Locks

For most games it is a good idea to restrict what people can do. In Evennia such restrictions are applied and checked by something called locks. All Evennia entities (Commands, Objects, Scripts, Players, Help System, messages and channels) are accessed through locks.

A lock can be thought of as an “access rule” restricting a particular use of an Evennia entity. Whenever another entity wants that kind of access the lock will analyze that entity in different ways to determine if access should be granted or not. Evennia implements a “lockdown” philosophy - all entities are inaccessible unless you explicitly define a lock that allows some or full access.

Let’s take an example: An object has a lock on itself that restricts how people may “delete” that object. Apart from knowing that it restricts deletion, the lock also knows that only players with the specific ID of, say, 34 are allowed to delete it. So whenever a player tries to run @delete on the object, the @delete command makes sure to check if this player is really allowed to do so. It calls the lock, which in turn checks if the player’s id is 34. Only then will it allow @delete to go on with its job.

Setting and checking a lock

The in-game command for setting locks on objects is @lock:

> @lock obj = <lockstring>

The <lockstring> is a string of a certain form that defines the behaviour of the lock. We will go into more detail on how <lockstring> should look in the next section.

Code-wise, Evennia handles locks through what is usually called locks on all relevant entities. This is a handler that allows you to add, delete and check locks.

myobj.locks.add(<lockstring>)

One can call locks.check() to perform a lock check, but to hide the underlying implementation all objects also have a convenience function called access. This should preferably be used. In the example below, accessing_obj is the object requesting the ‘delete’ access whereas obj is the object that might get deleted. This is how it would (and do) look from inside the @delete command:

if not obj.access(accessing_obj, 'delete'):
    accessing_obj.msg("Sorry, you may not delete that.")
    return

Defining locks

Defining a lock (i.e. an access restriction) in Evennia is done by adding simple strings of lock definitions to the object’s locks property using obj.locks.add().

Here are some examples of lock strings (not including the quotes):

delete:id(34)   # only allow obj #34 to delete
edit:all()      # let everyone edit
# only those who are not "very_weak" or are Wizards may pick this up
get: not attr(very_weak) or perm(Wizards)

Formally, a lockstring has the following syntax:

access_type: [NOT] lockfunc1([arg1,..]) [AND|OR] [NOT] lockfunc2([arg1,...]) [...]

where [] marks optional parts. AND, OR and NOT are not case sensitive and excess spaces are ignored. lockfunc1, lockfunc2 etc are special lock functions available to the lock system.

So, a lockstring consists of the type of restriction (the access_type), a colon (:) and then an expression involving function calls that determine what is needed to pass the lock. Each function returns either True or False. AND, OR and NOT work as they do normally in Python. If the total result is True, the lock is passed.

You can create several lock types one after the other by separating them with a semicolon (;) in the lockstring. The string below yields the same result as the previous example:

delete:id(34);edit:all();get: not attr(very_weak) or perm(Wizard)

Valid access_types

An access_type, the first part of a lockstring, defines what kind of capability a lock controls, such as “delete” or “edit”. You may in principle name your access_type anything as long as it is unique for the particular object. The name of the access types is not case-sensitive.

If you want to make sure the lock is used however, you should pick access_type names that you (or the default command set) actually checks for, as in the example of @delete above that uses the ‘delete’ access_type.

Below are the access_types checked by the default commandset.

  • Commands
  • cmd - this defines who may call this command at all.
  • Objects:
  • control - who is the “owner” of the object. Can set locks, delete it etc. Defaults to the creator of the object.
  • call - who may call object-commands on this object.
  • examine - who may examine this object’s properties.
  • delete - who may delete the object.
  • edit - who may edit properties and attributes of the object.
  • view - if the look command will display/list this object
  • get- who may pick up the object and carry it around.
  • puppet - who may “become” this object and control it as their “character”.
  • attrcreate - who may create new attributes on the object (default True)
  • Characters:
  • Same as for Objects
  • Exits:
  • Same as for Objects
  • traverse - who may pass the exit.
  • Players:
  • examine - who may examine the player’s properties.
  • delete - who may delete the player.
  • edit - who may edit the player’s attributes and properties.
  • msg - who may send messages to the player.
  • boot - who may boot the player.
  • Attributes: (only checked by obj.secure_attr)
  • attrread - see/access attribute
  • attredit - change/delete attribute
  • Channels:
  • control - who is administrating the channel. This means the ability to delete the channel, boot listeners etc.
  • send - who may send to the channel.
  • listen - who may subscribe and listen to the channel.
  • HelpEntry:
  • examine - who may view this help entry (usually everyone)
  • edit - who may edit this help entry.

