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<div class="section" id="python-classes-and-objects">
<h1>Python Classes and objects<a class="headerlink" href="#python-classes-and-objects" title="Permalink to this headline"></a></h1>
<p>We have now learned how to run some simple Python code from inside (and outside) your game server.
We have also taken a look at what our game dir looks and what is where. Now we’ll start to use it.</p>
<div class="section" id="importing-things">
<h2>Importing things<a class="headerlink" href="#importing-things" title="Permalink to this headline"></a></h2>
<p>No one writes something as big as an online game in one single huge file. Instead one breaks up the
code into separate files (modules). Each module is dedicated to different purposes. Not only does
it make things cleaner, organized and easier to understand. It also makes it easier to re-use code -
you just import the resources you need and know you only get just what you requested. This makes
it much easier to find errors and to know what code is good and which has issues.</p>
<blockquote>
<div><p>Evennia itself uses your code in the same way - you just tell it where a particular type of code is,
and it will import and use it (often instead of its defaults).</p>
</div></blockquote>
<p>We have already successfully imported things, for example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; py import world.test ; world.test.hello_world(me)
Hello World!
</pre></div>
</div>
<p>In this example, on your hard drive, the files looks like this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">mygame</span><span class="o">/</span>
<span class="n">world</span><span class="o">/</span>
<span class="n">test</span><span class="o">.</span><span class="n">py</span> <span class="o">&lt;-</span> <span class="n">inside</span> <span class="n">this</span> <span class="n">file</span> <span class="ow">is</span> <span class="n">a</span> <span class="n">function</span> <span class="n">hello_world</span>
</pre></div>
</div>
<p>If you followed earlier tutorial lessons, the <code class="docutils literal notranslate"><span class="pre">mygame/world/test.py</span></code> file should look like this (if
not, make it so):</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2</pre></div></td><td class="code"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">hello_world</span><span class="p">(</span><span class="n">who</span><span class="p">):</span>
<span class="n">who</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;Hello World!&quot;</span><span class="p">)</span>
</pre></div>
</td></tr></table></div>
<div class="sidebar">
<p class="sidebar-title">Remember:</p>
<ul class="simple">
<li><p>Indentation matters in Python</p></li>
<li><p>So does capitalization</p></li>
<li><p>Use 4 <cite>spaces</cite> to indent, not tabs</p></li>
<li><p>Empty lines are fine</p></li>
<li><p>Anything on a line after a <cite>#</cite> is a <cite>comment</cite>, ignored by Python</p></li>
</ul>
</div>
<p>The <em>python_path</em> describes the relation between Python resources, both between and inside
Python <em>modules</em> (that is, files ending with .py). A python-path separates each part of the
path <code class="docutils literal notranslate"><span class="pre">.</span></code> and always skips the <code class="docutils literal notranslate"><span class="pre">.py</span></code> file endings. Also, Evennia already knows to start looking
for python resources inside <code class="docutils literal notranslate"><span class="pre">mygame/</span></code> so this should never be specified. Hence</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">world.test</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">import</span></code> Python instruction loads <code class="docutils literal notranslate"><span class="pre">world.test</span></code> so you have it available. You can now go “into”
this module to get to the function you want:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">world</span><span class="o">.</span><span class="n">test</span><span class="o">.</span><span class="n">hello_world</span><span class="p">(</span><span class="n">me</span><span class="p">)</span>
</pre></div>
</div>
<p>Using <code class="docutils literal notranslate"><span class="pre">import</span></code> like this means that you have to specify the full <code class="docutils literal notranslate"><span class="pre">world.test</span></code> every time you want
to get to your function. Here’s a more powerful form of import:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">world.test</span> <span class="kn">import</span> <span class="n">hello_world</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span> <span class="pre">...</span></code> is very, very common as long as you want to get something with a longer
python path. It imports <code class="docutils literal notranslate"><span class="pre">hello_world</span></code> directly, so you can use it right away!</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> &gt; py from world.test import hello_world ; hello_world(me)
Hello World!
