Attributes

In-game
> set obj/myattr = "test"
In-code, using the .db wrapper
obj.db.foo = [1, 2, 3, "bar"]
value = obj.db.foo
In-code, using the .attributes handler
obj.attributes.add("myattr", 1234, category="bar")
value = attributes.get("myattr", category="bar")
In-code, using AttributeProperty at class level
from evennia import DefaultObject
from evennia import AttributeProperty

class MyObject(DefaultObject):
    foo = AttributeProperty(default=[1, 2, 3, "bar"])
    myattr = AttributeProperty(100, category='bar')

Attributes allow you to to store arbitrary data on objects and make sure the data survives a server reboot. An Attribute can store pretty much any Python data structure and data type, like numbers, strings, lists, dicts etc. You can also store (references to) database objects like characters and rooms.

Working with Attributes

Attributes are usually handled in code. All Typeclassed entities (Accounts, Objects, Scripts and Channels) can (and usually do) have Attributes associated with them. There are three ways to manage Attributes, all of which can be mixed.

Using .db

The simplest way to get/set Attributes is to use the .db shortcut. This allows for setting and getting Attributes that lack a category (having category None)

import evennia

obj = evennia.create_object(key="Foo")

obj.db.foo1 = 1234
obj.db.foo2 = [1, 2, 3, 4]
obj.db.weapon = "sword"
obj.db.self_reference = obj   # stores a reference to the obj

# (let's assume a rose exists in-game)
rose = evennia.search_object(key="rose")[0]  # returns a list, grab 0th element
rose.db.has_thorns = True

# retrieving
val1 = obj.db.foo1
val2 = obj.db.foo2
weap = obj.db.weapon
myself = obj.db.self_reference  # retrieve reference from db, get object back

is_ouch = rose.db.has_thorns

# this will return None, not AttributeError!
not_found = obj.db.jiwjpowiwwerw

# returns all Attributes on the object
obj.db.all

# delete an Attribute
del obj.db.foo2

Trying to access a non-existing Attribute will never lead to an AttributeError. Instead you will get None back. The special .db.all will return a list of all Attributes on the object. You can replace this with your own Attribute all if you want, it will replace the default all functionality until you delete it again.

Using .attributes

If you want to group your Attribute in a category, or don’t know the name of the Attribute beforehand, you can make use of the AttributeHandler, available as .attributes on all typeclassed entities. With no extra keywords, this is identical to using the .db shortcut (.db is actually using the AttributeHandler internally):

is_ouch = rose.attributes.get("has_thorns")

obj.attributes.add("helmet", "Knight's helmet")
helmet = obj.attributes.get("helmet")

# you can give space-separated Attribute-names (can't do that with .db)
obj.attributes.add("my game log", "long text about ...")

By using a category you can separate same-named Attributes on the same object to help organization.

# store (let's say we have gold_necklace and ringmail_armor from before)
obj.attributes.add("neck", gold_necklace, category="clothing")
obj.attributes.add("neck", ringmail_armor, category="armor")

# retrieve later - we'll get back gold_necklace and ringmail_armor
neck_clothing = obj.attributes.get("neck", category="clothing")
neck_armor = obj.attributes.get("neck", category="armor")

If you don’t specify a category, the Attribute’s category will be None and can thus also be found via .db. None is considered a category of its own, so you won’t find None-category Attributes mixed with Attributes having categories.

Here are the methods of the AttributeHandler. See the AttributeHandler API for more details.

  • has(...) - this checks if the object has an Attribute with this key. This is equivalent to doing obj.db.attrname except you can also check for a specific `category.

  • get(...) - this retrieves the given Attribute. You can also provide a default value to return if the Attribute is not defined (instead of None). By supplying an accessing_object to the call one can also make sure to check permissions before modifying anything. The raise_exception kwarg allows you to raise an AttributeError instead of returning None when you access a non-existing Attribute. The strattr kwarg tells the system to store the Attribute as a raw string rather than to pickle it. While an optimization this should usually not be used unless the Attribute is used for some particular, limited purpose.

  • add(...) - this adds a new Attribute to the object. An optional lockstring can be supplied here to restrict future access and also the call itself may be checked against locks.

  • remove(...) - Remove the given Attribute. This can optionally be made to check for permission before performing the deletion. - clear(...) - removes all Attributes from object.

