7. Making objects persistent

Now that we have learned a little about how to find things in the Evennia library, let’s use it.

In the Python classes and objects lesson we created the dragons Fluffy, Cuddly and Smaug and made them fly and breathe fire. So far our dragons are short-lived - whenever we restart the server or quit() out of python mode they are gone.

This is what you should have in mygame/typeclasses/monsters.py so far:

class Monster:
    """
    This is a base class for Monsters.
    """
 
    def __init__(self, key):
        self.key = key 

    def move_around(self):
        print(f"{self.key} is moving!")


class Dragon(Monster):
    """
    This is a dragon-specific monster.
    """

    def move_around(self):
        super().move_around()
        print("The world trembles.")

    def firebreath(self):
        """ 
        Let our dragon breathe fire.
        """
        print(f"{self.key} breathes fire!")

7.1. Our first persistent object

At this point we should know enough to understand what is happening in mygame/typeclasses/objects.py. Let’s open it:

"""
module docstring
"""
from evennia import DefaultObject

class ObjectParent:
    """ 
    class docstring 
    """
    pass

class Object(ObjectParent, DefaultObject):
    """
    class docstring
    """
    pass

So we have a class Object that inherits from ObjectParent (which is empty) and DefaultObject, which we have imported from Evennia. The ObjectParent acts as a place to put code you want all of your Objects to have. We’ll focus on Object and DefaultObject for now.

The class itself doesn’t do anything (it just passes) but that doesn’t mean it’s useless. As we’ve seen, it inherits all the functionality of its parent. It’s in fact an exact replica of DefaultObject right now. Once we know what kind of methods and resources are available on DefaultObject we could add our own and change the way it works!

One thing that Evennia classes offers and which you don’t get with vanilla Python classes is persistence - they survive a server reload since they are stored in the database.

Go back to mygame/typeclasses/monsters.py. Change it as follows:

from typeclasses.objects import Object

class Monster(Object):
    """
    This is a base class for Monsters.
    """
    def move_around(self):
        print(f"{self.key} is moving!")


class Dragon(Monster):
    """
    This is a dragon-specific Monster.
    """

    def move_around(self):
        super().move_around()
        print("The world trembles.")

    def firebreath(self):
        """ 
        Let our dragon breathe fire.
        """
        print(f"{self.key} breathes fire!")

Don’t forget to save. We removed Monster.__init__ and made Monster inherit from Evennia’s Object (which in turn inherits from Evennia’s DefaultObject, as we saw). By extension, this means that Dragon also inherits from DefaultObject, just from further away!

7.1.1. Making a new object by calling the class

First reload the server as usual. We will need to create the dragon a little differently this time:

Keyword arguments

Keyword arguments (like db_key="Smaug") is a way to name the input arguments to a function or method. They make things easier to read but also allows for conveniently setting defaults for values not given explicitly. We saw them previously in use for .format().

> py
> from typeclasses.monsters import Dragon
> smaug = Dragon(db_key="Smaug", db_location=here)
> smaug.save()
> smaug.move_around()
Smaug is moving!
The world trembles.

Smaug works the same as before, but we created him differently: first we used Dragon(db_key="Smaug", db_location=here) to create the object, and then we used smaug.save() afterwards.

here

The here used in db_location=here is a shortcut for your current location. This here (similar to me) is only available to use in the py command; you can’t use it in other Python code you write unless you define it yourself.

> quit()
Python Console is closing.
> look 

You should now see that Smaug is in the room with you. Woah!

> reload 
> look 

He’s still there… What we just did was to create a new entry in the database for Smaug. We gave the object its name (key) and set its location to our current location.

To make use of Smaug in code we must first find him in the database. For an object in the current location we can easily do this in py by using me.search():

> py smaug = me.search("Smaug") ; smaug.firebreath()
Smaug breathes fire!  

7.1.2. Creating using create_object

Creating Smaug like we did above is nice because it’s similar to how we created non-database bound Python instances before. But you need to use db_key instead of key and you also have to remember to call .save() afterwards. Evennia has a helper function that is more common to use, called create_object. Let’s recreate Cuddly this time:

> py evennia.create_object('typeclasses.monsters.Monster', key="Cuddly", location=here)
> look 

Boom, Cuddly should now be in the room with you, a little less scary than Smaug. You specify the python-path to the code you want and then set the key and location (if you had the Monster class already imported, you could have passed that too). Evennia sets things up and saves for you.

