evennia/docs/source/Howto/StartingTutorial/Python-basic-tutorial-part-two.md

# Python basic tutorial part two

[In the first part](Howto/StartingTutorial/Python-basic-introduction) of this Python-for-Evennia basic tutorial we learned
how to run some simple Python code from inside the game. We also made our first new *module*
containing a *function* that we called. Now we're going to start exploring the very important
subject of *objects*.

**Contents:**
- [On the subject of objects](Howto/StartingTutorial/Python-basic-tutorial-part-two#on-the-subject-of-objects)
- [Exploring the Evennia library](Howto/StartingTutorial/Python-basic-tutorial-part-two#exploring-the-evennia-library)
- [Tweaking our Character class](Howto/StartingTutorial/Python-basic-tutorial-part-two#tweaking-our-character-class)
- [The Evennia shell](Howto/StartingTutorial/Python-basic-tutorial-part-two#the-evennia-shell)
- [Where to go from here](Howto/StartingTutorial/Python-basic-tutorial-part-two#where-to-go-from-here)

### On the subject of objects

In the first part of the tutorial we did things like

    > py me.msg("Hello World!")

To learn about functions and imports we also passed that `me` on to a function `hello_world` in
another module.

Let's learn some more about this `me` thing we are passing around all over the place. In the
following we assume that we named our superuser Character "Christine".

    > py me
    Christine
    > py me.key
    Christine

These returns look the same at first glance, but not if we examine them more closely:

    > py type(me)
    <class 'typeclasses.characters.Character'>
    > py type(me.key)
    <type str>

> Note: In some MU clients, such as Mudlet and MUSHclient simply returning `type(me)`, you may not
see the proper return from the above commands. This is likely due to the HTML-like tags `<...>`,
being swallowed by the client.

The `type` function is, like `print`, another in-built function in Python. It
tells us that we (`me`) are of the *class* `typeclasses.characters.Character`.
Meanwhile `me.key` is a *property* on us, a string. It holds the name of this
object.

> When you do `py me`, the `me` is defined in such a way that it will use its `.key` property to
represent itself. That is why the result is the same as when doing `py me.key`. Also, remember that
as noted in the first part of the tutorial, the `me` is *not* a reserved Python word; it was just
defined by the Evennia developers as a convenient short-hand when creating the `py` command. So
don't expect `me` to be available elsewhere.

A *class* is like a "factory" or blueprint. From a class you then create individual *instances*. So
if class is`Dog`, an instance of `Dog` might be `fido`. Our in-game persona is of a class
`Character`. The superuser `christine` is an *instance* of the `Character` class (an instance is
also often referred to as an *object*). This is an important concept in *object oriented
programming*. You are wise to [familiarize yourself with it](https://en.wikipedia.org/wiki/Class-
based_programming) a little.

> In other terms:
> * class: A description of a thing, all the methods (code) and data (information)
> * object: A thing, defined as an *instance* of a class.
>
> So in "Fido is a Dog", "Fido" is an object--a unique thing--and "Dog" is a class. Coders would
also say, "Fido is an instance of Dog". There can be other dogs too, such as Butch and Fifi. They,
too, would be instances of Dog.
>
> As another example: "Christine is a Character", or "Christine is an instance of
typeclasses.characters.Character". To start, all characters will be instances of
typeclass.characters.Character.
>
> You'll be writing your own class soon! The important thing to know here is how classes and objects
relate.

The string `'typeclasses.characters.Character'` we got from the `type()` function is not arbitrary.
You'll recognize this from when we _imported_ `world.test` in part one. This is a _path_ exactly
describing where to find the python code describing this class. Python treats source code files on
your hard drive (known as *modules*) as well as folders (known as *packages*) as objects that you
access with the `.` operator. It starts looking at a place that Evennia has set up for you - namely
the root of your own game directory.