  • all(category=None) - returns all Attributes (of the given category) attached to this object.

Examples:

try:
  # raise error if Attribute foo does not exist
  val = obj.attributes.get("foo", raise_exception=True):
except AttributeError:
   # ...

# return default value if foo2 doesn't exist
val2 = obj.attributes.get("foo2", default=[1, 2, 3, "bar"])

# delete foo if it exists (will silently fail if unset, unless
# raise_exception is set)
obj.attributes.remove("foo")

# view all clothes on obj
all_clothes = obj.attributes.all(category="clothes")

Using AttributeProperty

The third way to set up an Attribute is to use an AttributeProperty. This is done on the class level of your typeclass and allows you to treat Attributes a bit like Django database Fields. Unlike using .db and .attributes, an AttributeProperty can’t be created on the fly, you must assign it in the class code.

# mygame/typeclasses/characters.py

from evennia import DefaultCharacter
from evennia.typeclasses.attributes import AttributeProperty

class Character(DefaultCharacter):

    strength = AttributeProperty(10, category='stat')
    constitution = AttributeProperty(11, category='stat')
    agility = AttributeProperty(12, category='stat')
    magic = AttributeProperty(13, category='stat')

    sleepy = AttributeProperty(False, autocreate=False)
    poisoned = AttributeProperty(False, autocreate=False)

    def at_object_creation(self):
      # ...

When a new instance of the class is created, new Attributes will be created with the value and category given.

With AttributeProperty’s set up like this, one can access the underlying Attribute like a regular property on the created object:

char = create_object(Character)

char.strength   # returns 10
char.agility = 15  # assign a new value (category remains 'stat')

char.db.magic  # returns None (wrong category)
char.attributes.get("agility", category="stat")  # returns 15

char.db.sleepy # returns None because autocreate=False (see below)

Warning

Be careful to not assign AttributeProperty’s to names of properties and methods already existing on the class, like ‘key’ or ‘at_object_creation’. That could lead to very confusing errors.

The autocreate=False (default is True) used for sleepy and poisoned is worth a closer explanation. When False, no Attribute will be auto-created for these AttributProperties unless they are explicitly set.

The advantage of not creating an Attribute is that the default value given to AttributeProperty is returned with no database access unless you change it. This also means that if you want to change the default later, all entities previously create will inherit the new default.

The drawback is that without a database precense you can’t find the Attribute via .db and .attributes.get (or by querying for it in other ways in the database):

char.sleepy   # returns False, no db access

char.db.sleepy   # returns None - no Attribute exists
char.attributes.get("sleepy")  # returns None too

char.sleepy = True  # now an Attribute is created
char.db.sleepy   # now returns True!
char.attributes.get("sleepy")  # now returns True

char.sleepy  # now returns True, involves db access

You can e.g. del char.strength to set the value back to the default (the value defined in the AttributeProperty).

See the AttributeProperty API for more details on how to create it with special options, like giving access-restrictions.

Properties of Attributes

An Attribute object is stored in the database. It has the following properties:

  • key - the name of the Attribute. When doing e.g. obj.db.attrname = value, this property is set to attrname.

  • value - this is the value of the Attribute. This value can be anything which can be pickled - objects, lists, numbers or what have you (see this section for more info). In the example obj.db.attrname = value, the value is stored here.

  • category - this is an optional property that is set to None for most Attributes. Setting this allows to use Attributes for different functionality. This is usually not needed unless you want to use Attributes for very different functionality (Nicks is an example of using Attributes in this way). To modify this property you need to use the Attribute Handler

  • strvalue - this is a separate value field that only accepts strings. This severely limits the data possible to store, but allows for easier database lookups. This property is usually not used except when re-using Attributes for some other purpose (Nicks use it). It is only accessible via the Attribute Handler.

There are also two special properties:

  • attrtype - this is used internally by Evennia to separate Nicks, from Attributes (Nicks use Attributes behind the scenes).

  • model - this is a natural-key describing the model this Attribute is attached to. This is on the form appname.modelclass, like objects.objectdb. It is used by the Attribute and NickHandler to quickly sort matches in the database. Neither this nor attrtype should normally need to be modified.

Non-database attributes are not stored in the database and have no equivalence to category nor strvalue, attrtype or model.

