Command Sets

Command Sets are intimately linked with Commands and you should be familiar with Commands before reading this page. The two pages were split for ease of reading.

A Command Set (often referred to as a CmdSet or cmdset) is the basic unit for storing one or more Commands. A given Command can go into any number of different command sets. Storing Command classes in a command set is the only way to make the commands available to use in your game.

When storing a CmdSet on an object, will you make the commands in that command set available to the object. An example is the default command set stored on new Characters. This command set contains all the useful commands, from look and inventory to @dig and @reload (permissions then limit which players may use them, but that’s a separate topic).

When a player enters a command, cmdsets from the Player, Character, its location and elsewhere are pulled together into a merge stack. This stack is merged together in a specific order to create a single “merged” cmdset, representing the pool of commands available at that very moment.

An example would be a Window object that has a cmdset with two commands in it: look through window and open window. The command set would be visible to players in the room with the window, allowing them to use those commands only there. You could imagine all sorts of clever uses of this, like a Television object which had multiple commands for looking at it, switching channels and so on. The tutorial world coming with Evennia showcases a dark room that replaces certain critical commands with its on versions because the Character cannot see.

If you want a quick start into defining your first commands and use them with command sets, you can head over to the Adding Command Tutorial which steps through things without the explanations.

Defining Command Sets

A CmdSet is, as most things in Evennia, defined as a Python class inheriting from the correct parent (evennia.CmdSet, which is a shortcut to evennia.commands.cmdset.CmdSet). The CmdSet class only needs to define one method, called at_cmdset_creation(). All other class parameters are optional, but are used for more advanced set manipulation and coding (see the merge rules section).

# file mygame/commands/

from evennia import CmdSet

# this is a theoretical custom module with commands we
# created previously: mygame/commands/
from commands import mycommands

class MyCmdSet(CmdSet):
    def at_cmdset_creation(self):
        The only thing this method should need
        to do is to add commands to the set.

The CmdSet’s add() method can also take another CmdSet as input. In this case all the commands from that CmdSet will be appended to this one as if you added them line by line:

def at_cmdset_creation():
    self.add(AdditionalCmdSet) # adds all command from this set

If you added your command to an existing cmdset (like to the default cmdset), that set is already loaded into memory. You need to make the server aware of the code changes:


You should now be able to use the command.

If you created a new, fresh cmdset, this must be added to an object in order to make the commands within available. A simple way to temporarily test a cmdset on yourself is use the @py command to execute a python snippet:

@py self.cmdset.add('commands.mycmdset.MyCmdSet')

This will stay with you until you @reset or @shutdown the server, or you run

@py self.cmdset.delete('commands.mycmdset.MyCmdSet')

Above a specific Cmdset class is removed. Calling delete without arguments will remove the latest added cmdset.

Note: Command sets added using cmdset.add are by default not persistent in the database.

If you want the cmdset to survive a reload, you can do

@py self.cmdset.add(commands.mycmdset.MyCmdSet, permanent=True)

or you could add the cmdset as the default cmdset:

@py self.cmdset.add_default(commands.mycmdset.MyCmdSet)

An object can only have one “default” cmdset (but can also have none). This is meant as a safe fall-back even if all other cmdsets fail or are removed. It is always persistent and will not be affected by cmdset.delete(). To remove a default cmdset you must explicitly call cmdset.remove_default().

Command sets are often added to an object in its at_object_creation method. For more examples of adding commands, read the Step by step tutorial. Generally you can customize which command sets are added to your objects by using self.cmdset.add() or self.cmdset.add_default().

Important: Commands are identified uniquely by key or alias (see Commands). If any overlap exists, two commands are considered identical. Adding a Command to a command set that already has an identical command will replace the previous command. This is very important in order to easily overload default Evennia commands with your own, but you need be aware of this or you may accidentally “hide” your own command in your command set because you add a new one that happen to have a matching alias.

Properties on command sets

There are a few extra flags one can set on CmdSets in order to modify how they work. All are optional and will be set to defaults otherwise. Since many of these relate to merging cmdsets you might want to read up on next section for some of these to make sense.