So to take an example, whenever an exit is to be traversed, a lock of the type traverse will be checked. Defining a suitable lock type for an exit object would thus involve a lockstring traverse: <lock functions>.

Lock functions

You are not allowed to use just any function in your lock definition; you are in fact only allowed to use those functions defined in one of the modules given in settings.LOCK_FUNC_MODULES. All functions in any of those modules will automatically be considered a valid lock function. The default ones are found in evennia/locks/lockfuncs.py or as properties on evennia.lockfuncs.

A lock function must always accept at least two arguments - the accessing object (this is the object wanting to get access) and the accessed object (this is the object with the lock). Those two are fed automatically as the first two arguments the function when the lock is checked. Any arguments explicitly given in the lock definition will appear as extra arguments.

# A simple example lock function. Called with e.g. `id(34)`

def id(accessing_obj, accessed_obj, *args, **kwargs):
    if args:
        wanted_id = args[0]
        return accessing_obj.id == wanted_id
    return False

(Using the * and ** syntax causes Python to magically put all extra arguments into a list args and all keyword arguments into a dictionary kwargs respectively. If you are unfamiliar with how *args and **kwargs work, see the Python manuals). Some useful default lockfuncs (see src/locks/lockfuncs.py for more):

  • true()/all() - give access to everyone
  • false()/none()/superuser() - give access to none. Superusers bypass the check entirely and are thus the only ones who will pass this check.
  • perm(perm) - this tries to match a given permission property, on a Player firsthand, on a Character second. See below.
  • perm_above(perm) - like perm but requires a “higher” permission level than the one given.
  • id(num)/dbref(num) - checks so the access_object has a certain dbref/id.
  • attr(attrname) - checks if a certain Attribute exists on accessing_object.
  • attr(attrname, value) - checks so an attribute exists on accessing_object and has the given value.
  • attr_gt(attrname, value) - checks so accessing_object has a value larger (>) than the given value.
  • attr_ge, attr_lt, attr_le, attr_ne - corresponding for >=, <, <= and !=.
  • holds(objid) - checks so the accessing objects contains an object of given name or dbref.
  • inside() - checks so the accessing object is inside the accessed object (the inverse of holds()).
  • pperm(perm), pid(num)/pdbref(num) - same as perm, id/dbref but always looks for permissions and dbrefs of Players, not on Characters.
  • serversetting(settingname, value) - Only returns True if Evennia has a given setting or a setting set to a given value.

Checking simple strings

Sometimes you don’t really need to look up a certain lock, you just want to check a lockstring. A common use is inside Commands, in order to check if a user has a certain permission. The lockhandler has a method check_lockstring(accessing_obj, lockstring, bypass_superuser=False) that allows this.

# inside command definition
if not self.caller.locks.check_lockstring(self.caller, "dummy:perm(Wizards)"):
    self.caller.msg("You must be Wizard or higher to do this!")
    return

Note here that the access_type can be left to a dummy value since this method does not actually do a Lock lookup.

Default locks

Evennia sets up a few basic locks on all new objects and players (if we didn’t, noone would have any access to anything from the start). This is all defined in the root Typeclasses of the respective entity, in the hook method basetype_setup() (which you usually don’t want to edit unless you want to change how basic stuff like rooms and exits store their internal variables). This is called once, before at_object_creation, so just put them in the latter method on your child object to change the default. Also creation commands like @create changes the locks of objects you create - for example it sets the control lock_type so as to allow you, its creator, to control and delete the object.