</pre></div>
</div>
<p>Let’s say your <code class="docutils literal notranslate"><span class="pre">test.py</span></code> module had a bunch of interesting functions. You could then import them
all one by one:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">world.test</span> <span class="kn">import</span> <span class="n">hello_world</span><span class="p">,</span> <span class="n">my_func</span><span class="p">,</span> <span class="n">awesome_func</span>
</pre></div>
</div>
<p>If there were <em>a lot</em> of functions, you could instead just import <code class="docutils literal notranslate"><span class="pre">test</span></code> and get the function
from there when you need (without having to give the full <code class="docutils literal notranslate"><span class="pre">world.test</span></code> every time):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; from world import test ; test.hello_world(me
Hello World!
</pre></div>
</div>
<p>You can also <em>rename</em> stuff you import. Say for example that the module you import to already
has a function <code class="docutils literal notranslate"><span class="pre">hello_world</span></code> but we also want to use the one from <code class="docutils literal notranslate"><span class="pre">world/test.py</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">world.test</span> <span class="kn">import</span> <span class="n">hello_world</span> <span class="k">as</span> <span class="n">test_hello_world</span>
</pre></div>
</div>
<p>The form <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span> <span class="pre">...</span> <span class="pre">as</span> <span class="pre">...</span></code> renames the import.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; from world.test import hello_world as hw ; hw(me)
Hello World!
</pre></div>
</div>
<blockquote>
<div><p>Avoid renaming unless it’s to avoid a name-collistion like above - you want to make things as
easy to read as possible, and renaming adds another layer of potential confusion.</p>
</div></blockquote>
<p>In <a class="reference internal" href="Python-basic-introduction.html"><span class="doc">the basic intro to Python</span></a> we learned how to open the in-game
multi-line interpreter.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">&gt;</span> <span class="n">py</span>
<span class="n">Evennia</span> <span class="n">Interactive</span> <span class="n">Python</span> <span class="n">mode</span>
<span class="n">Python</span> <span class="mf">3.7</span><span class="o">.</span><span class="mi">1</span> <span class="p">(</span><span class="n">default</span><span class="p">,</span> <span class="n">Oct</span> <span class="mi">22</span> <span class="mi">2018</span><span class="p">,</span> <span class="mi">11</span><span class="p">:</span><span class="mi">21</span><span class="p">:</span><span class="mi">55</span><span class="p">)</span>
<span class="p">[</span><span class="n">GCC</span> <span class="mf">8.2</span><span class="o">.</span><span class="mi">0</span><span class="p">]</span> <span class="n">on</span> <span class="n">Linux</span>
<span class="p">[</span><span class="n">py</span> <span class="n">mode</span> <span class="o">-</span> <span class="n">quit</span><span class="p">()</span> <span class="n">to</span> <span class="n">exit</span><span class="p">]</span>
</pre></div>
</div>
<p>You now only need to import once to use the imported function over and over.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; from world.test import hello_world
&gt; hello_world()
Hello World!
&gt; hello_world()
Hello World!
&gt; hello_world()
Hello World!
&gt; quit()
Closing the Python console.
</pre></div>
</div>
<p>The same goes when writing code in a module - in most Python modules you will see a bunch of
imports at the top, resources that are then used by all code in that module.</p>
</div>
<div class="section" id="on-classes-and-objects">
<h2>On classes and objects<a class="headerlink" href="#on-classes-and-objects" title="Permalink to this headline"></a></h2>
<p>Now that we know about imports, let look at a real Evennia module and try to understand it.</p>
<p>Open <code class="docutils literal notranslate"><span class="pre">mygame/typeclasses/objects.py</span></code> in your text editor of choice.</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
3
4
5
6
7
8
9
10</pre></div></td><td class="code"><div class="highlight"><pre><span></span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd">module docstring</span>
<span class="sd">&quot;&quot;&quot;</span>
<span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">DefaultObject</span>
<span class="k">class</span> <span class="nc">Object</span><span class="p">(</span><span class="n">DefaultObject</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> class docstring</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="k">pass</span>
</pre></div>
</td></tr></table></div>
<div class="sidebar">
<p class="sidebar-title">Docstrings vs Comments</p>
<p>A docstring is not the same as a comment (created by <cite>#</cite>). A