  • key (string) - an identifier for the cmdset. This is optional but should be unique; it is used for display in lists but also to identify special merging behaviours using the key_mergetype dictionary below. - mergetype (string) - one of “Union”, “Intersect”, “Replace” or “Remove”.
  • priority (int) - This defines the merge order of the merge stack - cmdsets will merge in rising order of priority with the highest priority set merging last. During a merger, the commands from the set with the higher priority will have precedence (just what happens depends on the merge type). If priority is identical, the order in the merge stack determines preference. The priority value must be greater or equal to -100. Most in-game sets should usually have priorities between 0 and 100. Evennia default sets have priorities as follows (these can be changed if you want a different distribution):
    • EmptySet: -101 (should be lower than all other sets)
    • SessionCmdSet: -20
    • PlayerCmdSet: -10
    • CharacterCmdSet: 0
    • ExitCmdSet: 101 (generally should always be available)
    • ChannelCmdSet: 101 (should usually always be available) - since exits never accept arguments, there is no collision between exits named the same as a channel even though the commands “collide”.
  • key_mergetype (dict) - a dict of key:mergetype pairs. This allows this cmdset to merge differently with certain named cmdsets. If the cmdset to merge with has a key matching an entry in key_mergetype, it will not be merged according to the setting in mergetype but according to the mode in this dict. Please note that this is more complex than it may seem due to the merge order of command sets. Please review that section before using key_mergetype.
  • duplicates (bool/None default None) - this determines what happens when merging same-priority cmdsets containing same-key commands together. Thedupicate option will only apply when merging the cmdset with this option onto one other cmdset with the same priority. The resulting cmdset will not retain this duplicate setting.
    • None (default): No duplicates are allowed and the cmdset being merged “onto” the old one will take precedence. The result will be unique commands. However, the system will assume this value to be True for cmdsets on Objects, to avoid dangerous clashes. This is usually the safe bet.
    • False: Like None except the system will not auto-assume any value for cmdsets defined on Objects.
    • True: Same-named, same-prio commands will merge into the same cmdset. This will lead to a multimatch error (the user will get a list of possibilities in order to specify which command they meant). This is is useful e.g. for on-object cmdsets (example: There is a red button and a green button in the room. Both have a press button command, in cmdsets with the same priority. This flag makes sure that just writing press button will force the Player to define just which object’s command was intended).
  • no_objs this is a flag for the cmdhandler that builds the set of commands available at every moment. It tells the handler not to include cmdsets from objects around the player (nor from rooms or inventory) when building the merged set. Exit commands will still be included. This option can have three values:
    • None (default): Passthrough of any value set explicitly earlier in the merge stack. If never set explicitly, this acts as False.
    • True/False: Explicitly turn on/off. If two sets with explicit no_objs are merged, priority determines what is used.
  • no_exits - this is a flag for the cmdhandler that builds the set of commands available at every moment. It tells the handler not to include cmdsets from exits. This flag can have three values:
    • None (default): Passthrough of any value set explicitly earlier in the merge stack. If never set explicitly, this acts as False.
    • True/False: Explicitly turn on/off. If two sets with explicit no_exits are merged, priority determines what is used.
  • no_channels (bool) - this is a flag for the cmdhandler that builds the set of commands available at every moment. It tells the handler not to include cmdsets from available in-game channels. This flag can have three values:
    • None (default): Passthrough of any value set explicitly earlier in the merge stack. If never set explicitly, this acts as False.
    • True/False: Explicitly turn on/off. If two sets with explicit no_channels are merged, priority determines what is used.

Command Sets searched

When a user issues a command, it is matched against the merged command sets available to the player at the moment. Which those are may change at any time (such as when the player walks into the room with the Window object described earlier).

