<pclass="last">You are reading an old version of the Evennia documentation. <ahref="https://www.evennia.com/docs/latest/index.html">The latest version is here</a></p>.
<h1><spanclass="section-number">3. </span>Intro to using Python with Evennia<aclass="headerlink"href="#intro-to-using-python-with-evennia"title="Permalink to this headline">¶</a></h1>
<p>Time to dip our toe into some coding! Evennia is written and extended in <aclass="reference external"href="https://python.org">Python</a>. Python is a mature and professional programming language that is very fast to work with.</p>
<p>That said, even though Python is widely considered easy to learn, we can only cover the basics in these lessons. While we will hopefully get you started with the most important bits you need, you may likely need to compliment with some learning on your own. Luckily there’s a vast amount of free online learning resources available for Python. See our <aclass="reference internal"href="../../../Links.html"><spanclass="doc std std-doc">link section</span></a> for some examples.</p>
<blockquote>
<div><p>While this will be quite basic if you are an experienced developer, you may want to at least stay around for the first few sections where we cover how to run Python from inside Evennia.</p>
</div></blockquote>
<p>First, if you were quelling yourself to play the tutorial world, make sure to get your
<h2><spanclass="section-number">3.1. </span>Evennia Hello world<aclass="headerlink"href="#evennia-hello-world"title="Permalink to this headline">¶</a></h2>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> Command (or <codeclass="docutils literal notranslate"><spanclass="pre">!</span></code>, which is an alias) allows you as a superuser to execute raw Python from in-game. This is useful for quick testing. From the game’s input line, enter the following:</p>
<p>The line with <codeclass="docutils literal notranslate"><spanclass="pre">></span></code> indicates input to enter in-game, while the lines below are the
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">print(...)</span></code><em>function</em> is the basic, in-built way to output text in Python. We are sending “Hello World” as a single <em>argument</em> to this function. If we were to send multiple arguments, they’d be separated by commas.</p>
<p>The quotes <codeclass="docutils literal notranslate"><spanclass="pre">"..."</span></code> mean that you are inputting a <em>string</em> (i.e. text). You could also have used single-quotes <codeclass="docutils literal notranslate"><spanclass="pre">'...'</span></code> - Python accepts both.</p>
<blockquote>
<div><p>A third way to enter Python strings is to use triple-quotes (<codeclass="docutils literal notranslate"><spanclass="pre">"""..."""</span></code> or <codeclass="docutils literal notranslate"><spanclass="pre">'''...'''</span></code>. This is used for longer strings stretching across multiple lines. When we insert code directly to <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> like this we can only use one line though.</p>
</div></blockquote>
</section>
<sectionid="making-some-text-graphics">
<h2><spanclass="section-number">3.2. </span>Making some text ‘graphics’<aclass="headerlink"href="#making-some-text-graphics"title="Permalink to this headline">¶</a></h2>
<p>When making a text-game you will, unsurprisingly, be working a lot with text. Even if you have the occational button or even graphical element, the normal process is for the user to input commands as text and get text back. As we saw above, a piece of text is called a <em>string</em> in Python and is enclosed in either single- or double-quotes.</p>
<p>Strings can be added together:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py print("This is a " + "breaking change.")
This is a breaking change.
</pre></div>
</div>
<p>A string multiplied with a number will repeat that string as many times:</p>
<h3><spanclass="section-number">3.2.1. </span>.format()<aclass="headerlink"href="#format"title="Permalink to this headline">¶</a></h3>
<asideclass="sidebar">
<pclass="sidebar-title">Functions and Methods</p>
<ulclass="simple">
<li><p>Function: Something that performs an action when you call it with zero or more <codeclass="docutils literal notranslate"><spanclass="pre">arguments</span></code>. A function is stand-alone in a python module, like <codeclass="docutils literal notranslate"><spanclass="pre">print()</span></code></p></li>
<li><p>Method: A function that sits “on” an object. It is accessed via the <codeclass="docutils literal notranslate"><spanclass="pre">.</span></code> operator, like <codeclass="docutils literal notranslate"><spanclass="pre">obj.msg()</span></code> or, in this case, <codeclass="docutils literal notranslate"><spanclass="pre"><string>.format()</span></code>.</p></li>
</ul>
</aside>
<p>While combining different strings is useful, even more powerful is the ability to modify the contents of the string in-place. There are several ways to do this in Python and we’ll show two of them here. The first is to use the <codeclass="docutils literal notranslate"><spanclass="pre">.format</span></code><em>method</em> of the string:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py print("This is a {} idea!".format("good"))
This is a good idea!