Open and look at your game folder (named `mygame` if you exactly followed the Getting Started
instructions) in a file editor or in a new terminal/console. Locate the file
`mygame/typeclasses/characters.py`

```
mygame/
    typeclasses
        characters.py
```

This represents the first part of the python path - `typeclasses.characters` (the `.py` file ending
is never included in the python path). The last bit, `.Character` is the actual class name inside
the `characters.py` module. Open that file in a text editor and you will see something like this:

```python
"""
(Doc string for module)
"""

from evennia import DefaultCharacter

class Character(DefaultCharacter):
    """
    (Doc string for class)
    """
    pass

```

There is `Character`, the last part of the path. Note how empty this file is. At first glance one
would think a Character had no functionality at all. But from what we have used already we know it
has at least the `key` property and the method `msg`! Where is the code? The answer is that this
'emptiness' is an illusion caused by something called *inheritance*. Read on.

Firstly, in the same way as the little `hello.py` we did in the first part of the tutorial, this is
an example of full, multi-line Python code. Those triple-quoted strings are used for strings that
have line breaks in them. When they appear on their own like this, at the top of a python module,
class or similar they are called *doc strings*. Doc strings are read by Python and is used for
producing online help about the function/method/class/module. By contrast, a line starting with `#`
is a *comment*. It is ignored completely by Python and is only useful to help guide a human to
understand the code.

The line

```python
    class Character(DefaultCharacter):
```

means that the class `Character` is a *child* of the class `DefaultCharacter`. This is called
*inheritance* and is another fundamental concept. The answer to the question "where is the code?" is
that the code is *inherited* from its parent, `DefaultCharacter`. And that in turn may inherit code
from *its* parent(s) and so on. Since our child, `Character` is empty, its functionality is *exactly
identical* to that of its parent. The moment we add new things to Character, these will take
precedence. And if we add something that already existed in the parent, our child-version will
*override* the version in the parent. This is very practical: It means that we can let the parent do
the heavy lifting and only tweak the things we want to change. It also means that we could easily
have many different Character classes, all inheriting from `DefaultCharacter` but changing different
things. And those can in turn also have children ...

Let's go on an expedition up the inheritance tree.

### Exploring the Evennia library

Let's figure out how to tweak `Character`. Right now we don't know much about `DefaultCharacter`
though. Without knowing that we won't know what to override. At the top of the file you find

```python
from evennia import DefaultCharacter
```

This is an `import` statement again, but on a different form to what we've seen before. `from ...
import ...` is very commonly used and allows you to precisely dip into a module to extract just the
component you need to use. In this case we head into the `evennia` package to get
`DefaultCharacter`.

Where is `evennia`? To find it you need to go to the `evennia` folder (repository) you originally
cloned from us. If you open it, this is how it looks:

```
evennia/
   __init__.py
   bin/
   CHANGELOG.txt etc.
   ...
   evennia/
   ...
```
There are lots of things in there. There are some docs but most of those have to do with the
distribution of Evennia and does not concern us right now. The `evennia` subfolder is what we are
looking for. *This* is what you are accessing when you do `from evennia import ...`. It's set up by
Evennia as a good place to find modules when the server starts. The exact layout of the Evennia
library [is covered by our directory overview](Coding/Directory-Overview#evennia-library-layout). You can
also explore it [online on github](https://github.com/evennia/evennia/tree/master/evennia).

The structure of the library directly reflects how you import from it.

- To, for example, import [the text justify
function](https://github.com/evennia/evennia/blob/master/evennia/utils/utils.py#L201) from
`evennia/utils/utils.py` you would do `from evennia.utils.utils import justify`. In your code you
could then just call `justify(...)` to access its functionality.
- You could also do `from evennia.utils import utils`. In code you would then have to write
`utils.justify(...)`. This is practical if want a lot of stuff from that `utils.py` module and don't
want to import each component separately.
- You could also do `import evennia`. You would then have to enter the full
`evennia.utils.utils.justify(...)` every time you use it. Using `from` to only import the things you
need is usually easier and more readable.
- See [this overview](http://effbot.org/zone/import-confusion.htm) about the different ways to
import in Python.

Now, remember that our `characters.py` module did `from evennia import DefaultCharacter`. But if we
look at the contents of the `evennia` folder, there is no `DefaultCharacter` anywhere! This is
because Evennia gives a large number of optional "shortcuts", known as [the "flat" API](Evennia-
API). The intention is to make it easier to remember where to find stuff. The flat API is defined in
that weirdly named `__init__.py` file. This file just basically imports useful things from all over
Evennia so you can more easily find them in one place.

