In this lesson we'll learn how to create our own Evennia [Commands](../../../Components/Commands.md) If you are new to Python you'll also learn some more basics about how to manipulate strings and get information out of Evennia.
[previous lesson about it](./Beginner-Tutorial-Python-classes-and-objects.md)! A Command inherits from `evennia.Command` or from one of the alternative command- classes, such as `MuxCommand` which is what most default commands use.
All Commands are grouped in another class called a _Command Set_. Think of a Command Set as a bag holding many different commands. One CmdSet could for example hold all commands for combat, another for building etc.
Command-Sets are then associated with objects, for example with your Character. Doing so makes the commands in that cmdset available to the object. By default, Evennia groups all character-commands into one big cmdset called the `CharacterCmdSet`. It sits on `DefaultCharacter` (and thus, through inheritance, on `typeclasses.characters.Character`).
We can see that we import `Command` from `evennia` and use the `from ... import ... as ...` form to rename it to `BaseCommand`. This is so we can let our child class also be named `Command` to make it easier to reference. The class itself doesn't do anything, it just has `pass`. So in the same way as `Object` and `Character` in the previous lessons, this class is identical to its parent.
We could modify this module directly, but let's work in a separate module just for the heck of it. Open a new file `mygame/commands/mycommands.py` and add the following code:
Our `EchoCmdSet` class must have an `at_cmdset_creation` method, named exactly like this - this is what Evennia will be looking for when setting up the cmdset later, so if you didn't set it up, it will use the parent's version, which is empty. Inside we add the command class to the cmdset by `self.add()`. If you wanted to add more commands to this CmdSet you could just add more lines of `self.add` after this.
`echo` works! You should be getting a long list of outputs. The reason for this is that your `echo` function is not really "doing" anything yet and the default function is then to show all useful resources available to you when you use your Command. Let's look at some of those listed:
These are all properties you can access with `.` on the Command instance, such as `.key`, `.args` and so on. Evennia makes these available to you and they will be different every time a command is run. The most important ones we will make use of now are:
The reason our command doesn't do anything yet is because it's missing a `func` method. This is what Evennia looks for to figure out what a Command actually does. Modify your `CmdEcho` class:
First we added a docstring. This is always a good thing to do in general, but for a Command class, it will also automatically become the in-game help entry!
In a Command class, the `self.msg()` acts as a convenient shortcut for `self.caller.msg()`. Not only is it shorter, it also has some advantages because the command can include more metadata with the message. So using `self.msg()` is usually better. For this tutorial though, `self.caller.msg()` is more explicit in showing what is going on.
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Next we add the `func` method. It has one active line where it makes use of some of those variables the Command class offers to us. If you did the [basic Python tutorial](./Beginner-Tutorial-Python-basic-introduction.md), you will recognize `.msg` - this will send a message to the object it is attached to us - in this case `self.caller`, that is, us. We grab `self.args` and includes that in the message.
Since we haven't changed `MyCmdSet`, that will work as before. Reload and re-add this command to ourselves to try out the new version:
Note that there is an extra space before `Woo!`. That is because self.args contains _everything_ after the command name, including spaces. Evennia will happily understand if you skip that space too:
There are ways to force Evennia to _require_ an initial space, but right now we want to just ignore it since it looks a bit weird for our echo example. Tweak the code:
The only difference is that we called `.strip()` on `self.args`. This is a helper method available on all strings - it strips out all whitespace before and after the string. Now the Command-argument will no longer have any space in front of it.
- **Line 15** has our first _conditional_, an `if` statement. This is written on the form `if <condition>:` and only if that condition is 'truthy' will the indented code block under the `if` statement run. To learn what is truthy in Python it's usually easier to learn what is "falsy":
- **Line 16**'s condition is `not args`. The `not`_inverses_ the result, so if `args` is the empty string (falsy), the whole conditional becomes truthy. Let's continue in the code:
- **Lines 19-20**: A feature of `.search` is that it will already inform `self.caller` if it couldn't find the target. In that case, `target` will be `None` and we should just directly `return`.
Don't panic; tracebacks are your friends - they are to be read bottom-up and usually describe exactly where your problem is. Refer to [The Python introduction lesson](./Beginner-Tutorial-Python-basic-introduction.md) for more hints. If you get stuck, reach out to the Evennia community for help.