</pre></div>
</div>
<p>A method can be thought of as a resource “on” another object. The method knows on which object it sits and can thus affect it in various ways. You access it with the period <codeclass="docutils literal notranslate"><spanclass="pre">.</span></code>. In this case, the string has a resource <codeclass="docutils literal notranslate"><spanclass="pre">format(...)</span></code> that modifies it. More specifically, it replaced the <codeclass="docutils literal notranslate"><spanclass="pre">{}</span></code> marker inside the string with the value passed to the format. You can do so many times:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py print("This is a {} idea!".format("good"))
This is a good idea!
</pre></div>
</div>
<p>or</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py print("This is the {} and {} {} idea!".format("first", "second", "great"))
This is the first and second great idea!
</pre></div>
</div>
<blockquote>
<div><p>Note the double-parenthesis at the end - the first closes the <codeclass="docutils literal notranslate"><spanclass="pre">format(...</span></code> method and the outermost closes the <codeclass="docutils literal notranslate"><spanclass="pre">print(...</span></code>. Not closing them will give you a scary <codeclass="docutils literal notranslate"><spanclass="pre">SyntaxError</span></code>. We will talk a little more about errors in the next section, for now just fix until it prints as expected.</p>
</div></blockquote>
<p>Here we passed three comma-separated strings as <em>arguments</em> to the string’s <codeclass="docutils literal notranslate"><spanclass="pre">format</span></code> method. These replaced the <codeclass="docutils literal notranslate"><spanclass="pre">{}</span></code> markers in the same order as they were given.</p>
<p>The input does not have to be strings either:</p>
<p>To separate two Python instructions on the same line, you use the semi-colon, <codeclass="docutils literal notranslate"><spanclass="pre">;</span></code>. Try this:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py a = "awesome sauce" ; print("This is {}!".format(a))
This is awesome sauce!
</pre></div>
</div>
<divclass="admonition warning">
<pclass="admonition-title">Warning</p>
<p>MUD clients and semi-colon</p>
<p>Some MUD clients use the semi-colon <codeclass="docutils literal notranslate"><spanclass="pre">;</span></code> to split client-inputs
into separate sends. If so, the above will give an error. Most clients allow you to run in ‘verbatim’ mode or to remap to use some other separator than <codeclass="docutils literal notranslate"><spanclass="pre">;</span></code>. If you still have trouble, use the Evennia web client.</p>
</div>
<p>What happened here was that we <em>assigned</em> the string <codeclass="docutils literal notranslate"><spanclass="pre">"awesome</span><spanclass="pre">sauce"</span></code> to a <em>variable</em> we chose to name <codeclass="docutils literal notranslate"><spanclass="pre">a</span></code>. In the next statement, Python remembered what <codeclass="docutils literal notranslate"><spanclass="pre">a</span></code> was and we passed that into <codeclass="docutils literal notranslate"><spanclass="pre">format()</span></code> to get the output. If you replaced the value of <codeclass="docutils literal notranslate"><spanclass="pre">a</span></code> with something else in between, <em>that</em> would be printed instead.</p>
<p>Here’s the stat-example again, moving the stats to variables (here we just set them, but in a real game they may be changed over time, or modified by circumstance):</p>
<p>The point is that even if the values of the stats change, the print() statement would not change - it just keeps pretty-printing whatever is given to it.</p>
<p>the <codeclass="docutils literal notranslate"><spanclass="pre">key=value</span></code> pairs we add are called <em>keyword arguments</em> for the <codeclass="docutils literal notranslate"><spanclass="pre">format()</span></code> method. Each named argument will go to the matching <codeclass="docutils literal notranslate"><spanclass="pre">{key}</span></code> in the string. When using keywords, the order we add them doesn’t matter. We have no <codeclass="docutils literal notranslate"><spanclass="pre">{dext}</span></code> and two <codeclass="docutils literal notranslate"><spanclass="pre">{stren}</span></code> in the string, and that works fine.</p>
</section>
<sectionid="f-strings">
<h3><spanclass="section-number">3.2.2. </span>f-strings<aclass="headerlink"href="#f-strings"title="Permalink to this headline">¶</a></h3>
<p>Using <codeclass="docutils literal notranslate"><spanclass="pre">.format()</span></code> is powerful (and there is a <aclass="reference external"href="https://www.w3schools.com/python/ref_string_format.asp">lot more</a> you can do with it). But the <em>f-string</em> can be even more convenient. An f-string looks like a normal string … except there is an <codeclass="docutils literal notranslate"><spanclass="pre">f</span></code> front of it, like this:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>f"this is now an f-string."
</pre></div>
</div>
<p>An f-string on its own is just like any other string. But let’s redo the example we did before, using an f-string:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py a = "awesome sauce" ; print(f"This is {a}!")
This is awesome sauce!