If you want to find Smaug from anywhere (not just in the same room), you can use Evennia’s search_object function:

> cuddly = evennia.search_object("Cuddly")[0] ; cuddly.move_around()
Cuddly is moving!

The [0] is because search_object always returns a list of zero, one or more found objects. The [0] means that we want the first element of this list (counting in Python always starts from 0). If there were multiple Cuddlies we could get the second one with [1].

7.1.3. Creating using create-command

Finally, you can also create a new dragon using the familiar builder-commands we explored a few lessons ago:

> create/drop Fluffy:typeclasses.monsters.Dragon

Fluffy is now in the room. After learning about how objects are created you’ll realize that all this command really does is to parse your input, figure out that /drop means to “give the object the same location as the caller”, and then do a call very similar to

evennia.create_object("typeclasses.monsters.Dragon", key="Cuddly", location=here)

That’s pretty much all there is to the mighty create command! The rest is just parsing for the command to understand just what the user wants to create.

7.2. Typeclasses

The Object (and DefafultObject class we inherited from above is what we refer to as a Typeclass. This is an Evennia thing. The instance of a typeclass saves itself to the database when it is created, and after that you can just search for it to get it back.

We use the term typeclass or typeclassed to differentiate these types of classes and objects from the normal Python classes, whose instances go away on a reload.

The number of typeclasses in Evennia are so few they can be learned by heart:

Evennia base typeclass	mygame.typeclasses child	description
evennia.DefaultObject	typeclasses.objects.Object	Everything with a location
evennia.DefaultCharacter (child of DefaultObject)	typeclasses.characters.Character	Player avatars
evennia.DefaultRoom (child of DefaultObject)	typeclasses.rooms.Room	In-game locations
evennia.DefaultExit (chld of DefaultObject)	typeclasses.exits.Exit	Links between rooms
evennia.DefaultAccount	typeclasses.accounts.Account	A player account
evennia.DefaultChannel	typeclasses.channels.Channel	In-game comms
evennia.DefaultScript	typeclasses.scripts.Script	Entities with no location

The child classes under mygame/typeclasses/ are meant for you to conveniently modify and work with. Every class inheriting (at any distance) from a Evennia base typeclass is also considered a typeclass.

from somewhere import Something 
from evennia import DefaultScript 

class MyOwnClass(Something): 
    # not inheriting from an Evennia core typeclass, so this 
    # is just a 'normal' Python class inheriting from somewhere
    pass 

class MyOwnClass2(DefaultScript):
    # inherits from one of the core Evennia typeclasses, so 
    # this is also considered a 'typeclass'.
    pass

Why invent the name ‘typeclass’?

We separate ‘regular classes’ from ‘typeclasses’ because while typeclasses act almost like normal Python classes, there are some differences. We will gloss over those differences for now, but they are worth to read up on when you want to do more advanced things later.

Notice that the classes in mygame/typeclasses/ are not inheriting from each other. For example, Character is inheriting from evennia.DefaultCharacter and not from typeclasses.objects.Object. So if you change Object you will not cause any change in the Character class. If you want that you can easily just change the child classes to inherit in that way instead; Evennia doesn’t care.

As seen with our Dragon example, you don’t have to modify these modules directly. You can just make your own modules and import the base class.

7.2.1. Examining objects

When you do

> create/drop giantess:typeclasses.monsters.Monster
You create a new Monster: giantess.

or

> py evennia.create_object("typeclasses.monsters.Monster", key="Giantess", location=here)

You are specifying exactly which typeclass you want to use to build the Giantess. Let’s examine the result:

> examine giantess
------------------------------------------------------------------------------- 
Name/key: Giantess (#14)
Typeclass: Monster (typeclasses.monsters.Monster)
Location: Limbo (#2)
Home: Limbo (#2)
Permissions: <None>
Locks: call:true(); control:id(1) or perm(Admin); delete:id(1) or perm(Admin);
   drop:holds(); edit:perm(Admin); examine:perm(Builder); get:all();
   puppet:pperm(Developer); tell:perm(Admin); view:all()
Persistent attributes:
 desc = You see nothing special. 
------------------------------------------------------------------------------- 

We used the examine command briefly in the lesson about building in-game. Now these lines may be more useful to us:

• Name/key - The name of this thing. The value (#14) is probably different for you. This is the unique ‘primary key’ or dbref for this entity in the database.