docstring is not ignored by Python but is an integral part of the thing
it is documenting (the module and the class in this case).</p>
</div>
<p>The real file is much longer but we can ignore the multi-line strings (<code class="docutils literal notranslate"><span class="pre">&quot;&quot;&quot;</span> <span class="pre">...</span> <span class="pre">&quot;&quot;&quot;</span></code>). These serve
as documentation-strings, or <em>docstrings</em> for the module (at the top) and the <code class="docutils literal notranslate"><span class="pre">class</span></code> below.</p>
<p>Below the module doc string we have the import. In this case we are importing a resource
from the core <code class="docutils literal notranslate"><span class="pre">evennia</span></code> library itself. We will dive into this later, for now we just treat this
as a black box.</p>
<p>Next we have a <code class="docutils literal notranslate"><span class="pre">class</span></code> named <code class="docutils literal notranslate"><span class="pre">Object</span></code>, which <em>inherits</em> from <code class="docutils literal notranslate"><span class="pre">DefaultObject</span></code>. This class doesn’t
actually do anything on its own, its only code (except the docstring) is <code class="docutils literal notranslate"><span class="pre">pass</span></code> which means,
well, to pass and don’t do anything.</p>
<p>We will get back to this module in the <a class="reference internal" href="Learning-Typeclasses.html"><span class="doc">next lesson</span></a>. First we need to do a
little detour to understand what a ‘class’, an ‘object’ or ‘instance’ is. These are fundamental
things to understand before you can use Evennia efficiently.</p>
<div class="sidebar">
<p class="sidebar-title">OOP</p>
<p>Classes, objects, instances and inheritance are fundamental to Python. This and some
other concepts are often clumped together under the term Object-Oriented-Programming (OOP).</p>
</div>
<div class="section" id="classes-and-instances">
<h3>Classes and instances<a class="headerlink" href="#classes-and-instances" title="Permalink to this headline"></a></h3>
<p>A ‘class’ can be seen as a ‘template’ for a ‘type’ of object. The class describes the basic functionality
of everyone of that class. For example, we could have a class <code class="docutils literal notranslate"><span class="pre">Monster</span></code> which has resources for moving itself
from room to room.</p>
<p>Open a new file <code class="docutils literal notranslate"><span class="pre">mygame/typeclasses/monsters.py</span></code>. Add the following simple class:</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6
7</pre></div></td><td class="code"><div class="highlight"><pre><span></span>
<span class="k">class</span> <span class="nc">Monster</span><span class="p">:</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;Monster&quot;</span>
<span class="k">def</span> <span class="nf">move_around</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">key</span><span class="si">}</span><span class="s2"> is moving!&quot;</span><span class="p">)</span>
</pre></div>
</td></tr></table></div>
<p>Above we have defined a <code class="docutils literal notranslate"><span class="pre">Monster</span></code> class with one variable <code class="docutils literal notranslate"><span class="pre">key</span></code> (that is, the name) and one
<em>method</em> on it. A method is like a function except it sits “on” the class. It also always has
at least one argument (almost always written as <code class="docutils literal notranslate"><span class="pre">self</span></code> although you could in principle use
another name), which is a reference back to itself. So when we print <code class="docutils literal notranslate"><span class="pre">self.key</span></code> we are referring
back to the <code class="docutils literal notranslate"><span class="pre">key</span></code> on the class.</p>
<div class="sidebar">
<p class="sidebar-title">Terms</p>
<ul class="simple">
<li><p>A <cite>class</cite> is a code template describing a ‘type’ of something</p></li>
<li><p>An <cite>object</cite> is an <cite>instance</cite> of a <cite>class</cite>. Like using a mold to cast tin soldiers, one class can be <cite>instantiated</cite> into any number of object-instances.</p></li>
</ul>
</div>
<p>A class is just a template. Before it can be used, we must create an <em>instance</em> of the class. If
<code class="docutils literal notranslate"><span class="pre">Monster</span></code> is a class, then an instance is Fluffy, the individual red dragon. You instantiate
by <em>calling</em> the class, much like you would a function:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">fluffy</span> <span class="o">=</span> <span class="n">Monster</span><span class="p">()</span>
</pre></div>
</div>
<p>Let’s try it in-game (we use multi-line mode, it’s easier)</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; py
&gt; from typeclasses.monsters import Monster
&gt; fluffy = Monster()
&gt; fluffy.move_around()
Monster is moving!