We could [just look at the documenation](github:evennia#typeclasses) to find out where we can look
at our `DefaultCharacter` parent. But for practice, let's figure it out. Here is where
`DefaultCharacter` [is imported
from](https://github.com/evennia/evennia/blob/master/evennia/__init__.py#L188) inside `__init__.py`:

```python
from .objects.objects import DefaultCharacter
```

The period at the start means that it imports beginning from the same location this module sits(i.e.
the `evennia` folder). The full python-path accessible from the outside is thus
`evennia.objects.objects.DefaultCharacter`. So to import this into our game it'd be perfectly valid
to do

```python
from evennia.objects.objects import DefaultCharacter
```

Using

```python
from evennia import DefaultCharacter
```

is the same thing, just a little easier to remember.

> To access the shortcuts of the flat API you *must* use `from evennia import
> ...`. Using something like `import evennia.DefaultCharacter` will not work.
> See [more about the Flat API here](Coding/Evennia-API).


### Tweaking our Character class

In the previous section we traced the parent of our `Character` class to be
`DefaultCharacter` in
[evennia/objects/objects.py](https://github.com/evennia/evennia/blob/master/evennia/objects/objects.py).
Open that file and locate the `DefaultCharacter` class. It's quite a bit down
in this module so you might want to search using your editor's (or browser's)
search function. Once you find it, you'll find that the class starts like this:

```python

class DefaultCharacter(DefaultObject):
    """
    This implements an Object puppeted by a Session - that is, a character
    avatar controlled by an account.
    """

    def basetype_setup(self):
        """
        Setup character-specific security.
        You should normally not need to overload this, but if you do,
        make sure to reproduce at least the two last commands in this
        method (unless you want to fundamentally change how a
        Character object works).
        """
        super().basetype_setup()
        self.locks.add(";".join(["get:false()",     # noone can pick up the character
                                 "call:false()"]))  # no commands can be called on character from
outside
        # add the default cmdset
        self.cmdset.add_default(settings.CMDSET_CHARACTER, permanent=True)

    def at_after_move(self, source_location, **kwargs):
        """
        We make sure to look around after a move.
        """
        if self.location.access(self, "view"):
            self.msg(self.at_look(self.location))

    def at_pre_puppet(self, account, session=None, **kwargs):
        """
        Return the character from storage in None location in `at_post_unpuppet`.
        """

    # ...

```

... And so on (you can see the full [class online
here](https://github.com/evennia/evennia/blob/master/evennia/objects/objects.py#L1915)). Here we
have functional code! These methods may not be directly visible in `Character` back in our game dir,
but they are still available since `Character` is a child of `DefaultCharacter` above. Here is a
brief summary of the methods we find in `DefaultCharacter` (follow in the code to see if you can see
roughly where things happen)::

- `basetype_setup` is called by Evennia only once, when a Character is first created. In the
`DefaultCharacter` class it sets some particular [Locks](Component/Locks) so that people can't pick up and
puppet Characters just like that. It also adds the [Character Cmdset](Component/Command-Sets) so that
Characters always can accept command-input (this should usually not be modified - the normal hook to
override is `at_object_creation`, which is called after `basetype_setup` (it's in the parent)).
- `at_after_move` makes it so that every time the Character moves, the `look` command is
automatically fired (this would not make sense for just any regular Object).
- `at_pre_puppet` is called when an Account begins to puppet this Character. When not puppeted, the
Character is hidden away to a `None` location. This brings it back to the location it was in before.
Without this, "headless" Characters would remain in the game world just standing around.
- `at_post_puppet` is called when puppeting is complete. It echoes a message to the room that his
Character has now connected.
- `at_post_unpuppet` is called once stopping puppeting of the Character. This hides away the
Character to a `None` location again.
- There are also some utility properties which makes it easier to get some time stamps from the
Character.