• Typeclass: This show the typeclass we specified, and the path to it.

• Location: We are in Limbo. If you moved elsewhere you’ll see that instead. Also the #dbref of Limbo is shown.

• Home: All objects with a location (inheriting from DefaultObject) must have a home location. This is a backup to move the object to if its current location is deleted.

• Permissions: Permissions are like the inverse to Locks - they are like keys to unlock access to other things. The giantess have no such keys (maybe fortunately). The Permissions has more info.

• Locks: Locks are the inverse of Permissions - specify what criterion other objects must fulfill in order to access the giantess object. This uses a very flexible mini-language. For examine, the line examine:perm(Builders) is read as “Only those with permission Builder or higher can examine this object”. Since we are the superuser we pass (even bypass) such locks with ease. See the Locks documentation for more info.

• Persistent attributes: This allows for storing arbitrary, persistent data on the typeclassed entity. We’ll get to those in the next section.

Note how the Typeclass line describes exactly where to find the code of this object? This is very useful for understanding how any object in Evennia works.

7.2.2. Default typeclasses

What happens if we create an object and don’t specify its typeclass though?

> create/drop box 
You create a new Object: box.

or

> py create.create_object(None, key="box", location=here)

Now check it out:

> examine box  

You will find that the Typeclass line now reads

Typeclass: Object (typeclasses.objects.Object) 

So when you didn’t specify a typeclass, Evennia used a default, more specifically the (so far) empty Object class in mygame/typeclasses/objects.py. This is usually what you want, especially since you can tweak that class as much as you like.

But the reason Evennia knows to fall back to this class is not hard-coded - it’s a setting. The default is in evennia/settings_default.py, with the name BASE_OBJECT_TYPECLASS, which is set to typeclasses.objects.Object.

Changing things

While it’s tempting to change folders around to your liking, this can make it harder to follow tutorials and may confuse if you are asking others for help. So don’t overdo it unless you really know what you are doing.

So if you wanted the creation commands and methods to default to some other class you could add your own BASE_OBJECT_TYPECLASS line to mygame/server/conf/settings.py. The same is true for all the other typeclasseses, like characters, rooms and accounts. This way you can change the layout of your game dir considerably if you wanted. You just need to tell Evennia where everything is.

7.3. Modifying ourselves

Let’s try to modify ourselves a little. Open up mygame/typeclasses/characters.py.

"""
(module docstring)
"""
from evennia import DefaultCharacter

class Character(DefaultCharacter):
    """
    (class docstring)
    """
    pass

This looks quite familiar now - an empty class inheriting from the Evennia base typeclass (it’s even easier than Object since there is no equvalent ParentObject mixin class here). As you would expect, this is also the default typeclass used for creating Characters if you don’t specify it. You can verify it:

> examine me
------------------------------------------------------------------------------
Name/key: YourName (#1)
Session id(s): #1
Account: YourName
Account Perms: <Superuser> (quelled)
Typeclass: Character (typeclasses.characters.Character)
Location: Limbo (#2)
Home: Limbo (#2)
Permissions: developer, player
Locks:      boot:false(); call:false(); control:perm(Developer); delete:false();
      drop:holds(); edit:false(); examine:perm(Developer); get:false();
      msg:all(); puppet:false(); tell:perm(Admin); view:all()
Stored Cmdset(s):
 commands.default_cmdsets.CharacterCmdSet [DefaultCharacter] (Union, prio 0)
Merged Cmdset(s):
   ...
Commands available to YourName (result of Merged CmdSets):
   ...
Persistent attributes:
 desc = This is User #1.
 prelogout_location = Limbo
Non-Persistent attributes:
 last_cmd = None
------------------------------------------------------------------------------

Yes, the examine command understands me. You got a lot longer output this time. You have a lot more going on than a simple Object. Here are some new fields of note:

• Session id(s): This identifies the Session (that is, the individual connection to a player’s game client).