Managing Attributes in-game

Attributes are mainly used by code. But one can also allow the builder to use Attributes to ‘turn knobs’ in-game. For example a builder could want to manually tweak the “level” Attribute of an enemy NPC to lower its difficuly.

When setting Attributes this way, you are severely limited in what can be stored - this is because giving players (even builders) the ability to store arbitrary Python would be a severe security problem.

In game you can set an Attribute like this:

set myobj/foo = "bar"

To view, do

set myobj/foo

or see them together with all object-info with

examine myobj

The first set-example will store a new Attribute foo on the object myobj and give it the value “bar”. You can store numbers, booleans, strings, tuples, lists and dicts this way. But if you store a list/tuple/dict they must be proper Python structures and may only contain strings or numbers. If you try to insert an unsupported structure, the input will be converted to a string.

set myobj/mybool = True
set myobj/mybool = True
set myobj/mytuple = (1, 2, 3, "foo")
set myobj/mylist = ["foo", "bar", 2]
set myobj/mydict = {"a": 1, "b": 2, 3: 4}
set mypobj/mystring = [1, 2, foo]   # foo is invalid Python (no quotes)

For the last line you’ll get a warning and the value instead will be saved as a string "[1, 2, foo]".

Locking and checking Attributes

While the set command is limited to builders, individual Attributes are usually not locked down. You may want to lock certain sensitive Attributes, in particular for games where you allow player building. You can add such limitations by adding a lock string to your Attribute. A NAttribute have no locks.

The relevant lock types are

  • attrread - limits who may read the value of the Attribute

  • attredit - limits who may set/change this Attribute

You must use the AttributeHandler to assign the lockstring to the Attribute:

lockstring = "attread:all();attredit:perm(Admins)"
obj.attributes.add("myattr", "bar", lockstring=lockstring)"

If you already have an Attribute and want to add a lock in-place you can do so by having the AttributeHandler return the Attribute object itself (rather than its value) and then assign the lock to it directly:

     lockstring = "attread:all();attredit:perm(Admins)"
     obj.attributes.get("myattr", return_obj=True).locks.add(lockstring)

Note the return_obj keyword which makes sure to return the Attribute object so its LockHandler could be accessed.

A lock is no good if nothing checks it – and by default Evennia does not check locks on Attributes. To check the lockstring you provided, make sure you include accessing_obj and set default_access=False as you make a get call.

    # in some command code where we want to limit
    # setting of a given attribute name on an object
    attr = obj.attributes.get(attrname,
                              return_obj=True,
                              accessing_obj=caller,
                              default=None,
                              default_access=False)
    if not attr:
        caller.msg("You cannot edit that Attribute!")
        return
    # edit the Attribute here

The same keywords are available to use with obj.attributes.set() and obj.attributes.remove(), those will check for the attredit lock type.

What types of data can I save in an Attribute?

The database doesn’t know anything about Python objects, so Evennia must serialize Attribute values into a string representation before storing it to the database. This is done using the pickle module of Python.

The only exception is if you use the strattr keyword of the AttributeHandler to save to the strvalue field of the Attribute. In that case you can only save strings and those will not be pickled).

Storing single objects

With a single object, we mean anything that is not iterable, like numbers, strings or custom class instances without the __iter__ method.

  • You can generally store any non-iterable Python entity that can be pickled.

  • Single database objects/typeclasses can be stored, despite them normally not being possible to pickle. Evennia will convert them to an internal representation using theihr classname, database-id and creation-date with a microsecond precision. When retrieving, the object instance will be re-fetched from the database using this information.

  • If you ‘hide’ a db-obj as a property on a custom class, Evennia will not be able to find it to serialize it. For that you need to help it out (see below).

Valid assignments
# Examples of valid single-value  attribute data:
obj.db.test1 = 23
obj.db.test1 = False
# a database object (will be stored as an internal representation)
obj.db.test2 = myobj

As mentioned, Evennia will not be able to automatically serialize db-objects ‘hidden’ in arbitrary properties on an object. This will lead to an error when saving the Attribute.

