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<li><a class="reference internal" href="#">Parsing command arguments, theory and best practices</a><ul>
<li><a class="reference internal" href="#what-are-command-arguments">What are command arguments?</a></li>
<li><a class="reference internal" href="#working-with-strings">Working with strings</a><ul>
<li><a class="reference internal" href="#self-args">self.args</a></li>
<li><a class="reference internal" href="#stripping">Stripping</a></li>
<li><a class="reference internal" href="#convert-arguments-to-numbers">Convert arguments to numbers</a></li>
<li><a class="reference internal" href="#working-with-several-arguments">Working with several arguments</a></li>
<li><a class="reference internal" href="#optional-arguments">Optional arguments</a></li>
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<section class="tex2jax_ignore mathjax_ignore" id="parsing-command-arguments-theory-and-best-practices">
<h1>Parsing command arguments, theory and best practices<a class="headerlink" href="#parsing-command-arguments-theory-and-best-practices" title="Permalink to this headline"></a></h1>
<p>This tutorial will elaborate on the many ways one can parse command arguments. The first step after
<a class="reference internal" href="Beginner-Tutorial/Part1/Beginner-Tutorial-Adding-Commands.html"><span class="doc std std-doc">adding a command</span></a> usually is to parse its arguments. There are lots of
ways to do it, but some are indeed better than others and this tutorial will try to present them.</p>
<p>If you’re a Python beginner, this tutorial might help you a lot. If you’re already familiar with
Python syntax, this tutorial might still contain useful information. There are still a lot of
things I find in the standard library that come as a surprise, though they were there all along.
This might be true for others.</p>
<p>In this tutorial we will:</p>
<ul class="simple">
<li><p>Parse arguments with numbers.</p></li>
<li><p>Parse arguments with delimiters.</p></li>
<li><p>Take a look at optional arguments.</p></li>
<li><p>Parse argument containing object names.</p></li>
</ul>
<section id="what-are-command-arguments">
<h2>What are command arguments?<a class="headerlink" href="#what-are-command-arguments" title="Permalink to this headline"></a></h2>
<p>I’m going to talk about command arguments and parsing a lot in this tutorial. So let’s be sure we
talk about the same thing before going any further:</p>
<blockquote>
<div><p>A command is an Evennia object that handles specific user input.</p>
</div></blockquote>
<p>For instance, the default <code class="docutils literal notranslate"><span class="pre">look</span></code> is a command. After having created your Evennia game, and
connected to it, you should be able to type <code class="docutils literal notranslate"><span class="pre">look</span></code> to see what’s around. In this context, <code class="docutils literal notranslate"><span class="pre">look</span></code> is
a command.</p>
<blockquote>
<div><p>Command arguments are additional text passed after the command.</p>
</div></blockquote>
<p>Following the same example, you can type <code class="docutils literal notranslate"><span class="pre">look</span> <span class="pre">self</span></code> to look at yourself. In this context, <code class="docutils literal notranslate"><span class="pre">self</span></code>
is the text specified after <code class="docutils literal notranslate"><span class="pre">look</span></code>. <code class="docutils literal notranslate"><span class="pre">&quot;</span> <span class="pre">self&quot;</span></code> is the argument to the <code class="docutils literal notranslate"><span class="pre">look</span></code> command.</p>
<p>Part of our task as a game developer is to connect user inputs (mostly commands) with actions in the
game. And most of the time, entering commands is not enough, we have to rely on arguments for
specifying actions with more accuracy.</p>
<p>Take the <code class="docutils literal notranslate"><span class="pre">say</span></code> command. If you couldn’t specify what to say as a command argument (<code class="docutils literal notranslate"><span class="pre">say</span> <span class="pre">hello!</span></code>),
you would have trouble communicating with others in the game. One would need to create a different
command for every kind of word or sentence, which is, of course, not practical.</p>
<p>Last thing: what is parsing?</p>
<blockquote>
<div><p>In our case, parsing is the process by which we convert command arguments into something we can work with.</p>
</div></blockquote>
<p>We don’t usually use the command argument as is (which is just text, of type <code class="docutils literal notranslate"><span class="pre">str</span></code> in Python). We
need to extract useful information. We might want to ask the user for a number, or the name of
another character present in the same room. We’re going to see how to do all that now.</p>
</section>
<section id="working-with-strings">
<h2>Working with strings<a class="headerlink" href="#working-with-strings" title="Permalink to this headline"></a></h2>
<p>In object terms, when you write a command in Evennia (when you write the Python class), the
arguments are stored in the <code class="docutils literal notranslate"><span class="pre">args</span></code> attribute. Which is to say, inside your <code class="docutils literal notranslate"><span class="pre">func</span></code> method, you can
access the command arguments in <code class="docutils literal notranslate"><span class="pre">self.args</span></code>.</p>
<section id="self-args">
<h3>self.args<a class="headerlink" href="#self-args" title="Permalink to this headline"></a></h3>
<p>To begin with, look at this example:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CmdTest</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Test command.</span>
<span class="sd"> Syntax:</span>
<span class="sd"> test [argument]</span>
<span class="sd"> Enter any argument after test.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;test&quot;</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You have entered: </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="si">}</span><span class="s2">.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>If you add this command and test it, you will receive exactly what you have entered without any
parsing:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">&gt;</span> <span class="n">test</span> <span class="n">Whatever</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="n">Whatever</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">test</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="o">.</span>
</pre></div>
</div>
<blockquote>
<div><p>The lines starting with <code class="docutils literal notranslate"><span class="pre">&gt;</span></code> indicate what you enter into your client. The other lines are what
you receive from the game server.</p>
</div></blockquote>
<p>Notice two things here:</p>
<ol class="simple">
<li><p>The left space between our command key (“test”, here) and our command argument is not removed.