</pre></div>
</div>
<p>We insert that <codeclass="docutils literal notranslate"><spanclass="pre">a</span></code> variable directly into the f-string using <codeclass="docutils literal notranslate"><spanclass="pre">{a}</span></code>. Fewer parentheses to
<p>In modern Python code, f-strings are more often used than <codeclass="docutils literal notranslate"><spanclass="pre">.format()</span></code> but to read code you need to be aware of both.</p>
<p>We will be exploring more complex string concepts when we get to creating Commands and need to parse and understand player input.</p>
</section>
<sectionid="colored-text">
<h3><spanclass="section-number">3.2.3. </span>Colored text<aclass="headerlink"href="#colored-text"title="Permalink to this headline">¶</a></h3>
<p>Python itself knows nothing about colored text, this is an Evennia thing. Evennia supports the standard color schemes of traditional MUDs.</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py print("|rThis is red text!|n This is normal color.")
</pre></div>
</div>
<p>Adding that <codeclass="docutils literal notranslate"><spanclass="pre">|r</span></code> at the start will turn our output bright red. <codeclass="docutils literal notranslate"><spanclass="pre">|R</span></code> will make it dark red. <codeclass="docutils literal notranslate"><spanclass="pre">|n</span></code>
gives the normal text color. You can also use RGB (Red-Green-Blue) values from 0-5 (Xterm256 colors):</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py print("|043This is a blue-green color.|[530|003 Now dark blue text on orange background.")
</pre></div>
</div>
<blockquote>
<div><p>If you don’t see the expected color, your client or terminal may not support Xterm256 (or
color at all). Use the Evennia webclient.</p>
</div></blockquote>
<p>Use the commands <codeclass="docutils literal notranslate"><spanclass="pre">color</span><spanclass="pre">ansi</span></code> or <codeclass="docutils literal notranslate"><spanclass="pre">color</span><spanclass="pre">xterm</span></code> to see which colors are available. Experiment! You can also read a lot more in the <aclass="reference internal"href="../../../Concepts/Colors.html"><spanclass="doc std std-doc">Colors</span></a> documentation.</p>
</section>
</section>
<sectionid="importing-code-from-other-modules">
<h2><spanclass="section-number">3.3. </span>Importing code from other modules<aclass="headerlink"href="#importing-code-from-other-modules"title="Permalink to this headline">¶</a></h2>
<p>As we saw in the previous sections, we used <codeclass="docutils literal notranslate"><spanclass="pre">.format</span></code> to format strings and <codeclass="docutils literal notranslate"><spanclass="pre">me.msg</span></code> to access the <codeclass="docutils literal notranslate"><spanclass="pre">msg</span></code> method on <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code>. This use of the full-stop character is used to access all sorts of resources, including that in other Python modules.</p>
<p>Keep your game running, then open a text editor of your choice. If your game folder is called
<codeclass="docutils literal notranslate"><spanclass="pre">mygame</span></code>, create a new text file <codeclass="docutils literal notranslate"><spanclass="pre">test.py</span></code> in the subfolder <codeclass="docutils literal notranslate"><spanclass="pre">mygame/world</span></code>. This is how the file
<p>If you make some error (we’ll cover how to handle errors below), make sure the text looks exactly like above and then run the <codeclass="docutils literal notranslate"><spanclass="pre">reload</span></code> command in-game for your changes to take effect.</p>
<p>… So as you can see, importing <codeclass="docutils literal notranslate"><spanclass="pre">world.test</span></code> actually means importing <codeclass="docutils literal notranslate"><spanclass="pre">world/test.py</span></code>. Think of the period <codeclass="docutils literal notranslate"><spanclass="pre">.</span></code> as replacing <codeclass="docutils literal notranslate"><spanclass="pre">/</span></code> (or <codeclass="docutils literal notranslate"><spanclass="pre">\</span></code> for Windows) in your path.</p>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">.py</span></code> ending of <codeclass="docutils literal notranslate"><spanclass="pre">test.py</span></code> is never included in this “Python-path”, but <em>only</em> files with that ending can be imported this way. Where is <codeclass="docutils literal notranslate"><spanclass="pre">mygame</span></code> in that Python-path? The answer is that Evennia has already told Python that your <codeclass="docutils literal notranslate"><spanclass="pre">mygame</span></code> folder is a good place to look for imports. So we should not include <codeclass="docutils literal notranslate"><spanclass="pre">mygame</span></code> in the path - Evennia handles this for us.</p>
<p>When you import the module, the top “level” of it will execute. In this case, it will immediately
<p>You will <em>not</em> see any output this or any subsequent times! This is not a bug. Rather it is because of how Python importing works - it stores all imported modules and will avoid importing them more than once. So your <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> will only run the first time, when the module is first imported.</p>
<p>Now we see it again. The <codeclass="docutils literal notranslate"><spanclass="pre">reload</span></code> wiped the server’s memory of what was imported, so it had to import it anew. You’d have to do this every time you wanted the hello-world to show, which is not very useful.</p>
<blockquote>
<div><p>We’ll get back to more advanced ways to import code in <aclass="reference internal"href="Beginner-Tutorial-Python-classes-and-objects.html#importing-things"><spanclass="std std-doc">a later lesson</span></a> - this is an important topic. But for now, let’s press on and resolve this particular problem.</p>