Invalid, ‘hidden’ dbobject
# example of storing an invalid, "hidden" dbobject in Attribute
class Container:
    def __init__(self, mydbobj):
        # no way for Evennia to know this is a database object!
        self.mydbobj = mydbobj

# let's assume myobj is a db-object
container = Container(myobj)
obj.db.mydata = container  # will raise error!

By adding two methods __serialize_dbobjs__ and __deserialize_dbobjs__ to the object you want to save, you can pre-serialize and post-deserialize all ‘hidden’ objects before Evennia’s main serializer gets to work. Inside these methods, use Evennia’s evennia.utils.dbserialize.dbserialize and dbunserialize functions to safely serialize the db-objects you want to store.

Fixing an invalid ‘hidden’ dbobj for storing in Attribute
from evennia.utils import dbserialize  # important

class Container:
    def __init__(self, mydbobj):
        # A 'hidden' db-object
        self.mydbobj = mydbobj

    def __serialize_dbobjs__(self):
        """This is called before serialization and allows
        us to custom-handle those 'hidden' dbobjs"""
        self.mydbobj = dbserialize.dbserialize(self.mydbobj

    def __deserialize_dbobjs__(self):
        """This is called after deserialization and allows you to
        restore the 'hidden' dbobjs you serialized before"""
        if isinstance(self.mydbobj, bytes):
            # make sure to check if it's bytes before trying dbunserialize
            self.mydbobj = dbserialize.dbunserialize(self.mydbobj)

# let's assume myobj is a db-object
container = Container(myobj)
obj.db.mydata = container  # will now work fine!

Note the extra check in __deserialize_dbobjs__ to make sure the thing you are deserializing is a bytes object. This is needed because the Attribute’s cache reruns deserializations in some situations when the data was already once deserialized. If you see errors in the log saying Could not unpickle data for storage: ..., the reason is likely that you forgot to add this check.

Storing multiple objects

This means storing objects in a collection of some kind and are examples of iterables, pickle-able entities you can loop over in a for-loop. Attribute-saving supports the following iterables:

  • Tuples, like (1,2,"test", <dbobj>).

  • Lists, like [1,2,"test", <dbobj>].

  • Dicts, like {1:2, "test":<dbobj>].

  • Sets, like {1,2,"test",<dbobj>}.

  • collections.OrderedDict, like OrderedDict((1,2), ("test", <dbobj>)).

  • collections.Deque, like deque((1,2,"test",<dbobj>)).

  • collections.DefaultDict like defaultdict(list).

  • Nestings of any combinations of the above, like lists in dicts or an OrderedDict of tuples, each containing dicts, etc.

  • All other iterables (i.e. entities with the __iter__ method) will be converted to a list. Since you can use any combination of the above iterables, this is generally not much of a limitation.

Any entity listed in the Single object section above can be stored in the iterable.

As mentioned in the previous section, database entities (aka typeclasses) are not possible to pickle. So when storing an iterable, Evennia must recursively traverse the iterable and all its nested sub-iterables in order to find eventual database objects to convert. This is a very fast process but for efficiency you may want to avoid too deeply nested structures if you can.

# examples of valid iterables to store
obj.db.test3 = [obj1, 45, obj2, 67]
# a dictionary
obj.db.test4 = {'str':34, 'dex':56, 'agi':22, 'int':77}
# a mixed dictionary/list
obj.db.test5 = {'members': [obj1,obj2,obj3], 'enemies':[obj4,obj5]}
# a tuple with a list in it
obj.db.test6 = (1, 3, 4, 8, ["test", "test2"], 9)
# a set
obj.db.test7 = set([1, 2, 3, 4, 5])
# in-situ manipulation
obj.db.test8 = [1, 2, {"test":1}]
obj.db.test8[0] = 4
obj.db.test8[2]["test"] = 5
# test8 is now [4,2,{"test":5}]

Note that if make some advanced iterable object, and store an db-object on it in a way such that it is not returned by iterating over it, you have created a ‘hidden’ db-object. See the previous section for how to tell Evennia how to serialize such hidden objects safely.

Retrieving Mutable objects

A side effect of the way Evennia stores Attributes is that mutable iterables (iterables that can be modified in-place after they were created, which is everything except tuples) are handled by custom objects called _SaverList, _SaverDict etc. These _Saver... classes behave just like the normal variant except that they are aware of the database and saves to it whenever new data gets assigned to them. This is what allows you to do things like self.db.mylist[7] = val and be sure that the new version of list is saved. Without this you would have to load the list into a temporary variable, change it and then re-assign it to the Attribute in order for it to save.