Permissions

A permission is simply a list of text strings stored in the handler permissions on Objects and Players. Permissions can be used as a convenient way to structure access levels and hierarchies. It is set by the @perm command. Permissions are especially handled by the perm() and pperm() lock functions listed above.

Let’s say we have a red_key object. We also have red chests that we want to unlock with this key.

@perm red_key = unlocks_red_chests

This gives the red_key object the permission “unlocks_red_chests”. Next we lock our red chests:

@lock red chest = unlock:perm(unlocks_red_chests)

What this lock will expect is to the fed the actual key object. The perm() lock function will check the permissions set on the key and only return true if the permission is the one given.

Finally we need to actually check this lock somehow. Let’s say the chest has an command open <key> sitting on itself. Somewhere in its code the command needs to figure out which key you are using and test if this key has the correct permission:

# self.obj is the chest
# and used_key is the key we used as argument to
# the command. The self.caller is the one trying
# to unlock the chest
if not self.obj.access(used_key, "unlock"):
    self.caller.msg("The key does not fit!")
    return

All new players are given a default set of permissions defined by settings.PERMISSION_PLAYER_DEFAULT.

Selected permission strings can be organized in a permission hierarchy by editing the tuple settings.PERMISSION_HIERARCHY. Evennia’s default permission hierarchy is as follows:

Immortals        # like superuser but affected by locks
Wizards          # can administrate players
Builders         # can edit the world
PlayerHelpers    # can edit help files
Players          # can chat and send tells (default level)

The main use of this is that if you use the lock function perm() mentioned above, a lock check for a particular permission in the hierarchy will also grant access to those with higher hierarchy access. So if you have the permission “Wizards” you will also pass a lock defined as perm(Builders) or any of those levels below “Wizards”.

When doing an access check from an Object or Character, the perm() lock function will always first use the permissions of any Player connected to that Object before checking for permissions on the Object. In the case of hierarchical permissions (Wizards, Builders etc), the Player permission will always be used (this stops a Player from escalating their permission by puppeting a high-level Character). If the permission looked for is not in the hierarchy, an exact match is required, first on the Player and if not found there (or if no Player is connected), then on the Object itself.

Here is how you use @perm to give a player more permissions:

@perm/player Tommy = Builders
@perm/player/del Tommy = Builders # remove it again

Note the use of the /player switch. It means you assign the permission to the Players Tommy instead of any Character that also happens to be named “Tommy”.

Putting permissions on the Player guarantees that they are kept, regardless of which Character they are currently puppeting. This is especially important to remember when assigning permissions from the hierarchy tree - as mentioned above, a Player’s permissions will overrule that of its character. So to be sure to avoid confusion you should generally put hierarchy permissions on the Player, not on their Characters (but see also quelling).

Below is an example of an object without any connected player

obj1.permissions = ["Builders", "cool_guy"]
obj2.locks.add("enter:perm_above(Players) and perm(cool_guy)")

obj2.access(obj1, "enter") # this returns True!

And one example of a puppet with a connected player:

player.permissions.add("Players")
puppet.permissions.add("Builders", "cool_guy")
obj2.locks.add("enter:perm_above(Players) and perm(cool_guy)")

obj2.access(puppet, "enter") # this returns False!

Superusers

There is normally only one superuser account and that is the one first created when starting Evennia (User #1). This is sometimes known as the “Owner” or “God” user. A superuser has more than full access - it completely bypasses all locks so no checks are even run. This allows for the superuser to always have access to everything in an emergency. But it also hides any eventual errors you might have made in your lock definitions. So when trying out game systems you should either use quelling (see below) or make a second Immortal-level character so your locks get tested correctly.

Quelling

The @quell command can be used to enforce the perm() lockfunc to ignore permissions on the Player and instead use the permissions on the Character only. This can be used e.g. by staff to test out things with a lower permission level. Return to the normal operation with @unquell. Note that quelling will use the smallest of any hierarchical permission on the Player or Character, so one cannot escalate one’s Player permission by quelling to a high-permission Character. Also the superuser can quell their powers this way, making them affectable by locks.