</div></blockquote>
<sectionid="our-first-own-function">
<h3><spanclass="section-number">3.3.1. </span>Our first own function<aclass="headerlink"href="#our-first-own-function"title="Permalink to this headline">¶</a></h3>
<p>We want to be able to print our hello-world message at any time, not just once after a server
reload. Change your <codeclass="docutils literal notranslate"><spanclass="pre">mygame/world/test.py</span></code> file to look like this:</p>
<p>If you are coming from some other language like Javascript or C you may be familiar with variables and functions mixing cases in names, like <codeclass="docutils literal notranslate"><spanclass="pre">helloWorld()</span></code>. While you <em>could</em> choose to name things this way, it will clash with other Python code - Python standard is to use lower-case and underscores <codeclass="docutils literal notranslate"><spanclass="pre">_</span></code> for all variables and methods.</p>
</aside>
<p>As we are moving to multi-line Python code, there are some important things to remember:</p>
<ulclass="simple">
<li><p>Capitalization matters in Python. It must be <codeclass="docutils literal notranslate"><spanclass="pre">def</span></code> and not <codeclass="docutils literal notranslate"><spanclass="pre">DEF</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">hello_world()</span></code> is not the same as <codeclass="docutils literal notranslate"><spanclass="pre">Hello_World()</span></code>.</p></li>
<li><p>Indentation matters in Python. The second line must be indented or it’s not valid code. You should also use a consistent indentation length. We <em>strongly</em> recommend that you, for your own sanity’s sake, set up your editor to always indent <em>4 spaces</em> (<strong>not</strong> a single tab-character) when you press the TAB key.</p></li>
</ul>
<p>So about that function. Line 1:</p>
<ulclass="simple">
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">def</span></code> is short for “define” and defines a <em>function</em> (or a <em>method</em>, if sitting on an object). This is a <aclass="reference external"href="https://docs.python.org/2.5/ref/keywords.html">reserved Python keyword</a>; try not to use these words anywhere else.</p></li>
<li><p>A function name can not have spaces but otherwise we could have called it almost anything. We call it <codeclass="docutils literal notranslate"><spanclass="pre">hello_world</span></code>. Evennia follows <aclass="reference internal"href="../../../Coding/Evennia-Code-Style.html"><spanclass="doc std std-doc">Python’s standard naming style</span></a> with lowercase letters and underscores. We recommend you do the same.</p></li>
<li><p>The colon (<codeclass="docutils literal notranslate"><spanclass="pre">:</span></code>) at the end of line 1 indicates that the header of the function is complete.</p></li>
</ul>
<p>Line 2:</p>
<ulclass="simple">
<li><p>The indentation marks the beginning of the actual operating code of the function (the function’s <em>body</em>). If we wanted more lines to belong to this function those lines would all have to start at least at this indentation level.</p></li>
</ul>
<p>Now let’s try this out. First <codeclass="docutils literal notranslate"><spanclass="pre">reload</span></code> your game to have it pick up our updated Python module, then import it.</p>
<p>Nothing happened! That is because the function in our module won’t do anything just by importing it (this is what we wanted). It will only act when we <em>call</em> it. So we need to first import the module and then access the function within:</p>
<p>There is our “Hello World”! As mentioned earlier, use use semi-colon to put multiple Python-statements on one line. Note also the previous warning about mud-clients using the <codeclass="docutils literal notranslate"><spanclass="pre">;</span></code> to their own ends.</p>
<p>So what happened there? First we imported <codeclass="docutils literal notranslate"><spanclass="pre">world.test</span></code> as usual. But this time the ‘top level’ of the module only defined a function. It didn’t actually execute the body of that function.</p>
<p>By adding <codeclass="docutils literal notranslate"><spanclass="pre">()</span></code> to the <codeclass="docutils literal notranslate"><spanclass="pre">hello_world</span></code> function we <em>call</em> it. That is, we execute the body of the function and print our text. We can now redo this as many times as we want without having to <codeclass="docutils literal notranslate"><spanclass="pre">reload</span></code> in between:</p>
<h2><spanclass="section-number">3.4. </span>Sending text to others<aclass="headerlink"href="#sending-text-to-others"title="Permalink to this headline">¶</a></h2>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> command is a standard Python structure. We can use that here in the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command since we can se the output. It’s great for debugging and quick testing. But if you need to send a text to an actual player, <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> won’t do, because it doesn’t know <em>who</em> to send to. Try this:</p>
<p>This looks the same as the <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> result, but we are now actually messaging a specific <em>object</em>, <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code>. The <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> is a shortcut to ‘us’, the one running the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command. It is not some special Python thing, but something Evennia just makes available in the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command for convenience (<codeclass="docutils literal notranslate"><spanclass="pre">self</span></code> is an alias).</p>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> is an example of an <em>Object instance</em>. Objects are fundamental in Python and Evennia. The <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> object also contains a lot of useful resources for doing things with that object. We access those resources with ‘<codeclass="docutils literal notranslate"><spanclass="pre">.</span></code>’.</p>
