud fart noise!

You can make this happen by tweaking your Sittable class having the return messages be replaceable by Attributes that you can set on the object you create. You want something like this:

> py 
> chair = evennia.create_object("typeclasses.sittables.Sittable", key="pallet", location=here)
> chair.do_sit(me)
You sit down on pallet.
> chair.do_stand(me)
You stand up from pallet.
> chair.db.msg_sitting_down = "You sit down and a whoopie cushion makes a loud fart noise!"
> chair.do_sit(me)
You sit down and a whoopie cushion makes a loud fart noise!

That is, if you are not setting the Attribute, you should get a default value. We leave this implementation up to the reader.

13.3. Adding commands

As we discussed in the lesson about adding Commands, there are two main ways to design the commands for sitting and standing up:

• You can store the commands on the chair so they are only available when a chair is in the room

• You can store the commands on the Character so they are always available and you must always specify which chair to sit on.

Both of these are very useful to know about, so in this lesson we’ll try both.

13.3.1. Command variant 1: Commands on the chair

This way to implement sit and stand puts new cmdsets on the Sittable itself. As we’ve learned before, commands on objects are made available to others in the room. This makes the command easy but instead adds some complexity in the management of the CmdSet.

This is how it could look if armchair is in the room (Extra credits: Change the sit message on the armchair to match this output instead of getting the default You sit in armchair!):

> sit
As you sit down in armchair, life feels easier.

What happens if there are sittables sofa and barstool also in the room? Evennia will automatically handle this for us and allow us to specify which one we want:

> sit
More than one match for 'sit' (please narrow target):
 sit-1 (armchair)
 sit-2 (sofa)
 sit-3 (barstool)
> sit-1
As you sit down in armchair, life feels easier.

To keep things separate we’ll make a new module mygame/commands/sittables.py:

Separate Commands and Typeclasses?

You can organize these things as you like. If you wanted you could put the sit-command + cmdset together with the Sittable typeclass in mygame/typeclasses/sittables.py. That has the advantage of keeping everything related to sitting in one place. But there is also some organizational merit to keeping all Commands in one place as we do here.

# in mygame/commands/sittables.py 

from evennia import Command, CmdSet

class CmdSit(Command):
    """
    Sit down.
    """
    key = "sit"
    def func(self):
        self.obj.do_sit(self.caller)

class CmdStand(Command):
     """
     Stand up.
     """
     key = "stand"
     def func(self):
         self.obj.do_stand(self.caller)


class CmdSetSit(CmdSet):
    priority = 1
    def at_cmdset_creation(self):
        self.add(CmdSit)
        self.add(CmdStand)

As seen, the commands are nearly trivial.

• Lines 11 and 19: The self.obj is the object to which we added the cmdset with this Command (so the chair). We just call the do_sit/stand on that object and pass the caller (the person sitting down). The Sittable will do the rest.

• Line 23: The priority = 1 on CmdSetSit means that same-named Commands from this cmdset merge with a bit higher priority than Commands from the on-Character-cmdset (which has priority = 0). This means that if you have a sit command on your Character and comes into a room with a chair, the sit command on the chair will take precedence.

We also need to make a change to our Sittable typeclass. Open mygame/typeclasses/sittables.py:

# in mygame/typeclasses/sittables.py

from typeclasses.objects import Object
from commands.sittables import CmdSetSit 

class Sittable(Object):
    """
    (docstring)
    """
    def at_object_creation(self):
        self.cmdset.add_default(CmdSetSit)
    # ... 

• Line 4: We must install the CmdSetSit .

• Line 10: The at_object_creation method will only be called once, when the object is first created.

• Line 11: We add the command-set as a ‘default’ cmdset with add_default. This makes it persistent also protects it from being deleted should another cmdset be added. See Command Sets for more info.

Make sure to reload to make the code changes available.

All new Sittables will now have your sit Command. Your existing armchair will not though. This is because at_object_creation will not re-run for already existing objects. We can update it manually:

> update armchair

We could also update all existing sittables (all on one line):

List comprehensions

[obj for obj in iterator] is an example of a list comprehension. Think of it as an efficient way to construct a new list all in one line. You can read more about list comprehensions here in the Python docs.

> py from typeclasses.sittables import Sittable ;
       [sittable.at_object_creation() for sittable in Sittable.objects.all()]

We should now be able to use sit while in the room with the armchair.

> sit
As you sit down in armchair, life feels easier.
> stand
You stand up from armchair.

One issue with placing the sit (or stand) Command “on” the chair is that it will not be available when in a room without a Sittable object:

> sit
Command 'sit' is not available. ...

This is practical but not so good-looking; it makes it harder for the user to know a sit action is at all possible. Here is a trick for fixing this. Let’s add another Command to the bottom of mygame/commands/sittables.py:

# after the other commands in mygame/commands/sittables.py
# ...

class CmdNoSitStand(Command):
    """
    Sit down or Stand up
    """
    key = "sit"
    aliases = ["stand"]

    def func(self):
        if self.cmdname == "sit":
            self.msg("You have nothing to sit on.")
        else:
            self.msg("You are not sitting down.")