</pre></div>
</div>
<p>We created an <em>instance</em> of <code class="docutils literal notranslate"><span class="pre">Monster</span></code>, which we stored in the variable <code class="docutils literal notranslate"><span class="pre">fluffy</span></code>. We then
called the <code class="docutils literal notranslate"><span class="pre">move_around</span></code> method on fluffy to get the printout.</p>
<blockquote>
<div><p>Note how we <em>didn’t</em> call the method as <code class="docutils literal notranslate"><span class="pre">fluffy.move_around(self)</span></code>. While the <code class="docutils literal notranslate"><span class="pre">self</span></code> has to be
there when defining the method, we <em>never</em> add it explicitly when we call the method (Python
will add the correct <code class="docutils literal notranslate"><span class="pre">self</span></code> for us automatically behind the scenes).</p>
</div></blockquote>
<p>Let’s create the sibling of Fluffy, Cuddly:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; cuddly = Monster()
&gt; cuddly.move_around()
Monster is moving!
</pre></div>
</div>
<p>We now have two dragons and they’ll hang around until with call <code class="docutils literal notranslate"><span class="pre">quit()</span></code> to exit this Python
instance. We can have them move as many times as we want. But no matter how many dragons we
create, they will all show the same printout since <code class="docutils literal notranslate"><span class="pre">key</span></code> is always fixed as “Monster”.</p>
<p>Let’s make the class a little more flexible:</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6
7
8</pre></div></td><td class="code"><div class="highlight"><pre><span></span>
<span class="k">class</span> <span class="nc">Monster</span><span class="p">:</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">key</span> <span class="o">=</span> <span class="n">key</span>
<span class="k">def</span> <span class="nf">move_around</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">key</span><span class="si">}</span><span class="s2"> is moving!&quot;</span><span class="p">)</span>
</pre></div>
</td></tr></table></div>
<p>The <code class="docutils literal notranslate"><span class="pre">__init__</span></code> is a special method that Python recognizes. If given, this handles extra arguments
when you instantiate a new Monster. We have it add an argument <code class="docutils literal notranslate"><span class="pre">key</span></code> that we store on <code class="docutils literal notranslate"><span class="pre">self</span></code>.</p>
<p>Now, for Evennia to see this code change, we need to reload the server. You can either do it this
way:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">&gt;</span> <span class="n">quit</span><span class="p">()</span>
<span class="n">Python</span> <span class="n">Console</span> <span class="ow">is</span> <span class="n">closing</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">reload</span>
</pre></div>
</div>
<p>Or you can use a separate terminal and restart from outside the game:</p>
<div class="sidebar">
<p class="sidebar-title">On reloading</p>
<p>Reloading with the python mode gets a little annoying since you need to redo everything
after every reload. Just keep in mind that during regular development you will not be
working this way. The in-game python mode is practical for quick fixes and experiments like
this, but actual code is normally written externally, in python modules.</p>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ evennia reload (or restart)
</pre></div>
</div>
<p>Either way you’ll need to go into <code class="docutils literal notranslate"><span class="pre">py</span></code> again:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; py
&gt; from typeclasses.monsters import Monster
fluffy = Monster(&quot;Fluffy&quot;)
fluffy.move_around()
Fluffy is moving!
</pre></div>
</div>
<p>Now we passed <code class="docutils literal notranslate"><span class="pre">&quot;Fluffy&quot;</span></code> as an argument to the class. This went into <code class="docutils literal notranslate"><span class="pre">__init__</span></code> and set <code class="docutils literal notranslate"><span class="pre">self.key</span></code>, which we
later used to print with the right name! Again, note that we didn’t include <code class="docutils literal notranslate"><span class="pre">self</span></code> when calling.</p>
</div>
<div class="section" id="what-s-so-good-about-objects">
<h3>What’s so good about objects?<a class="headerlink" href="#what-s-so-good-about-objects" title="Permalink to this headline"></a></h3>
<p>So far all we’ve seen a class do is to behave our first <code class="docutils literal notranslate"><span class="pre">hello_world</span></code> function but more complex. We
could just have made a function:</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2</pre></div></td><td class="code"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">monster_move_around</span><span class="p">(</span><span class="n">key</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">key</span><span class="si">}</span><span class="s2"> is moving!&quot;</span><span class="p">)</span>
</pre></div>
</td></tr></table></div>
<p>The difference between the function and an instance of a class (the object), is that the
object retains <em>state</em>. Once you called the function it forgets everything about what you called
it with last time. The object, on the other hand, remembers changes:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; fluffy.key = &quot;Cuddly&quot;
&gt; fluffy.move_around()
Cuddly is moving!