<p>One such resource is <codeclass="docutils literal notranslate"><spanclass="pre">msg</span></code>, which works like <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> except it sends the text to the object it
is attached to. So if we, for example, had an object <codeclass="docutils literal notranslate"><spanclass="pre">you</span></code>, doing <codeclass="docutils literal notranslate"><spanclass="pre">you.msg(...)</span></code> would send a message to the object <codeclass="docutils literal notranslate"><spanclass="pre">you</span></code>.</p>
<p>For now, <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">me.msg</span></code> behaves the same, just remember that <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> is mainly used for
debugging and <codeclass="docutils literal notranslate"><spanclass="pre">.msg()</span></code> will be more useful for you in the future.</p>
</section>
<sectionid="parsing-python-errors">
<h2><spanclass="section-number">3.5. </span>Parsing Python errors<aclass="headerlink"href="#parsing-python-errors"title="Permalink to this headline">¶</a></h2>
<p>Let’s try this new text-sending in the function we just created. Go back to
your <codeclass="docutils literal notranslate"><spanclass="pre">test.py</span></code> file and Replace the function with this instead:</p>
<p>Save your file and <codeclass="docutils literal notranslate"><spanclass="pre">reload</span></code> your server to tell Evennia to re-import new code,
<p>In regular use, tracebacks will often appear in the log rather than
in the game. Use <codeclass="docutils literal notranslate"><spanclass="pre">evennia</span><spanclass="pre">--log</span></code> to view the log in the terminal. Make
sure to scroll back if you expect an error and don’t see it. Use
<codeclass="docutils literal notranslate"><spanclass="pre">Ctrl-C</span></code> (or <codeclass="docutils literal notranslate"><spanclass="pre">Cmd-C</span></code> on Mac) to exit the log-view.</p>
</aside>
<p>This is called a <em>traceback</em>. Python’s errors are very friendly and will most of the time tell you exactly what and where things go wrong. It’s important that you learn to parse tracebacks so you know how to fix your code.</p>
<p>A traceback is to be read from the <em>bottom up</em>:</p>
<ulclass="simple">
<li><p>(line 3) An error of type <codeclass="docutils literal notranslate"><spanclass="pre">NameError</span></code> is the problem …</p></li>
<li><p>(line 3) … more specifically it is due to the variable <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> not being defined.</p></li>
<li><p>(line 2) This happened on the line <codeclass="docutils literal notranslate"><spanclass="pre">me.msg("Hello</span><spanclass="pre">world!")</span></code> …</p></li>
<li><p>(line 1) … which is on line <codeclass="docutils literal notranslate"><spanclass="pre">2</span></code> of the file <codeclass="docutils literal notranslate"><spanclass="pre">./world/test.py</span></code>.</p></li>
</ul>
<p>In our case the traceback is short. There may be many more lines above it, tracking just how
different modules called each other until the program got to the faulty line. That can
sometimes be useful information, but reading from the bottom is always a good start.</p>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">NameError</span></code> we see here is due to a module being its own isolated thing. It knows nothing about the environment into which it is imported. It knew what <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> is because that is a special <aclass="reference external"href="https://docs.python.org/2.5/ref/keywords.html">reserved Python keyword</a>. But <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> is <em>not</em> such a reserved word (as mentioned, it’s just something Evennia came up with for convenience in the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command). As far as the module is concerned <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> is an unfamiliar name, appearing out of nowhere. Hence the <codeclass="docutils literal notranslate"><spanclass="pre">NameError</span></code>.</p>
</section>
<sectionid="passing-arguments-to-functions">
<h2><spanclass="section-number">3.6. </span>Passing arguments to functions<aclass="headerlink"href="#passing-arguments-to-functions"title="Permalink to this headline">¶</a></h2>
<p>We know that <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> exists at the point when we run the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command, because we can do <codeclass="docutils literal notranslate"><spanclass="pre">py</span><spanclass="pre">me.msg("Hello</span><spanclass="pre">World!")</span></code> with no problem. So let’s <em>pass</em> that me along to the function so it knows what it should be. Go back to your <codeclass="docutils literal notranslate"><spanclass="pre">test.py</span></code> and change it to this:</p>
<p>We now added an <em>argument</em> to the function. We could have named it anything. Whatever <codeclass="docutils literal notranslate"><spanclass="pre">who</span></code> is, we will call a method <codeclass="docutils literal notranslate"><spanclass="pre">.msg()</span></code> on it.</p>
<p>As usual, <codeclass="docutils literal notranslate"><spanclass="pre">reload</span></code> the server to make sure the new code is available.</p>