• Line 9: This command responds both to sit and stand because we added stand to its aliases list. Command aliases have the same ‘weight’ as the key of the command, both equally identify the Command.

• Line 12: The .cmdname of a Command holds the name actually used to call it. This will be one of "sit" or "stand". This leads to different return messages.

We don’t need a new CmdSet for this, instead we will add this to the default Character cmdset. Open mygame/commands/default_cmdsets.py:

# in mygame/commands/default_cmdsets.py

# ...
from commands import sittables

class CharacterCmdSet(CmdSet):
    """
    (docstring)
    """
    def at_cmdset_creation(self):
        # ...
        self.add(sittables.CmdNoSitStand)

As usual, make sure to reload the server to have the new code recognized.

To test we’ll build a new location without any comfy armchairs and go there:

> tunnel n = kitchen
north
> sit
You have nothing to sit on.
> south
sit
As you sit down in armchair, life feels easier.

We now have a fully functioning sit action that is contained with the chair itself. When no chair is around, a default error message is shown.

How does this work? There are two cmdsets at play, both of which have a sit/stand Command - one on the Sittable (armchair) and the other on us (via the CharacterCmdSet). Since we set a priority=1 on the chair’s cmdset (and CharacterCmdSet has priority=0), there will be no command-collision: the chair’s sit takes precedence over the sit defined on us … until there is no chair around.

So this handles sit. What about stand? That will work just fine:

> stand
You stand up from armchair.
> north
> stand
You are not sitting down.

We have one remaining problem with stand though - what happens when you are sitting down and try to stand in a room with more than one Sittable:

> stand
More than one match for 'stand' (please narrow target):
 stand-1 (armchair)
 stand-2 (sofa)
 stand-3 (barstool)

Since all the sittables have the stand Command on them, you’ll get a multi-match error. This works … but you could pick any of those sittables to “stand up from”. That’s really weird.

With sit it was okay to get a choice - Evennia can’t know which chair we intended to sit on. But once we sit we sure know from which chair we should stand up from! We must make sure that we only get the command from the chair we are actually sitting on.

We will fix this with a Lock and a custom lock function. We want a lock on the stand Command that only makes it available when the caller is actually sitting on the chair that particular stand command is attached to.

First let’s add the lock so we see what we want. Open mygame/commands/sittables.py:

# in mygame/commands/sittables.py

# ...

class CmdStand(Command):
     """
     Stand up.
     """
     key = "stand"
     locks = "cmd:sitsonthis()"

     def func(self):
         self.obj.do_stand(self.caller)
# ...

• Line 10: This is the lock definition. It’s on the form condition:lockfunc. The cmd: type lock is checked by Evennia when determining if a user has access to a Command at all. We want the lock-function to only return True if this command is on a chair which the caller is sitting on. What will be checked is the sitsonthis lock function which doesn’t exist yet.

Open mygame/server/conf/lockfuncs.py to add it!

# mygame/server/conf/lockfuncs.py

"""
(module lockstring)
"""
# ...

def sitsonthis(accessing_obj, accessed_obj, *args, **kwargs):
    """
    True if accessing_obj is sitting on/in the accessed_obj.
    """
    return accessed_obj.obj.db.sitter == accessing_obj

# ...

Evennia knows that all functions in mygame/server/conf/lockfuncs should be possible to use in a lock definition.

All lock functions must acccept the same arguments. The arguments are required and Evennia will pass all relevant objects as needed.

Lockfuncs

Evennia provides a large number of default lockfuncs, such as checking permission-levels, if you are carrying or are inside the accessed object etc. There is no concept of ‘sitting’ in default Evennia however, so this we need to specify ourselves.

• accessing_obj is the one trying to access the lock. So us, in this case.

• accessed_obj is the entity we are trying to gain a particular type of access to. Since we define the lock on the CmdStand class, this is the command instance. We are however not interested in that, but the object the command is assigned to (the chair). The object is available on the Command as .obj. So here, accessed_obj.obj is the chair.

• args is a tuple holding any arguments passed to the lockfunc. Since we use sitsondthis() this will be empty (and if we add anything, it will be ignored).

• kwargs is a tuple of keyword arguments passed to the lockfuncs. This will be empty as well in our example.

Make sure you reload.

If you are superuser, it’s important that you quell yourself before trying this out. This is because the superuser bypasses all locks - it can never get locked out, but it means it will also not see the effects of a lock like this.

> quell
> stand
You stand up from armchair

None of the other sittables’ stand commands passed the lock and only the one we are actually sitting on did! This is a fully functional chair now!

Adding a Command to the chair object like this is powerful and is a good technique to know. It does come with some caveats though, as we’ve seen.