• Account shows, well the Account object associated with this Character and Session.

• Stored/Merged Cmdsets and Commands available is related to which Commands are stored on you. We will get to them in the next lesson. For now it’s enough to know these consitute all the commands available to you at a given moment.

• Non-Persistent attributes are Attributes that are only stored temporarily and will go away on next reload.

Look at the Typeclass field and you’ll find that it points to typeclasses.character.Character as expected. So if we modify this class we’ll also modify ourselves.

7.3.1. A method on ourselves

Let’s try something simple first. Back in mygame/typeclasses/characters.py:

class Character(DefaultCharacter):
    """
    (class docstring)
    """

    strength = 10
    dexterity = 12
    intelligence = 15

    def get_stats(self):
        """
        Get the main stats of this character
        """
        return self.strength, self.dexterity, self.intelligence

> reload 
> py self.get_stats()
(10, 12, 15)

Tuples and lists

• A list is written [a, b, c, d, ...]. It can be modified after creation.

• A tuple is written (a, b, c, ...). It cannot be modified once created.

We made a new method, gave it a docstring and had it return the RP-esque values we set. It comes back as a tuple (10, 12, 15). To get a specific value you could specify the index of the value you want, starting from zero:

> py stats = self.get_stats() ; print(f"Strength is {stats[0]}.")
Strength is 10.

7.3.2. Attributes

So what happens when we increase our strength? This would be one way:

> py self.strength = self.str + 1
> py self.strength
11

Here we set the strength equal to its previous value + 1. A shorter way to write this is to use Python’s += operator:

> py self.strength += 1
> py self.strength
12     
> py self.get_stats()
(12, 12, 15)

This looks correct! Try to change the values for dex and int too; it works fine. However:

> reload 
> py self.get_stats()
(10, 12, 15)

After a reload all our changes were forgotten. When we change properties like this, it only changes in memory, not in the database (nor do we modify the python module’s code). So when we reloaded, the ‘fresh’ Character class was loaded, and it still has the original stats we wrote in it.

In principle we could change the python code. But we don’t want to do that manually every time. And more importantly since we have the stats hardcoded in the class, every character instance in the game will have exactly the same str, dex and int now! This is clearly not what we want.

Evennia offers a special, persistent type of property for this, called an Attribute. Rework your mygame/typeclasses/characters.py like this:

class Character(DefaultCharacter):
    """
    (class docstring)
    """

    def get_stats(self):
        """
        Get the main stats of this character
        """
        return self.db.strength, self.db.dexterity, self.db.intelligence

Spaces in Attribute name?

What if you want spaces in your Attribute name? Or you want to assign the name of the Attribute on-the fly? Then you can use .attributes.add(name, value) instead, for example self.attributes.add("emotional intelligence", 10). You read it out again with self.attributes.get("emotional intelligence".

We removed the hard-coded stats and added added .db for every stat. The .db handler makes the stat into an an Evennia Attribute.

> reload 
> py self.get_stats()
(None, None, None) 

Since we removed the hard-coded values, Evennia don’t know what they should be (yet). So all we get back is None, which is a Python reserved word to represent nothing, a no-value. This is different from a normal python property:

> py me.strength
AttributeError: 'Character' object has no attribute 'strength'
> py me.db.strength
(nothing will be displayed, because it's None)

Trying to get an unknown normal Python property will give an error. Getting an unknown Evennia Attribute will never give an error, but only result in None being returned. This is often very practical.

Next, let us test out assigning those Attributes

> py me.db.strength, me.db.dexterity, me.db.intelligence = 10, 12, 15
> py me.get_stats()
(10, 12, 15)
> reload 
> py me.get_stats()
(10, 12, 15)

Now we set the Attributes to the right values, and they survive a server reload! Let’s modify the strength:

> py self.db.strength += 2 
> py self.get_stats()
(12, 12, 15)
> reload 
> py self.get_stats()
(12, 12, 15)

Also our change now survives a reload since Evennia automatically saves the Attribute to the database for us.