There is however an important thing to remember. If you retrieve your mutable iterable into another variable, e.g. mylist2 = obj.db.mylist, your new variable (mylist2) will still be a _SaverList. This means it will continue to save itself to the database whenever it is updated!

obj.db.mylist = [1, 2, 3, 4]
mylist = obj.db.mylist

mylist[3] = 5  # this will also update database

print(mylist)  # this is now [1, 2, 3, 5]
print(obj.db.mylist)  # now also [1, 2, 3, 5]

When you extract your mutable Attribute data into a variable like mylist, think of it as getting a snapshot of the variable. If you update the snapshot, it will save to the database, but this change will not propagate to any other snapshots you may have done previously.

obj.db.mylist = [1, 2, 3, 4]
mylist1 = obj.db.mylist
mylist2 = obj.db.mylist
mylist1[3] = 5

print(mylist1)  # this is now [1, 2, 3, 5]
print(obj.db.mylist)  # also updated to [1, 2, 3, 5]

print(mylist2)  # still [1, 2, 3, 4]  !

To avoid confusion with mutable Attributes, only work with one variable (snapshot) at a time and save back the results as needed.

You can also choose to “disconnect” the Attribute entirely from the database with the help of the .deserialize() method:

obj.db.mylist = [1, 2, 3, 4, {1: 2}]
mylist = obj.db.mylist.deserialize()

The result of this operation will be a structure only consisting of normal Python mutables (list instead of _SaverList, dict instead of _SaverDict and so on). If you update it, you need to explicitly save it back to the Attribute for it to save.

In-memory Attributes (NAttributes)

NAttributes (short of Non-database Attributes) mimic Attributes in most things except they are non-persistent - they will not survive a server reload.

  • Instead of .db use .ndb.

  • Instead of .attributes use .nattributes

  • Instead of AttributeProperty, use NAttributeProperty.

    rose.ndb.has_thorns = True
    is_ouch = rose.ndb.has_thorns

    rose.nattributes.add("has_thorns", True)
    is_ouch = rose.nattributes.get("has_thorns")

Differences between Attributes and NAttributes:

  • NAttributes are always wiped on a server reload.

  • They only exist in memory and never involve the database at all, making them faster to access and edit than Attributes.

  • NAttributes can store any Python structure (and database object) without limit. However, if you were to delete a database object you previously stored in an NAttribute, the NAttribute will not know about this and may give you a python object without a matching database entry. In comparison, an Attribute always checks this). If this is a concern, use an Attribute or check that the object’s .pk property is not None before saving it.

  • They can not be set with the standard set command (but they are visible with examine)

There are some important reasons we recommend using ndb to store temporary data rather than the simple alternative of just storing a variable directly on an object:

  • NAttributes are tracked by Evennia and will not be purged in various cache-cleanup operations the server may do. So using them guarantees that they’ll remain available at least as long as the server lives.

  • It’s a consistent style - .db/.attributes and .ndb/.nattributes makes for clean-looking code where it’s clear how long-lived (or not) your data is to be.

Persistent vs non-persistent

So persistent data means that your data will survive a server reboot, whereas with non-persistent data it will not …

… So why would you ever want to use non-persistent data? The answer is, you don’t have to. Most of the time you really want to save as much as you possibly can. Non-persistent data is potentially useful in a few situations though.

  • You are worried about database performance. Since Evennia caches Attributes very aggressively, this is not an issue unless you are reading and writing to your Attribute very often (like many times per second). Reading from an already cached Attribute is as fast as reading any Python property. But even then this is not likely something to worry about: Apart from Evennia’s own caching, modern database systems themselves also cache data very efficiently for speed. Our default database even runs completely in RAM if possible, alleviating much of the need to write to disk during heavy loads.

  • A more valid reason for using non-persistent data is if you want to lose your state when logging off. Maybe you are storing throw-away data that are re-initialized at server startup. Maybe you are implementing some caching of your own. Or maybe you are testing a buggy Script that does potentially harmful stuff to your character object. With non-persistent storage you can be sure that whatever is messed up, it’s nothing a server reboot can’t clear up.

  • NAttributes have no restrictions at all on what they can store, since they don’t need to worry about being saved to the database - they work very well for temporary storage.

  • You want to implement a fully or partly non-persistent world. Who are we to argue with your grand vision!