We’ll now try another way to add the sit/stand commands.

13.3.2. Command variant 2: Command on Character

Before we start with this, delete the chairs you’ve created:

> del armchair 
> del sofa 
> (etc)

The make the following changes:

• In mygame/typeclasses/sittables.py, comment out the entire at_object_creation method.

• In mygame/commands/default_cmdsets.py, comment out the line self.add(sittables.CmdNoSitStand).

This disables the on-object command solution so we can try an alternative. Make sure to reload so the changes are known to Evennia.

In this variation we will put the sit and stand commands on the Character instead of on the chair. This makes some things easier, but makes the Commands themselves more complex because they will not know which chair to sit on. We can’t just do sit anymore. This is how it will work:

> sit <chair>
You sit on chair.
> stand
You stand up from chair.

Open mygame/commands/sittables.py again. We’ll add a new sit-command. We name the class CmdSit2 since we already have CmdSit from the previous example. We put everything at the end of the module to keep it separate.

# in mygame/commands/sittables.py

from evennia import Command, CmdSet
from evennia import InterruptCommand

class CmdSit(Command):
    # ...

# ...

# new from here

class CmdSit2(Command):
    """
    Sit down.

    Usage:
        sit <sittable>

    """
    key = "sit"

    def parse(self):
        self.args = self.args.strip()
        if not self.args:
            self.caller.msg("Sit on what?")
            raise InterruptCommand

    def func(self):

        # self.search handles all error messages etc.
        sittable = self.caller.search(self.args)
        if not sittable:
            return
        try:
            sittable.do_sit(self.caller)
        except AttributeError:
            self.caller.msg("You can't sit on that!")

Raising exceptions

Raising an exception allows for immediately interrupting the current program flow. Python automatically raises error-exceptions when detecting problems with the code. It will be raised up through the sequence of called code (the ‘stack’) until it’s either caught with a try ... except or reaches the outermost scope where it’ll be logged or displayed. In this case Evennia knows to catch the InterruptCommand exception and stop the command execution early.

• Line 4: We need the InterruptCommand to be able to abort command parsing early (see below).

• Line 27: The parse method runs before the func method on a Command. If no argument is provided to the command, we want to fail early, already in parse, so func never fires. Just return is not enough to do that, we need to raise InterruptCommand. Evennia will see a raised InterruptCommand as a sign it should immediately abort the command execution.

• Line 32: We use the parsed command arguments as the target-chair to search for. As discussed in the search tutorial, self.caller.search() will handle error messages itself. So if it returns None, we can just return.

• Line 35-38: The try...except block ‘catches’ and exception and handles it. In this case we try to run do_sit on the object. If the object we found is not a Sittable, it will likely not have a do_sit method and an AttributeError will be raised. We should handle that case gracefully.

Let’s do the stand command while we are at it. Since the Command is external to the chair we need to figure out if we are sitting down or not.

# end of mygame/commands/sittables.py

class CmdStand2(Command):
    """
    Stand up.

    Usage:
        stand

    """
    key = "stand"

    def func(self):
        caller = self.caller
        # if we are sitting, this should be set on us
        sittable = caller.db.is_sitting
        if not sittable:
            caller.msg("You are not sitting down.")
        else:
            sittable.do_stand(caller)

• Line 17: We didn’t need the is_sitting Attribute for the first version of these Commands, but we do need it now. Since we have this, we don’t need to search and know just which chair we sit on. If we don’t have this Attribute set, we are not sitting anywhere.

• Line 21: We stand up using the sittable we found.

All that is left now is to make sit and stand available to us. This type of Command should be available to us all the time so we can put it in the default Cmdset on the Character. Open mygame/commands/default_cmdsets.py.

# in mygame/commands/default_cmdsets.py

# ...
from commands import sittables

class CharacterCmdSet(CmdSet):
    """
    (docstring)
    """
    def at_cmdset_creation(self):
        # ...
        self.add(sittables.CmdSit2)
        self.add(sittables.CmdStand2)

Make sure to reload.

Now let’s try it out:

> create/drop sofa : sittables.Sittable
> sit sofa
You sit down on sofa.
> stand
You stand up from sofa.
> north 
> sit sofa 
> You can't find 'sofa'.

Storing commands on the Character centralizes them, but you must instead search or store any external objects you want that command to interact on.

13.4. Conclusions

In this lesson we built ourselves a chair and even a sofa!

• We modified our Character class to avoid moving when sitting down.

• We made a new Sittable typeclass

• We tried two ways to allow a user to interact with sittables using sit and stand commands.

Eagle-eyed readers will notice that the stand command sitting “on” the chair (variant 1) would work just fine together with the sit command sitting “on” the Character (variant 2). There is nothing stopping you from mixing them, or even try a third solution that better fits what you have in mind.

This concludes the first part of the Beginner tutorial!