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">fluffy</span></code> object’s <code class="docutils literal notranslate"><span class="pre">key</span></code> was changed to “Cuddly” for as long as it’s around. This makes objects
extremely useful for representing and remembering collections of data - some of which can be other
objects in turn:</p>
<ul class="simple">
<li><p>A player character with all its stats</p></li>
<li><p>A monster with HP</p></li>
<li><p>A chest with a number of gold coins in it</p></li>
<li><p>A room with other objects inside it</p></li>
<li><p>The current policy positions of a political party</p></li>
<li><p>A rule with methods for resolving challenges or roll dice</p></li>
<li><p>A multi-dimenstional data-point for a complex economic simulation</p></li>
<li><p>And so much more!</p></li>
</ul>
</div>
<div class="section" id="classes-can-have-children">
<h3>Classes can have children<a class="headerlink" href="#classes-can-have-children" title="Permalink to this headline"></a></h3>
<p>Classes can <em>inherit</em> from each other. A “child” class will inherit everything from its “parent” class. But if
the child adds something with the same name as its parent, it will <em>override</em> whatever it got from its parent.</p>
<p>Let’s expand <code class="docutils literal notranslate"><span class="pre">mygame/typeclasses/monsters.py</span></code> with another class:</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26</pre></div></td><td class="code"><div class="highlight"><pre><span></span>
<span class="k">class</span> <span class="nc">Monster</span><span class="p">:</span>
<span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> This is a base class for Monster.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">key</span> <span class="o">=</span> <span class="n">key</span>
<span class="k">def</span> <span class="nf">move_around</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">key</span><span class="si">}</span><span class="s2"> is moving!&quot;</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Dragon</span><span class="p">(</span><span class="n">Monster</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> This is a dragon-specific monster.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="k">def</span> <span class="nf">move_around</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">key</span><span class="si">}</span><span class="s2"> flies through the air high above!&quot;</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">firebreath</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot; </span>
<span class="sd"> Let our dragon breathe fire.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">key</span><span class="si">}</span><span class="s2"> breathes fire!&quot;</span><span class="p">)</span>
</pre></div>
</td></tr></table></div>
<p>We added some docstrings for clarity. It’s always a good idea to add doc strings; you can do so also for methods,
as exemplified for the new <code class="docutils literal notranslate"><span class="pre">firebreath</span></code> method.</p>
<p>We created the new class <code class="docutils literal notranslate"><span class="pre">Dragon</span></code> but we also specified that <code class="docutils literal notranslate"><span class="pre">Monster</span></code> is the <em>parent</em> of <code class="docutils literal notranslate"><span class="pre">Dragon</span></code> but adding
the parent in parenthesis. <code class="docutils literal notranslate"><span class="pre">class</span> <span class="pre">Classname(Parent)</span></code> is the way to do this.</p>
<div class="sidebar">
<p class="sidebar-title">Multi-inheritance</p>
<p>It’s possible to add more comma-separated parents to a class. You should usually avoid
this until you <cite>really</cite> know what you are doing. A single parent will be enough for almost
every case you’ll need.</p>
</div>
<p>Let’s try out our new class. First <code class="docutils literal notranslate"><span class="pre">reload</span></code> the server and the do</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; py
&gt; from typeclasses.monsters import Dragon
&gt; smaug = Dragon(&quot;Smaug&quot;)
&gt; smaug.move_around()
Smaug flies through the air high above!
&gt; smaug.firebreath()
Smaug breathes fire!