7.3.3. Setting things on new Characters

Things are looking better, but one thing remains strange - the stats start out with a value None and we have to manually set them to something reasonable. In a later lesson we will investigate character-creation in more detail. For now, let’s give every new character some random stats to start with.

We want those stats to be set only once, when the object is first created. For the Character, this method is called at_object_creation.

# up by the other imports
import random 

class Character(DefaultCharacter):
    """
    (class docstring)
    """

    def at_object_creation(self):       
        self.db.strength = random.randint(3, 18)
        self.db.dexterity = random.randint(3, 18)
        self.db.intelligence = random.randint(3, 18)
    
    def get_stats(self):
        """
        Get the main stats of this character
        """
        return self.db.strength, self.db.dexterity, self.db.intelligence

We imported a new module, random. This is part of Python’s standard library. We used random.randint to set a random value from 3 to 18 to each stat. Simple, but for some classical RPGs this is all you need!

> reload 
> py self.get_stats()
(12, 12, 15)

init vs at_object_creation

For the Monster class we used __init__ to set up the class. We can’t use this for a typeclass because it will be called more than once, at the very least after every reload and maybe more depending on caching. Even if you are familiar with Python, avoid touching __init__ for typeclasses, the results will not be what you expect.

Hm, this is the same values we set before. They are not random. The reason for this is of course that, as said, at_object_creation only runs once, the very first time a character is created. Our character object was already created long before, so it will not be called again.

It’s simple enough to run it manually though:

> self.at_object_creation()
> py self.get_stats()
(5, 4, 8)

Lady luck didn’t smile on us for this example; maybe you’ll fare better. Evennia has a helper command update that re-runs the creation hook and also cleans up any other Attributes not re-created by at_object_creation:

> update self
> py self.get_stats()
(8, 16, 14)

7.3.4. Updating all Characters in a loop

AttributeProperties

There is another way to define Attributes on a class, known as AttributeProperties. They can make it easier to maintain static default Attribute values on a typeclass. We will show them off when we make our game later in this tutorial series.

Needless to say, you are wise to have a feel for what you want to go into the at_object_creation hook before you create a lot of objects (characters in this case).

Luckily you only need to update objects once, and you don’t have to go around and re-run the at_object_creation method on everyone manually. For this we’ll try out a Python loop. Let’s go into multi-line Python mode:

> py
> for a in [1, 2, "foo"]:   
>     print(a)
1
2
foo

A python for-loop allows us to loop over something. Above, we made a list of two numbers and a string. In every iteration of the loop, the variable a becomes one element in turn, and we print that.

For our list, we want to loop over all Characters, and want to call .at_object_creation on each. This is how this is done (still in python multi-line mode):

> from typeclasses.characters import Character
> for char in Character.objects.all()
>     char.at_object_creation()

Database queries

Character.objects.all() is an example of a database query expressed in Python. This will be converted into a database query under the hood. This syntax is part of Django’s query language. You don’t need to know Django to use Evennia, but if you ever need more specific database queries, this is always available when you need it. We’ll get back to database queries in a later lesson.

We import the Character class and then we use .objects.all() to get all Character instances. Simplified, .objects is a resource from which one can query for all Characters. Using .all() gets us a listing of all of them that we then immediately loop over. Boom, we just updated all Characters, including ourselves:

> quit()
Closing the Python console.
> self.get_stats()
(3, 18, 10)

7.4. Extra Credits

This principle is the same for other typeclasses. So using the tools explored in this lesson, try to expand the default room with an is_dark flag. It can be either True or False. Have all new rooms start with is_dark = False and make it so that once you change it, it survives a reload. Oh, and if you created any other rooms before, make sure they get the new flag too!

7.5. Conclusions

In this lesson we created database-persistent dragons by having their classes inherit from one Object, one of Evennia’s typeclasses. We explored where Evennia looks for typeclasses if we don’t specify the path explicitly. We then modified ourselves - via the Character class - to give us some simple RPG stats. This led to the need to use Evennia’s Attributes, settable via .db and to use a for-loop to update ourselves.

Typeclasses are a fundamental part of Evennia and we will see a lot of more uses of them in the course of this tutorial. But that’s enough of them for now. It’s time to take some action. Let’s learn about Commands.