evennia/docs/source/Components/Attributes.md

Attributes

:caption: In-game
> set obj/myattr = "test"

:caption: In-code
obj.db.foo = [1,2,3, "bar"]
value = obj.db.foo

obj.attributes.add("myattr", 1234, category="bar")
value = attributes.get("myattr", 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.

• What can be stored in an Attribute is a must-read also for experienced developers, to avoid getting surprised. Attributes can store almost everything but you need to know the quirks.
• NAttributes are the in-memory, non-persistent siblings of Attributes.
• Managing Attributes In-game for in-game builder commands.

Managing Attributes in Code

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

• Using the .db property shortcut
• Using the .attributes manager (AttributeManager)
• Using AttributeProperty for assigning Attributes in a way similar to Django fields

Using .db

The simplest way to get/set Attributes is to use the .db shortcut:

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 don't know the name of the Attribute beforehand you can also use the AttributeHandler, available as .attributes. 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 ...")

With the AttributeHandler you can also give Attributes a category. By using a category you can separate same-named Attributes on the same object which can 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. Note that None is also considered a category of its own, so you won't find None-category Attributes mixed with Attributes having categories.

When using .db, you will always use the None category.

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

There is a third way to set up an Attribute, and that is by setting up an AttributeProperty. This is done on the class level of your typeclass and allows you to treat Attributes a bit like Django database Fields.

# mygame/typeclasses/characters.py

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

class Character(DefaultCharacter):

    strength = AttributeProperty(default=10, category='stat', autocreate=True)
    constitution = AttributeProperty(default=10, category='stat', autocreate=True)
    agility = AttributeProperty(default=10, category='stat', autocreate=True)
    magic = AttributeProperty(default=10, category='stat', autocreate=True)
    
    sleepy = AttributeProperty(default=False)
    poisoned = AttributeProperty(default=False)
    
    def at_object_creation(self): 
      # ... 

These "Attribute-properties" will be made available to all instances of the class.

If you change the `default` of an `AttributeProperty` (and reload), it will 
change the default for _all_ instances of that class (it will not override 
explicitly changed values).

char = evennia.search_object(Character, key="Bob")[0]  # returns list, get 0th element

# get defaults 
strength = char.strength   # will get the default value 10

# assign new values (this will create/update new Attributes)
char.strength = 12
char.constitution = 16
char.agility = 8
char.magic = 2

# you can also do arithmetic etc 
char.magic += 2   # char.magic is now 4

# check Attributes 
strength = char.strength   # this is now 12
is_sleepy = char.sleepy 
is_poisoned = char.poisoned

del char.strength   # wipes the Attribute
strength  = char.strengh  # back to the default (10) again

See the AttributeProperty docs for more details on arguments.

An AttributeProperty will not create an Attribute by default. A new Attribute will be created (or an existing one retrieved/updated) will happen differently depending on how the autocreate keyword:

• If autocreate=False (default), an Attribute will be created only if the field is explicitly assigned a value (even if the value is the same as the default, such as char.strength = 10).
• If autocreate=True, an Attribute will be created as soon as the field is accessed in any way (So both strength = char.strength and char.strength = 10 will both make sure that an Attribute exists.

Example:

# in mygame/typeclasses/objects.py 

from evennia import create_object 
from evennia import DefaultObject
from evennia.typeclasses.attributes import AttributeProperty

class Object(DefaultObject):
  
    value_a = AttributeProperty(default="foo")
    value_b = AttributeProperty(default="bar", autocreate=True)
    
obj = evennia.create_object(key="Dummy")

# these will find NO Attributes! 
obj.db.value_a 
obj.attributes.get("value_a")
obj.db.value_b 
obj.attributes.get("value_b")

# get data from attribute-properties
vala = obj.value_a  # returns "foo"
valb = obj.value_b  # return "bar" AND creates the Attribute (autocreate)

# the autocreate property will now be found 
obj.db.value_a                      # still not found 
obj.attributes.get("value_a")       #       ''
obj.db.value_b                      # now returns "bar" 
obj.attributes.get("value_b")       #       ''

# assign new values 
obj.value_a = 10   # will now create a new Attribute 
obj.value_b = 12   # will update the existing Attribute 

# both are now found as Attributes 
obj.db.value_a                      # now returns 10
obj.attributes.get("value_a")       #       ''
obj.db.value_b                      # now returns 12
obj.attributes.get("value_b")       #       ''

If you always access your Attributes via the AttributeProperty this does not matter that much (it's also a bit of an optimization to not create an actual database Attribute unless the value changed). But until an Attribute has been created, AttributeProperty fields will not show up with the examine command or by using the .db or .attributes handlers - so this is a bit inconsistent. If this is important, you need to 'initialize' them by accessing them at least once ... something like this:

# ... 
class Character(DefaultCharacter):

    strength = AttributeProperty(12, autocreate=True)
    agility = AttributeProperty(12, autocreate=True)


    def at_object_creation(self):
        # initializing 
        self.strength   # by accessing it, the Attribute is auto-created
        self.agility    #             ''

If you created your `AttributeProperty` with a `category`, you *must* specify the 
category in `.attributes.get()` if you want to find it this way. Remember that 
`.db` always uses a `category` of `None`.

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 wil convert them to an internal representation using their 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.
• To convert the database object, Evennia must know it's there. If you hide a database object inside a non-iterable class, you will run into errors - this is not supported!

:caption: 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

:caption: Invalid, 'hidden' dbobject

# example of an invalid, "hidden" dbobject
class Container:
    def __init__(self, mydbobj):
        # no way for Evennia to know this is a database object!
        self.mydbobj = mydbobj
container = Container(myobj)
obj.db.invalid = container  # will cause error!

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>)).
• 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}]

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) # this is also [1,2,3,5]

To "disconnect" your extracted mutable variable from the database you simply need to convert the _Saver... iterable to a normal Python structure. So to convert a _SaverList, you use the list() function, for a _SaverDict you use dict() and so on.

     obj.db.mylist = [1,2,3,4]
     mylist = list(obj.db.mylist) # convert to normal list
     mylist[3] = 5
     print(mylist) # this is now [1,2,3,5]
     print(obj.db.mylist) # this is still [1,2,3,4]

A further problem comes with nested mutables, like a dict containing lists of dicts or something like that. Each of these nested mutables would be _Saver* structures connected to the database and disconnecting the outermost one of them would not disconnect those nested within. To make really sure you disonnect a nested structure entirely from the database, Evennia provides a special function evennia.utils.dbserialize.deserialize:

from evennia.utils.dbserialize import deserialize

decoupled_mutables = deserialize(nested_mutables)

The result of this operation will be a structure only consisting of normal Python mutables (list instead of _SaverList and so on).

Remember, this is only valid for mutable iterables. Immutable objects (strings, numbers, tuples etc) are already disconnected from the database from the onset.

     obj.db.mytup = (1,2,[3,4])
     obj.db.mytup[0] = 5 # this fails since tuples are immutable

     # this works but will NOT update database since outermost is a tuple
     obj.db.mytup[2][1] = 5
     print(obj.db.mytup[2][1]) # this still returns 4, not 5

     mytup1 = obj.db.mytup # mytup1 is already disconnected from database since outermost
                           # iterable is a tuple, so we can edit the internal list as we want
                           # without affecting the database.

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.

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.
• 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!