</pre></div>
</div>
<p>Because we didn’t implement <code class="docutils literal notranslate"><span class="pre">__init__</span></code> in <code class="docutils literal notranslate"><span class="pre">Dragon</span></code>, we got the one from <code class="docutils literal notranslate"><span class="pre">Monster</span></code> instead. But since we
implemented our own <code class="docutils literal notranslate"><span class="pre">move_around</span></code> in <code class="docutils literal notranslate"><span class="pre">Dragon</span></code>, it <em>overrides</em> the one in <code class="docutils literal notranslate"><span class="pre">Monster</span></code>. And <code class="docutils literal notranslate"><span class="pre">firebreath</span></code> is only
available for <code class="docutils literal notranslate"><span class="pre">Dragon</span></code>s of course. Having that on <code class="docutils literal notranslate"><span class="pre">Monster</span></code> would not have made much sense, since not every monster
can breathe fire.</p>
<p>One can also force a class to use resources from the parent even if you are overriding some of it. This is done
with the <code class="docutils literal notranslate"><span class="pre">super()</span></code> method. Modify your <code class="docutils literal notranslate"><span class="pre">Dragon</span></code> class as follows:</p>
<div class="highlight-python notranslate"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6
7
8
9</pre></div></td><td class="code"><div class="highlight"><pre><span></span><span class="c1"># ... </span>
<span class="k">class</span> <span class="nc">Dragon</span><span class="p">(</span><span class="n">Monster</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">move_around</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="n">move_around</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;The world trembles.&quot;</span><span class="p">)</span>
<span class="c1"># ...</span>
</pre></div>
</td></tr></table></div>
<blockquote>
<div><p>Keep <code class="docutils literal notranslate"><span class="pre">Monster</span></code> and the <code class="docutils literal notranslate"><span class="pre">firebreath</span></code> method, <code class="docutils literal notranslate"><span class="pre">#</span> <span class="pre">...</span></code> indicates the rest of the code is untouched.</p>
</div></blockquote>
<p>The <code class="docutils literal notranslate"><span class="pre">super().move_around()</span></code> line means that we are calling <code class="docutils literal notranslate"><span class="pre">move_around()</span></code> on the parent of the class. So in this
case, we will call <code class="docutils literal notranslate"><span class="pre">Monster.move_around</span></code> first, before doing our own thing.</p>
<p>Now <code class="docutils literal notranslate"><span class="pre">reload</span></code> the server and then:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; py
&gt; from typeclasses.monsters import Dragon
&gt; smaug = Dragon(&quot;Smaug&quot;)
&gt; smaug.move_around()
Smaug is moving!
The world trembles.
</pre></div>
</div>
<p>We can see that <code class="docutils literal notranslate"><span class="pre">Monster.move_around()</span></code> is calls first and prints “Smaug is moving!”, followed by the extra bit
about the trembling world we added in the <code class="docutils literal notranslate"><span class="pre">Dragon</span></code> class.</p>
<p>Inheritance is very powerful because it allows you to organize and re-use code while only adding the special things
you want to change. Evennia uses this concept a lot.</p>
</div>
</div>
<div class="section" id="summary">
<h2>Summary<a class="headerlink" href="#summary" title="Permalink to this headline"></a></h2>
<p>We have created our first dragons from classes. We have learned a little about how you <em>instantiate</em> a class
into an <em>object</em>. We have seen some examples of <em>inheritance</em> and we tested to <em>override</em> a method in the parent
with one in the child class. We also used <code class="docutils literal notranslate"><span class="pre">super()</span></code> to good effect.</p>
<p>We have used pretty much raw Python so far. In the coming lessons we’ll start to look at the extra bits that Evennia
provides. But first we need to learn just where to find everything.</p>
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<p><h3><a href="../../../index.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Python Classes and objects</a><ul>
<li><a class="reference internal" href="#importing-things">Importing things</a></li>
<li><a class="reference internal" href="#on-classes-and-objects">On classes and objects</a><ul>
<li><a class="reference internal" href="#classes-and-instances">Classes and instances</a></li>
<li><a class="reference internal" href="#what-s-so-good-about-objects">What’s so good about objects?</a></li>
<li><a class="reference internal" href="#classes-can-have-children">Classes can have children</a></li>
</ul>
</li>
<li><a class="reference internal" href="#summary">Summary</a></li>
</ul>
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