# 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 Object(DefaultObject):
        """
        class docstring
        """
        pass

So we have a class `Object` that inherits from `DefaultObject`, which we have imported from Evennia. The class itself doesn’t do anything (it just `pass`es) 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. If we knew what kind of methods and resources were available on `DefaultObject` we could add our own and change the way it works!

> Hint: We will get back to this, but to learn what resources an Evennia parent like `DefaultObject` offers, easiest is to peek at its API documentation. The docstring for the `Object` class can also help.

One thing that Evennia classes offers and which you don’t get with vanilla Python classes is persistence. As you’ve found, Fluffy, Cuddly and Smaug are gone once we reload the server. Let’s see if we can fix this.

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:

    > 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.

    > 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 (remember that `here` is just something available in the `py` command, you can’t use it elsewhere).

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`:

    > py fluffy = evennia.create_object('typeclases.monster.Monster', key="Fluffy", location=here)
    > look

Boom, Fluffy 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. Evennia sets things up and saves for you.

If you want to find Fluffy from anywhere, you can use Evennia’s `search_object` helper:

    > fluffy = evennia.search_object("Fluffy")[0] ; fluffy.move_around()
    Fluffy 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 Fluffies 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 Cuddly:typeclasses.monsters.Monster

Cuddly 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 akin to

    evennia.create_object("typeclasses.monsters.Monster", 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.DefaultObject`: This is the parent of all in-game entities - everything with a location. Evennia makes a few very useful child classes of this class:

  - `evennia.DefaultCharacter`: The default entity represening a player avatar in-game.

  - `evennia.DefaultRoom`: A location in the game world.

  - `evennia.DefaultExit`: A link between locations.

- `evennia.DefaultAccount`: The OOC representation of a player, holds password and account info.

- `evennia.DefaultChannel`: In-game channels. These could be used for all sorts of in-game communication.

- `evennia.DefaultScript`: Out-of-game objects, with no presence in the game world. Anything you want to create that needs to be persistent can be stored with these entities, such as combat state, economic systems or what have you.

If you take a look in `mygame/typeclasses/` you’ll find modules for each of these. Each contains an empty child class ready that already inherits from the right parent, ready for you to modify or build from:

- `mygame/typeclasses/objects.py` has `class Object(DefaultObject)`, a class directly inheriting the basic in-game entity, this works as a base for any object.

- `mygame/typeclasses/characters.py` has `class Character(DefaultCharacter)`

- `mygame/typeclasses/rooms.py` has `class Room(DefaultRoom)`

- `mygame/typeclasses/exits.py` has `class Exit(DefaultExit)`

- `mygame/typeclasses/accounts.py` has `class Account(DefaultAccount)`

- `mygame/typeclasses/channels.py` has `class Channel(DefaultChannel)`

- `mygame/typeclasses/scripts.py` has `class Script(DefaultScript)`

> 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 and defaults

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` is shown.

- 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).

- 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.

- 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.

What happens if we don’t specify the 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`.

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. 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
    ------------------------------------------------------------------------------

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)
        """

        str = 10
        dex = 12
        int = 15

        def get_stats(self):
            """
            Get the main stats of this character
            """
            return self.str, self.dex, self.int

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

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.str = self.str + 1
    > py self.str
    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.str += 1
    > py self.str
    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 to 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.str, self.db.dex, self.db.int

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 self.str
    AttributeError: 'Character' object has no attribute 'str'
    > py self.db.str
    (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.

    > py self.db.str, self.db.dex, self.db.int = 10, 12, 15
    > py self.get_stats()
    (10, 12, 15)
    > reload
    > py self.get_stats()
    (10, 12, 15)

Now we set the Attributes to the right values. We can see that things work the same as before, also after a server reload. Let’s modify the strength:

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

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 a 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.str = random.randint(3, 18)
            self.db.dex = random.randint(3, 18)
            self.db.int = random.randint(3, 18)

        def get_stats(self):
            """
            Get the main stats of this character
            """
            return self.db.str, self.db.dex, self.db.int

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)

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

Needless to say, for your game 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). But should it come to that you don’t want to have to go around and re-run the method on everyone manually. For the Python beginner, doing this will also give a chance to try out Python loops. We try them out in 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()

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.