<p>Now it worked. We <em>passed</em><codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> to our function. It will appear inside the function renamed as <codeclass="docutils literal notranslate"><spanclass="pre">who</span></code> and now the function works and prints as expected. Note how the <codeclass="docutils literal notranslate"><spanclass="pre">hello_world</span></code> function doesn’t care <em>what</em> you pass into it as long as it has a <codeclass="docutils literal notranslate"><spanclass="pre">.msg()</span></code> method on it. So you could reuse this function over and over for other suitable targets.</p>
<blockquote>
<div><p><strong>Extra Credit:</strong> As an exercise, try to pass something else into <codeclass="docutils literal notranslate"><spanclass="pre">hello_world</span></code>. Try for example
to pass the number <codeclass="docutils literal notranslate"><spanclass="pre">5</span></code> or the string <codeclass="docutils literal notranslate"><spanclass="pre">"foo"</span></code>. You’ll get errors telling you that they don’t have
the attribute <codeclass="docutils literal notranslate"><spanclass="pre">msg</span></code>. They don’t care about <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> itself not being a string or a number. If you are
familiar with other programming languages (especially C/Java) you may be tempted to start <em>validating</em><codeclass="docutils literal notranslate"><spanclass="pre">who</span></code> to make sure it’s of the right type before you send it. This is usually not recommended in Python. Python philosophy is to <aclass="reference external"href="https://docs.python.org/2/tutorial/errors.html">handle</a> the error if it happens
rather than to add a lot of code to prevent it from happening. See <aclass="reference external"href="https://en.wikipedia.org/wiki/Duck_typing">duck typing</a>
and the concept of <em>Leap before you Look</em>.</p>
</div></blockquote>
</section>
<sectionid="finding-others-to-send-to">
<h2><spanclass="section-number">3.7. </span>Finding others to send to<aclass="headerlink"href="#finding-others-to-send-to"title="Permalink to this headline">¶</a></h2>
<p>Let’s wrap up this first Python <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> crash-course by finding someone else to send to.</p>
<p>In Evennia’s <codeclass="docutils literal notranslate"><spanclass="pre">contrib/</span></code> folder (<codeclass="docutils literal notranslate"><spanclass="pre">evennia/contrib/tutorial_examples/mirror.py</span></code>) is a handy little object called the <codeclass="docutils literal notranslate"><spanclass="pre">TutorialMirror</span></code>. The mirror will echo whatever is being sent to it to
the room it is in.</p>
<p>On the game command-line, let’s create a mirror:</p>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">create</span></code> command was first used to create boxes in the
<aclass="reference internal"href="Beginner-Tutorial-Building-Quickstart.html"><spanclass="doc std std-doc">Building Stuff</span></a> tutorial. You should now recognize
that it uses a “python-path” to tell Evennia where to load the mirror’s code from.</p>
<p>What you are seeing is actually your own avatar in the game, the same thing that is available as <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> in the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command.</p>
<p>What we are aiming for now is the equivalent of <codeclass="docutils literal notranslate"><spanclass="pre">mirror.msg("Mirror</span><spanclass="pre">Mirror</span><spanclass="pre">on</span><spanclass="pre">the</span><spanclass="pre">wall")</span></code>. But the first thing that comes to mind will not work:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py mirror.msg("Mirror, Mirror on the wall ...")
NameError: name 'mirror' is not defined.
</pre></div>
</div>
<p>This is not surprising: Python knows nothing about “mirrors” or locations or anything. The <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> we’ve been using is, as mentioned, just a convenient thing the Evennia devs makes available to the <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command. They couldn’t possibly predict that you wanted to talk to mirrors.</p>
<p>Instead we will need to <em>search</em> for that <codeclass="docutils literal notranslate"><spanclass="pre">mirror</span></code> object before we can send to it. Make sure you are in the same location as the mirror and try:</p>
<p><codeclass="docutils literal notranslate"><spanclass="pre">me.search("name")</span></code> will, by default, search and <em>return</em> an object with the given name found in <em>the same location</em> as the <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> object is. If it can’t find anything you’ll see an error.</p>
<asideclass="sidebar">
<pclass="sidebar-title">Function returns</p>
<p>Whereas a function like <codeclass="docutils literal notranslate"><spanclass="pre">print</span></code> only prints its arguments, it’s very common
for functions/methods to <codeclass="docutils literal notranslate"><spanclass="pre">return</span></code> a result of some kind. Think of the function
as a machine - you put something in and out comes a result you can use. In the case of <codeclass="docutils literal notranslate"><spanclass="pre">me.search</span></code>, it will perform a database search and spit out the object it finds.</p>
<p>Wanting to find things in the same location is very common, but as we continue we’ll
find that Evennia provides ample tools for tagging, searching and finding things from all over your game.</p>
<p>Now that we know how to find the ‘mirror’ object, we just need to use that instead of <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code>!</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> py mirror = self.search("mirror") ; mirror.msg("Mirror, Mirror on the wall ...")