Reading the class we notice another thing:

```python
class DefaultCharacter(DefaultObject):
    # ...
```

This means that `DefaultCharacter` is in *itself* a child of something called `DefaultObject`! Let's
see what this parent class provides. It's in the same module as `DefaultCharacter`, you just need to
[scroll up near the
top](https://github.com/evennia/evennia/blob/master/evennia/objects/objects.py#L182):

```python
class DefaultObject(with_metaclass(TypeclassBase, ObjectDB)):
   # ...
```

This is a really big class where the bulk of code defining an in-game object resides. It consists of
a large number of methods, all of which thus also becomes available on the `DefaultCharacter` class
below *and* by extension in your `Character` class over in your game dir. In this class you can for
example find the `msg` method we have been using before.

> You should probably not expect to understand all details yet, but as an exercise, find and read
the doc string of `msg`.

> As seen, `DefaultObject` actually has multiple parents. In one of those the basic `key` property
is defined, but we won't travel further up the inheritance tree in this tutorial. If you are
interested to see them, you can find `TypeclassBase` in
[evennia/typeclasses/models.py](https://github.com/evennia/evennia/blob/master/evennia/typeclasses/models.py#L93)
and `ObjectDB` in
[evennia/objects/models.py](https://github.com/evennia/evennia/blob/master/evennia/objects/models.py#L121).
We will also not go into the details of [Multiple
Inheritance](https://docs.python.org/2/tutorial/classes.html#multiple-inheritance) or
[Metaclasses](http://www.onlamp.com/pub/a/python/2003/04/17/metaclasses.html) here. The general rule
is that if you realize that you need these features, you already know enough to use them.

Remember the `at_pre_puppet` method we looked at in `DefaultCharacter`? If you look at the
`at_pre_puppet` hook as defined in `DefaultObject` you'll find it to be completely empty (just a
`pass`). So if you puppet a regular object it won't be hiding/retrieving the object when you
unpuppet it. The `DefaultCharacter` class *overrides* its parent's functionality with a version of
its own. And since it's `DefaultCharacter` that our `Character` class inherits back in our game dir,
it's *that* version of `at_pre_puppet` we'll get. Anything not explicitly overridden will be passed
down as-is.

While it's useful to read the code, we should never actually modify anything inside the `evennia`
folder. Only time you would want that is if you are planning to release a bug fix or new feature for
Evennia itself. Instead you *override* the default functionality inside your game dir.

So to conclude our little foray into classes, objects and inheritance, locate the simple little
`at_before_say` method in the `DefaultObject` class:

```python
    def at_before_say(self, message, **kwargs):
        """
        (doc string here)
        """
        return message
```

If you read the doc string you'll find that this can be used to modify the output of `say` before it
goes out. You can think of it like this: Evennia knows the name of this method, and when someone
speaks, Evennia will make sure to redirect the outgoing message through this method. It makes it
ripe for us to replace with a version of our own.

> In the Evennia documentation you may sometimes see the term *hook* used for a method explicitly
meant to be overridden like this.

As you can see, the first argument to `at_before_say` is `self`. In Python, the first argument of a
method is *always a back-reference to the object instance on which the method is defined*. By
convention this argument is always called `self` but it could in principle be named anything. The
`self` is very useful. If you wanted to, say, send a message to the same object from inside
`at_before_say`, you would do `self.msg(...)`.

What can trip up newcomers is that you *don't* include `self` when you *call* the method. Try:

    > @py me.at_before_say("Hello World!")
    Hello World!

Note that we don't send `self` but only the `message` argument. Python will automatically add `self`
for us. In this case, `self` will become equal to the Character instance `me`.

By default the `at_before_say` method doesn't do anything. It just takes the `message` input and
`return`s it just the way it was (the `return` is another reserved Python word).

> We won't go into `**kwargs` here, but it (and its sibling `*args`) is also important to
understand, extra reading is [here for
`**kwargs`](https://stackoverflow.com/questions/1769403/understanding-kwargs-in-python).

Now, open your game folder and edit `mygame/typeclasses/characters.py`. Locate your `Character`
class and modify it as such:

```python
class Character(DefaultCharacter):
    """
    (docstring here)
    """
    def at_before_say(self, message, **kwargs):
        "Called before say, allows for tweaking message"
        return f"{message} ..."
```

So we add our own version of `at_before_say`, duplicating the `def` line from the parent but putting
new code in it. All we do in this tutorial is to add an ellipsis (`...`) to the message as it passes
through the method.