The currently valid command sets are collected from the following sources:

  • The cmdsets stored on the currently active Session. Default is the empty SessionCmdSet with merge priority -20.
  • The cmdsets defined on the Player. Default is the PlayerCmdSet with merge priority -10.
  • All cmdsets on the Character/Object (assuming the Player is currently puppeting such a Character/Object). Merge priority 0.
  • The cmdsets of all objects carried by the puppeted Character (checks the call lock). Will not be included if no_objs option is active in the merge stack.
  • The cmdsets of the Character’s current location (checks the call lock). Will not be included if no_objs option is active in the merge stack.
  • The cmdsets of objects in the current location (checks the call lock). Will not be included if no_objs option is active in the merge stack.
  • The cmdsets of Exits in the location. Merge priority +101. Will not be included if no_exits or no_objs option is active in the merge stack.
  • The channel cmdset containing commands for posting to all channels the player or character is currently connected to. Merge priority +101. Will not be included if no_channels option is active in the merge stack.

Note that an object does not have to share its commands with its surroundings. A Character’s cmdsets should not be shared for example, or all other Characters would get multi-match errors just by being in the same room. The ability of an object to share its cmdsets is managed by its call lock. For example, Character objects defaults to call:false() so that any cmdsets on them can only be accessed by themselves, not by other objects around them. Another example might be to lock an object with call:inside() to only make their commands available to objects inside them, or cmd:holds() to make their commands available only if they are held.

Adding and merging command sets

Note: This is an advanced topic. It’s very useful to know about, but you might want to skip it if this is your first time learning about commands.

CmdSets have the special ability that they can be merged together into new sets. Which of the ingoing commands end up in the merged set is defined by the merge rule and the relative priorities of the two sets. Removing the latest added set will restore things back to the way it was before the addition.

CmdSets are non-destructively stored in a stack inside the cmdset handler on the object. This stack is parsed to create the “combined” cmdset active at the moment. CmdSets from other sources are also included in the merger such as those on objects in the same room (like buttons to press) or those introduced by state changes (such as when entering a menu). The cmdsets are all ordered after priority and then merged together in reverse order. That is, the higher priority will be merged “onto” lower-prio ones. By defining a cmdset with a merge-priority between that of two other sets, you will make sure it will be merged in between them.
The very first cmdset in this stack is called the Default cmdset and is protected from accidental deletion. Running obj.cmdset.delete() will never delete the default set. Instead one should add new cmdsets on top of the default to “hide” it, as described below. Use the special obj.cmdset.delete_default() only if you really know what you are doing.

CmdSet merging is an advanced feature useful for implementing powerful game effects. Imagine for example a player entering a dark room. You don’t want the player to be able to find everything in the room at a glance - maybe you even want them to have a hard time to find stuff in their backpack! You can then define a different CmdSet with commands that override the normal ones. While they are in the dark room, maybe the look and inv commands now just tell the player they cannot see anything! Another example would be to offer special combat commands only when the player is in combat. Or when being on a boat. Or when having taken the super power-up. All this can be done on the fly by merging command sets.

Merge rules

Basic rule is that command sets are merged in reverse priority order. That is, lower-prio sets are merged first and higher prio sets are merged “on top” of them. Think of it like a layered cake with the highest priority on top.

To further understand how sets merge, we need to define some examples. Let’s call the first command set A and the second B. We assume B is the command set already active on our object and we will merge A onto B. In code terms this would be done by object.cdmset.add(A). Remember, B is already active on object from before.

We let the A set have higher priority than B. A priority is simply an integer number. As seen in the list above, Evennia’s default cmdsets have priorities in the range -101 to 120. You are usually safe to use a priority of 0 or 1 for most game effects.

In our examples, both sets contain a number of commands which we’ll identify by numbers, like A1, A2 for set A and B1, B2, B3, B4 for B. So for that example both sets contain commands with the same keys (or aliases) “1” and “2” (this could for example be “look” and “get” in the real game), whereas commands 3 and 4 are unique to B. To describe a merge between these sets, we would write A1,A2 + B1,B2,B3,B4 = ? where ? is a list of commands that depend on which merge type A has, and which relative priorities the two sets have. By convention, we read this statement as “New command set A is merged onto the old command set B to form ?”.

