When performing actions in Evennia it is often important that you store data for later. If you write a menu system, you have to keep track of the current location in the menu tree so that the player can give correct subsequent commands. If you are writing a combat system, you might have a combattant’s next roll get easier dependent on if their opponent failed. Your characters will probably need to store roleplaying-attributes like strength and agility. And so on.

Typeclassed game entities (Players, Objects, Scripts and Channels) always have Attributes associated with them. Attributes are used to store any type of data ‘on’ such entities. This is different from storing data in properties already defined on entities (such as key or location) - these have very specific names and require very specific types of data (for example you couldn’t assign a python list to the key property no matter how hard you tried). Attributes come into play when you want to assign arbitrary data to arbitrary names.

The .db and .ndb shortcuts

To save persistent data on a Typeclassed object you normally use the db (DataBase) operator. Let’s try to save some data to a Rose (an Object):

# saving
rose.db.has_thorns = True
# getting it back
is_ouch = rose.db.has_thorns

This looks like any normal Python assignment, but that db makes sure that an Attribute is created behind the scenes and is stored in the database. Your rose will continue to have thorns throughout the life of the server now, until you deliberately remove them.

To be sure to save non-persistently, i.e. to make sure NOT to create a database entry, you use ndb (NonDataBase). It works in the same way:

# saving
rose.ndb.has_thorns = True
# getting it back
is_ouch = rose.ndb.has_thorns

Technically, ndb has nothing to do with Attributes, despite how similar they look. No Attribute object is created behind the scenes when using ndb. In fact the database is not invoked at all since we are not interested in persistence. There is however an important reason to use ndb to store data rather than to just store variables direct on entities - ndb-stored data is tracked by the server and will not be purged in various cache-cleanup operations Evennia may do while it runs. Data stored on ndb (as well as db) will also be easily listed by example the @examine command.

You can also del properties on db and ndb as normal. This will for example delete an Attribute:

del rose.db.has_thorns

Both db and ndb defaults to offering an all() method on themselves. This returns all associated attributes or non-persistent properties.

list_of_all_rose_attributes = rose.db.all()
list_of_all_rose_ndb_attrs = rose.ndb.all()

If you use all as the name of an attribute, this will be used instead. Later deleting your custom all will return the default behaviour.

The AttributeHandler

The .db and .ndb properties are very convenient but if you don’t know the name of the Attribute beforehand they cannot be used. Behind the scenes .db actually accesses the AttributeHandler which sits on typeclassed entities as the .attributes property. .ndb does the same for the .nattributes property.

The handlers have normal access methods that allow you to manage and retrieve Attributes and NAttributes:

  • has(...) - this checks if the object has an Attribute with this key. This is equivalent to doing obj.db.key.
  • get(...) - this retrieves the given Attribute. Normally the value property of the Attribute is returned, but the method takes keywords for returning the Attribute object itself. By supplying an accessing_object to the call one can also make sure to check permissions before modifying anything.
  • 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(...) - returns all Attributes (of the given category) attached to this object.

See this section for more about locking down Attribute access and editing. The Nattribute offers no concept of access control.

Some examples:

import evennia
obj = evennia.search_object("MyObject")

obj.attributes.add("test", "testvalue")
print obj.db.test                 # prints "testvalue"
print obj.attributes.get("test")  #       "
print obj.attributes.all()        # prints [<AttributeObject>]

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 have no equivalence to category nor strvalue, attrtype or model.

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 (see next section), 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!

What types of data can I save in an Attribute?

None of the following affects NAttributes, which does not invoke the database at all. There are no restrictions to what can be stored in a NAttribute.

The database doesn’t know anything about Python objects, so Evennia must serialize Attribute values into a string representation in order to store 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 which 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 as any other in the Attribute. These can normally not be pickled, but Evennia will behind the scenes convert them to an internal representation using their classname, database-id and creation-date with a microsecond precision, guaranteeing you get the same object back when you access the Attribute later.
  • If you hide a database object inside a non-iterable custom class (like stored as a variable inside it), Evennia will not know it’s there and won’t convert it safely. Storing classes with such hidden database objects is not supported and will lead to errors!
# 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

# example of an invalid, "hidden" dbobject
class Invalid(object):
    def __init__(self, dbobj):
        # no way for Evennia to know this is a dbobj
        self.dbobj = dbobj
invalid = Invalid(myobj)
obj.db.invalid = invalid # 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.OrdereredDict, like OrderendDict((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 will still be stored and returned as a list [1,2,3,4,5]!
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! While this can be convenient in some cases, it is important to keep in mind so you are not confused by the results.

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. In the case of “nested” structure, you only have to convert the “outermost” _Saver.. entity in order to cut the entire structure’s connection to the database.

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]

Remember, this is only valid for mutable iterables. Immutable objects (strings, numbers, tuples etc) are already disconnected from the database from the onset. So making the outermost iterable into a tuple is another way to stop any changes to the structure from updating the database.

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.

Locking and checking Attributes

Attributes are normally not locked down by default, but you can easily change that for individual Attributes (like those that may be game-sensitive in games with user-level building).

First you need to set a lock string on your Attribute. Lock strings are specified 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 cannot use the db handler to modify Attribute object (such as setting a lock on them) - The db handler will return the Attribute’s value, not the Attribute object itself. Instead you use the AttributeHandler and set it to return the object instead of the value:

lockstring = "attread:all();attredit:perm(Wizards)"
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. You have to add a check to your commands/code wherever it fits (such as before setting an Attribute).

# in some command code where we want to limit
# setting of a given attribute name on an object
attr = obj.attributes.get(attrname,
if not attr:
    caller.msg("You cannot edit that Attribute!")
# 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.