Note that `f` in front of the string, it means we turned the string into a 'formatted string'. We
can now easily inject stuff directly into the string by wrapping them in curly brackets `{ }`. In
this example, we put the incoming `message` into the string, followed by an ellipsis. This is only
one way to format a string. Python has very powerful [string
formatting](https://docs.python.org/2/library/string.html#format-specification-mini-language) and
you are wise to learn it well, considering your game will be mainly text-based.

> You could also copy & paste the relevant method from `DefaultObject` here to get the full doc
string. For more complex methods, or if you only want to change some small part of the default
behavior, copy & pasting will eliminate the need to constantly look up the original method and keep
you sane.

In-game, now try

    > @reload
    > say Hello
    You say, "Hello ..."

An ellipsis `...` is added to what you said! This is a silly example but you have just made your
first code change to core functionality - without touching any of Evennia's original code! We just
plugged in our own version of the `at_before_say` method and it replaced the default one. Evennia
happily redirected the message through our version and we got a different output.

> For sane overriding of parent methods you should also be aware of Python's
[super](https://docs.python.org/3/library/functions.html#super), which allows you to call the
methods defined on a parent in your child class.

### The Evennia shell

Now on to some generally useful tools as you continue learning Python and Evennia. We have so far
explored using `py` and have inserted Python code directly in-game. We have also modified Evennia's
behavior by overriding default functionality with our own. There is a third way to conveniently
explore Evennia and Python - the Evennia shell.

Outside of your game, `cd` to your mygame folder and make sure any needed virtualenv is running.
Next:

    > pip install ipython      # only needed once

The [`IPython`](https://en.wikipedia.org/wiki/IPython) program is just a nicer interface to the
Python interpreter - you only need to install it once, after which Evennia will use it
automatically.

    > evennia shell

If you did this call from your game dir you will now be in a Python prompt managed by the IPython
program.

    IPython ...
    ...
    In [1]: 
IPython has some very nice ways to explore what Evennia has to offer.

    > import evennia
    > evennia.<TAB>

That is, write `evennia.` and press the Tab key. You will be presented with a list of all available
resources in the Evennia Flat API. We looked at the `__init__.py` file in the `evennia` folder
earlier, so some of what you see should be familiar. From the IPython prompt, do:

    > from evennia import DefaultCharacter
    > DefaultCharacter.at_before_say?

Don't forget that you can use `<TAB>` to auto-complete code as you write. Appending a single `?` to
the end will show you the doc-string for `at_before_say` we looked at earlier. Use `??` to get the
whole source code.

Let's look at our over-ridden version instead. Since we started the `evennia shell` from our game
dir we can easily get to our code too:

    > from typeclasses.characters import Character
    > Character.at_before_say??

This will show us the changed code we just did. Having a window with IPython running is very
convenient for quickly exploring code without having to go digging through the file structure!

### Where to go from here

This should give you a running start using Python with Evennia. If you are completely new to
programming or Python you might want to look at a more formal Python tutorial. You can find links
and resources [on our link page](Links).

We have touched upon many of the concepts here but to use Evennia and to be able to follow along in
the code, you will need basic understanding of Python
[modules](http://docs.python.org/2/tutorial/modules.html),
[variables](http://www.tutorialspoint.com/python/python_variable_types.htm), [conditional
statements](http://docs.python.org/tutorial/controlflow.html#if-statements),
[loops](http://docs.python.org/tutorial/controlflow.html#for-statements),
[functions](http://docs.python.org/tutorial/controlflow.html#defining-functions), [lists,
dictionaries, list comprehensions](http://docs.python.org/tutorial/datastructures.html) and [string
formatting](http://docs.python.org/tutorial/introduction.html#strings). You should also have a basic
understanding of [object-oriented
programming](http://www.tutorialspoint.com/python/python_classes_objects.htm) and what Python
[Classes](http://docs.python.org/tutorial/classes.html) are.

Once you have familiarized yourself, or if you prefer to pick Python up as you go, continue to one
of the beginning-level [Evennia tutorials](Tutorials) to gradually build up your understanding.

Good luck!