Below are the available merge types and how they work. Names are partly borrowed from Set theory.

  • Union (default) - The two cmdsets are merged so that as many commands as possible from each cmdset ends up in the merged cmdset. Same-key commands are merged by priority.

    # Union
    A1,A2 + B1,B2,B3,B4 = A1,A2,B3,B4
  • Intersect - Only commands found in both cmdsets (i.e. which have the same keys) end up in the merged cmdset, with the higher-priority cmdset replacing the lower one’s commands.

    # Intersect
    A1,A3,A5 + B1,B2,B4,B5 = A1,A5
  • Replace - The commands of the higher-prio cmdset completely replaces the lower-priority cmdset’s commands, regardless of if same-key commands exist or not.

    # Replace
    A1,A3 + B1,B2,B4,B5 = A1,A3
  • Remove - The high-priority command sets removes same-key commands from the lower-priority cmdset. They are not replaced with anything, so this is a sort of filter that prunes the low-prio set using the high-prio one as a template.

    # Remove
    A1,A3 + B1,B2,B3,B4,B5 = B2,B4,B5

Besides priority and mergetype, a command-set also takes a few other variables to control how they merge:

  • duplicates (bool) - determines what happens when two sets of equal priority merge. Default is that the new set in the merger (i.e. A above) automatically takes precedence. But if duplicates is true, the result will be a merger with more than one of each name match. This will usually lead to the player receiving a multiple-match error higher up the road, but can be good for things like cmdsets on non-player objects in a room, to allow the system to warn that more than one ‘ball’ in the room has the same ‘kick’ command defined on it and offer a chance to select which ball to kick … Allowing duplicates only makes sense for Union and Intersect, the setting is ignored for the other mergetypes.

  • key_mergetypes (dict) - allows the cmdset to define a unique mergetype for particular cmdsets, identified by their cmdset key. Format is {CmdSetkey:mergetype}. Example: {'Myevilcmdset','Replace'} which would make sure for this set to always use ‘Replace’ on the cmdset with the key Myevilcmdset only, no matter what the main mergetype is set to.

    Warning: The key_mergetypes dictionary can only work on the cmdset we merge onto. When using key_mergetypes it is thus important to consider the merge priorities - you must make sure that you pick a priority between the cmdset you want to detect and the next higher one, if any. That is, if we define a cmdset with a high priority and set it to affect a cmdset that is far down in the merge stack, we would not “see” that set when it’s time for us to merge. Example: Merge stack is A(prio=-10), B(prio=-5), C(prio=0), D(prio=5). We now merge a cmdset E(prio=10) onto this stack, with a key_mergetype={"B":"Replace"}. But priorities dictate that we won’t be merged onto B, we will be merged onto E (which is a merger of the lower-prio sets at this point). Since we are merging onto E and not B, our key_mergetype directive won’t trigger. To make sure it works we must make sure we merge onto B. Setting E’s priority to, say, -4 will make sure to merge it onto B and affect it appropriately.

More advanced cmdset example:

from commands import mycommands

class MyCmdSet(CmdSet):

    key = "MyCmdSet"
    priority = 4
    mergetype = "Replace"
    key_mergetypes = {'MyOtherCmdSet':'Union'}

    def at_cmdset_creation(self):
        The only thing this method should need
        to do is to add commands to the set.

Assorted notes

It is very important to remember that two commands are compared both by their key properties and by their aliases properties. If either keys or one of their aliases match, the two commands are considered the same. So consider these two Commands:

  • A Command with key “kick” and alias “fight”
  • A Command with key “punch” also with an alias “fight”

During the cmdset merging (which happens all the time since also things like channel commands and exits are merged in), these two commands will be considered identical since they share alias. It means only one of them will remain after the merger. Each will also be compared with all other commands having any combination of the keys and/or aliases “kick”, “punch” or “fight”.

… So avoid duplicate aliases, it will only cause confusion.