Tags

A common task of a game designer is to organize and find groups of objects and do operations on them. A classic example is to have a weather script affect all “outside” rooms. Another would be for a player casting a magic spell that affects every location “in the dungeon”, but not those “outside”. Another would be to quickly find everyone joined with a particular guild or everyone currently dead.

Tags are short text labels that you attach to objects so as to easily be able to retrieve and group them. An Evennia entity can be tagged with any number of Tags. On the database side, Tag entities are shared between all objects with that tag. This makes them very efficient but also fundamentally different from Attributes, each of which always belongs to one single object.

In Evennia, Tags are technically also used to implement Aliases (alternative names for objects) and Permissions (simple strings for Locks to check for).

Properties of Tags (and Aliases and Permissions)

Tags are unique. This means that there is only ever one Tag object with a given key and category. When Tags are assigned to game entities, these entities are actually sharing the same Tag. This means that Tags are not suitable for storing information about a single object - use an Attribute for this instead. Tags are a lot more limited than Attributes but this also makes them very quick to lookup in the database - this is the whole point.

Tags have the following properties, stored in the database:

  • key - the name of the Tag. This is the main property to search for when looking up a Tag.
  • category - this category allows for retrieving only specific subsets of tags used for different purposes. You could have one category of tags for “zones”, another for “outdoor locations”, for example.
  • data - this is an optional text field with information about the tag. Remember that Tags are shared between entities, so this field cannot hold any object-specific information. Usually it would be used to hold info about the group of entities the Tag is tagging - possibly used for contextual help like a tool tip. It is not used by default.

There are also two special properties. These should usually not need to be changed or set, it is used internally by Evennia to implement various other uses it makes of the Tag object:

  • model - this holds a natural-key description of the model object that this tag deals with, on the form application.modelclass, for example objects.objectdb. It used by the TagHandler of each entity type for correctly storing the data behind the scenes.
  • tagtype - this is a “top-level category” of sorts for the inbuilt children of Tags, namely Aliases and Permissions. The Taghandlers using this special field are especially intended to free up the category property for any use you desire.

Using Tags

You can tag any typeclassed object, namely Objects, Players, Scripts and Channels. General tags are added by the Taghandler. The tag handler is accessed as a property tags on the relevant entity:

mychair.tags.add("furniture")
myroom.tags.add("dungeon#01")
myscript.tags.add("weather", category="climate")
myplayer.tags.add("guestaccount")

mychair.tags.all()  # returns a list of Tags
mychair.tags.remove("furniture")
mychair.tags.clear()

Adding a new tag will either create a new Tag or re-use an already existing one. When using remove, the Tag is not deleted but are just disconnected from the tagged object. This makes for very quick operations. The clear method removes (disconnects) all Tags from the object. You can also use the default @tag command:

@tag mychair = furniture

A main use of Tag is to use it to search for tagged database entities. You can retrieve all objects with a given Tag like this in code:

import evennia

# search for objects (most common)
objs = evennia.search_tag("furniture")
dungeon = evennia.search_tag("dungeon#01")

# search for scripts
weather = evennia.search_tag_script("weather")
climates = evennia.search_tag_script(category="climate")

# search for players
players = evennia.search_tag_player("guestaccount")

There is also an in-game command that deals with assigning and using (Object-) tags:

@tag/search furniture

Using Aliases and Permissions

Aliases and Permissions are implemented using normal TagHandlers that simply save Tags with a different tagtype. These handlers are named aliases and permissions on all Objects. They are used in the same way as Tags above:

boy.aliases.add("rascal")
boy.permissions.add("Builders")
boy.permissions.remove("Builders")

all_aliases = boy.aliases.all()

and so on. Similarly to how @tag works in-game, there is also the @perm command for assigning permissions and @alias command for aliases.

Using Tag categories

Generally, tags are enough on their own for grouping objects. Having no tag category is perfectly fine and the normal operation. Simply adding a new Tag for grouping objects is often better than making a new category. So think hard before deciding you really need to categorize your Tags.

That said, tag categories can be useful if you build some game system that uses tags. You can then use tag categories to make sure to separate tags created with this system from any other tags created elsewhere. You can then supply custom search methods that only find objects tagged with tags of that category. An example of this
is found in the Zone tutorial.