More Lock definition examples

examine: attr(eyesight, excellent) or perm(Builders)

You are only allowed to do examine on this object if you have ‘excellent’ eyesight (that is, has an Attribute eyesight with the value excellent defined on yourself) or if you have the “Builders” permission string assigned to you.

open: holds('the green key') or perm(Builder)

This could be called by the open command on a “door” object. The check is passed if you are a Builder or has the right key in your inventory.

cmd: perm(Builders)

Evennia’s command handler looks for a lock of type cmd to determine if a user is allowed to even call upon a particular command or not. When you define a command, this is the kind of lock you must set. See the default command set for lots of examples. If a character/player don’t pass the cmd lock type the command will not even appear in their help list.

cmd: not perm(no_tell)

“Permissions” can also be used to block users or implement highly specific bans. The above example would be be added as a lock string to the tell command. This will allow everyone not having the “permission” no_tell to use the tell command. You could easily give a player the “permission” no_tell to disable their use of this particular command henceforth.

dbref = caller.id
lockstring = "control:id(%s);examine:perm(Builders);delete:id(%s) or perm(Wizards);get:all()" % (dbref, dbref)
new_obj.locks.add(lockstring)

This is how the @create command sets up new objects. In sequence, this permission string sets the owner of this object be the creator (the one running @create). Builders may examine the object whereas only Wizards and the creator may delete it. Everyone can pick it up.

A complete example of setting locks on an object

Assume we have two objects - one is ourselves (not superuser) and the other is an Object called box.

> @create/drop box
> @desc box = "This is a very big and heavy box."

We want to limit which objects can pick up this heavy box. Let’s say that to do that we require the would-be lifter to to have an attribute strength on themselves, with a value greater than 50. We assign it to ourselves to begin with.

> @set self/strength = 45

Ok, so for testing we made ourselves strong, but not strong enough. Now we need to look at what happens when someone tries to pick up the the box - they use the get command (in the default set). This is defined in evennia/commands/default/general.py. In its code we find this snippet:

if not obj.access(caller, 'get'):
    if obj.db.get_err_msg:
        caller.msg(obj.db.get_err_msg)
    else:
        caller.msg("You can't get that.")
    return

So the get command looks for a lock with the type get (not so surprising). It also looks for an Attribute on the checked object called get_err_msg in order to return a customized error message. Sounds good! Let’s start by setting that on the box:

> @set box/get_err_msg = You are not strong enough to lift this box.

Next we need to craft a Lock of type get on our box. We want it to only be passed if the accessing object has the attribute strength of the right value. For this we would need to create a lock function that checks if attributes have a value greater than a given value. Luckily there is already such a one included in evennia (see evennia/locks/lockfuncs.py), called attr_gt.

So the lock string will look like this: get:attr_gt(strength, 50). We put this on the box now:

@lock box = get:attr_gt(strength, 50)

Try to get the object and you should get the message that we are not strong enough. Increase your strength above 50 however and you’ll pick it up no problem. Done! A very heavy box!

If you wanted to set this up in python code, it would look something like this:

from evennia import create_object

   # create, then set the lock
   box = create_object(None, key="box")
   box.locks.add("get:attr_gt(strength, 50)")

   # or we can assign locks in one go right away
   box = create_object(None, key="box", locks="get:attr_gt(strength, 50)")

   # set the attributes
   box.db.desc = "This is a very big and heavy box."
   box.db.get_err_msg = "You are not strong enough to lift this box."

   # one heavy box, ready to withstand all but the strongest...

On Django’s permission system

Django also implements a comprehensive permission/security system of its own. The reason we don’t use that is because it is app-centric (app in the Django sense). Its permission strings are of the form appname.permstring and it automatically adds three of them for each database model in the app - for the app evennia/object this would be for example ‘object.create’, ‘object.admin’ and ‘object.edit’. This makes a lot of sense for a web application, not so much for a MUD, especially when we try to hide away as much of the underlying architecture as possible.

The django permissions are not completely gone however. We use it for validating passwords during login. It is also used exclusively for managing Evennia’s web-based admin site, which is a graphical front-end for the database of Evennia. You edit and assign such permissions directly from the web interface. It’s stand-alone from the permissions described above.