mirror echoes back to you:
"Mirror, Mirror on the wall ..."
</pre></div>
</div>
<p>The mirror is useful for testing because its <codeclass="docutils literal notranslate"><spanclass="pre">.msg</span></code> method just echoes whatever is sent to it back to the room. More common would be to talk to a player character, in which case the text you sent would have appeared in their game client.</p>
</section>
<sectionid="multi-line-py">
<h2><spanclass="section-number">3.8. </span>Multi-line py<aclass="headerlink"href="#multi-line-py"title="Permalink to this headline">¶</a></h2>
<p>So far we have use <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> in single-line mode, using <codeclass="docutils literal notranslate"><spanclass="pre">;</span></code> to separate multiple inputs. This is very convenient when you want to do some quick testing. But you can also start a full multi-line Python interactive interpreter inside Evennia.</p>
<p>Note that we didn’t need to put <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> in front now. The system will also echo your input (that’s the bit after the <codeclass="docutils literal notranslate"><spanclass="pre">>>></span></code>). For brevity in this tutorual we’ll turn the echo off. First exit <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> and then start again with the <codeclass="docutils literal notranslate"><spanclass="pre">/noecho</span></code> flag.</p>
Evennia Interactive Python mode (no echoing of prompts)
Python 3.11.0 (default, Nov 22 2022, 11:21:56)
[GCC 8.2.0] on Linux
[py mode - quit() to exit]
</pre></div>
</div>
<asideclass="sidebar">
<pclass="sidebar-title">interactive py</p>
<ulclass="simple">
<li><p>Start with <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code>.</p></li>
<li><p>Use <codeclass="docutils literal notranslate"><spanclass="pre">py/noecho</span></code> if you don’t want your input to be echoed for every line.</p></li>
<li><p><em>All</em> your inputs will now be interpreted as Python code.</p></li>
<li><p>Exit with <codeclass="docutils literal notranslate"><spanclass="pre">quit()</span></code>.</p></li>
</ul>
</aside>
<p>We can now enter multi-line Python code:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> a = "Test"
<li><p>Definining a function with <codeclass="docutils literal notranslate"><spanclass="pre">def</span></code> means we are starting a new code block. Python works so that you mark the content
of the block with indention. So the next line must be manually indented (4 spaces is a good standard) in order
for Python to know it’s part of the function body.</p></li>
<li><p>We expand the <codeclass="docutils literal notranslate"><spanclass="pre">hello_world</span></code> function with another argument <codeclass="docutils literal notranslate"><spanclass="pre">txt</span></code>. This allows us to send any text, not just
“Hello World” over and over.</p></li>
<li><p>To tell <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> that no more lines will be added to the function body, we end with an empty input. When the normal prompt returns, we know we are done.</p></li>
</ul>
<p>Now we have defined a new function. Let’s try it out:</p>
<divclass="highlight-none notranslate"><divclass="highlight"><pre><span></span>> hello_world(me, "Hello world to me!")
Hello world to me!
</pre></div>
</div>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> is still available to us, so we pass that as the <codeclass="docutils literal notranslate"><spanclass="pre">who</span></code> argument, along with a little longer
string. Let’s combine this with searching for the mirror.</p>
<h2><spanclass="section-number">3.9. </span>Other ways to test Python code<aclass="headerlink"href="#other-ways-to-test-python-code"title="Permalink to this headline">¶</a></h2>
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> command is very powerful for experimenting with Python in-game. It’s great for quick testing.
But you are still limited to working over telnet or the webclient, interfaces that doesn’t know anything
about Python per-se.</p>
<p>Outside the game, go to the terminal where you ran Evennia (or any terminal where the <codeclass="docutils literal notranslate"><spanclass="pre">evennia</span></code> command
is available).</p>
<ulclass="simple">
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">cd</span></code> to your game dir.</p></li>
<p>A Python shell opens. This works like <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> did inside the game, with the exception that you don’t have
<codeclass="docutils literal notranslate"><spanclass="pre">me</span></code> available out of the box. If you want <codeclass="docutils literal notranslate"><spanclass="pre">me</span></code>, you need to first find yourself:</p>
> me = evennia.search_object("YourChar")[0]
</pre></div>
</div>
<p>Here we make use of one of evennia’s search functions, available by importing <codeclass="docutils literal notranslate"><spanclass="pre">evennia</span></code> directly.
We will cover more advanced searching later, but suffice to say, you put your own character name instead of
“YourChar” above.</p>
<blockquote>
<div><p>The <codeclass="docutils literal notranslate"><spanclass="pre">[0]</span></code> at the end is because <codeclass="docutils literal notranslate"><spanclass="pre">.search_object</span></code> returns a list of objects and we want to
get at the first of them (counting starts from 0).</p>
</div></blockquote>
<p>Use <codeclass="docutils literal notranslate"><spanclass="pre">Ctrl-D</span></code> (<codeclass="docutils literal notranslate"><spanclass="pre">Cmd-D</span></code> on Mac) or <codeclass="docutils literal notranslate"><spanclass="pre">quit()</span></code> to exit the Python console.</p>
</section>
<sectionid="ipython">
<h2><spanclass="section-number">3.10. </span>ipython<aclass="headerlink"href="#ipython"title="Permalink to this headline">¶</a></h2>
<p>The default Python shell is quite limited and ugly. It’s <em>highly</em> recommended to install <codeclass="docutils literal notranslate"><spanclass="pre">ipython</span></code> instead. This
is a much nicer, third-party Python interpreter with colors and many usability improvements.</p>
<p>If <codeclass="docutils literal notranslate"><spanclass="pre">ipython</span></code> is installed, <codeclass="docutils literal notranslate"><spanclass="pre">evennia</span><spanclass="pre">shell</span></code> will use it automatically.</p>
IPython 7.4.0 -- An enhanced Interactive Python. Type '?' for help
In [1]: You now have Tab-completion:
> import evennia
> evennia.<TAB>
</pre></div>
</div>
<p>That is, enter <codeclass="docutils literal notranslate"><spanclass="pre">evennia.</span></code> and then press the TAB key - you will be given a list of all the resources
available on the <codeclass="docutils literal notranslate"><spanclass="pre">evennia</span></code> object. This is great for exploring what Evennia has to offer. For example,
use your arrow keys to scroll to <codeclass="docutils literal notranslate"><spanclass="pre">search_object()</span></code> to fill it in.</p>
<p>Adding a <codeclass="docutils literal notranslate"><spanclass="pre">?</span></code> and pressing return will give you the full documentation for <codeclass="docutils literal notranslate"><spanclass="pre">.search_object</span></code>. Use <codeclass="docutils literal notranslate"><spanclass="pre">??</span></code> if you
want to see the entire source code.</p>
<p>As for the normal python interpreter, use <codeclass="docutils literal notranslate"><spanclass="pre">Ctrl-D</span></code>/<codeclass="docutils literal notranslate"><spanclass="pre">Cmd-D</span></code> or <codeclass="docutils literal notranslate"><spanclass="pre">quit()</span></code> to exit ipython.</p>
<divclass="admonition important">
<pclass="admonition-title">Important</p>
<p>Persistent code</p>
<p>Common for both <codeclass="docutils literal notranslate"><spanclass="pre">py</span></code> and <codeclass="docutils literal notranslate"><spanclass="pre">python</span></code>/<codeclass="docutils literal notranslate"><spanclass="pre">ipython</span></code> is that the code you write is not persistent - it will
be gone after you shut down the interpreter (but ipython will remember your input history). For making long-lasting
Python code, we need to save it in a Python module, like we did for <codeclass="docutils literal notranslate"><spanclass="pre">world/test.py</span></code>.</p>
</div>
</section>
<sectionid="conclusions">
<h2><spanclass="section-number">3.11. </span>Conclusions<aclass="headerlink"href="#conclusions"title="Permalink to this headline">¶</a></h2>
<p>This covers quite a lot of basic Python usage. We printed and formatted strings, defined our own
first function, fixed an error and even searched and talked to a mirror! Being able to access
python inside and outside of the game is an important skill for testing and debugging, but in
practice you will be writing most your code in Python modules.</p>
<p>To that end we also created a first new Python module in the <codeclass="docutils literal notranslate"><spanclass="pre">mygame/</span></code> game dir, then imported and used it. Now let’s look at the rest of the stuff you’ve got going on inside that <codeclass="docutils literal notranslate"><spanclass="pre">mygame/</span></code> folder …</p>
<pclass="last">You are reading an old version of the Evennia documentation. <ahref="https://www.evennia.com/docs/latest/index.html">The latest version is here</a></p>.