That’s why there are two spaces in our output at line 2. Try entering something like “testok”.</p></li>
<li><p>Even if you don’t enter command arguments, the command will still be called with an empty string
in <code class="docutils literal notranslate"><span class="pre">self.args</span></code>.</p></li>
</ol>
<p>Perhaps a slight modification to our code would be appropriate to see what’s happening. We will
force Python to display the command arguments as a debug string using a little shortcut.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CmdTest</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Test command.</span>
<span class="sd"> Syntax:</span>
<span class="sd"> test [argument]</span>
<span class="sd"> Enter any argument after test.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;test&quot;</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You have entered: </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="si">!r}</span><span class="s2">.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>The only line we have changed is the last one, and we have added <code class="docutils literal notranslate"><span class="pre">!r</span></code> between our braces to tell
Python to print the debug version of the argument (the repr-ed version). Let’s see the result:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">&gt;</span> <span class="n">test</span> <span class="n">Whatever</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s1">&#39; Whatever&#39;</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">test</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s1">&#39;&#39;</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">test</span> <span class="n">And</span> <span class="n">something</span> <span class="k">with</span> <span class="s1">&#39;?</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s2">&quot; And something with &#39;?&quot;</span><span class="o">.</span>
</pre></div>
</div>
<p>This displays the string in a way you could see in the Python interpreter. It might be easier to
read… to debug, anyway.</p>
<p>I insist so much on that point because it’s crucial: the command argument is just a string (of type
<code class="docutils literal notranslate"><span class="pre">str</span></code>) and we will use this to parse it. What you will see is mostly not Evennia-specific, it’s
Python-specific and could be used in any other project where you have the same need.</p>
</section>
<section id="stripping">
<h3>Stripping<a class="headerlink" href="#stripping" title="Permalink to this headline"></a></h3>
<p>As you’ve seen, our command arguments are stored with the space. And the space between the command
and the arguments is often of no importance.</p>
<blockquote>
<div><p>Why is it ever there?</p>
</div></blockquote>
<p>Evennia will try its best to find a matching command. If the user enters your command key with
arguments (but omits the space), Evennia will still be able to find and call the command. You might
have seen what happened if the user entered <code class="docutils literal notranslate"><span class="pre">testok</span></code>. In this case, <code class="docutils literal notranslate"><span class="pre">testok</span></code> could very well be a
command (Evennia checks for that) but seeing none, and because there’s a <code class="docutils literal notranslate"><span class="pre">test</span></code> command, Evennia
calls it with the arguments <code class="docutils literal notranslate"><span class="pre">&quot;ok&quot;</span></code>.</p>
<p>But most of the time, we don’t really care about this left space, so you will often see code to
remove it. There are different ways to do it in Python, but a command use case is the <code class="docutils literal notranslate"><span class="pre">strip</span></code>
method on <code class="docutils literal notranslate"><span class="pre">str</span></code> and its cousins, <code class="docutils literal notranslate"><span class="pre">lstrip</span></code> and <code class="docutils literal notranslate"><span class="pre">rstrip</span></code>.</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">strip</span></code>: removes one or more characters (either spaces or other characters) from both ends of the
string.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">lstrip</span></code>: same thing but only removes from the left end (left strip) of the string.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">rstrip</span></code>: same thing but only removes from the right end (right strip) of the string.</p></li>
</ul>
<p>Some Python examples might help:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="s1">&#39; this is &#39;</span><span class="o">.</span><span class="n">strip</span><span class="p">()</span> <span class="c1"># remove spaces by default</span>
<span class="go">&#39;this is&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="s2">&quot; What if I&#39;m right? &quot;</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span> <span class="c1"># strip spaces from the left</span>
<span class="go">&quot;What if I&#39;m right? &quot;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="s1">&#39;Looks good to me...&#39;</span><span class="o">.</span><span class="n">strip</span><span class="p">(</span><span class="s1">&#39;.&#39;</span><span class="p">)</span> <span class="c1"># removes &#39;.&#39;</span>
<span class="go">&#39;Looks good to me&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="s1">&#39;&quot;Now, what is it?&quot;&#39;</span><span class="o">.</span><span class="n">strip</span><span class="p">(</span><span class="s1">&#39;&quot;?&#39;</span><span class="p">)</span> <span class="c1"># removes &#39;&quot;&#39; and &#39;?&#39; from both ends</span>
<span class="go">&#39;Now, what is it&#39;</span>
</pre></div>
</div>
<p>Usually, since we don’t need the space separator, but still want our command to work if there’s no
separator, we call <code class="docutils literal notranslate"><span class="pre">lstrip</span></code> on the command arguments:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">CmdTest</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Test command.</span>
<span class="sd"> Syntax:</span>
<span class="sd"> test [argument]</span>
<span class="sd"> Enter any argument after test.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;test&quot;</span>
<span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;Parse arguments, just strip them.&quot;&quot;&quot;</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You have entered: </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="si">!r}</span><span class="s2">.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<blockquote>
<div><p>We are now beginning to override the command’s <code class="docutils literal notranslate"><span class="pre">parse</span></code> method, which is typically useful just for
argument parsing. This method is executed before <code class="docutils literal notranslate"><span class="pre">func</span></code> and so <code class="docutils literal notranslate"><span class="pre">self.args</span></code> in <code class="docutils literal notranslate"><span class="pre">func()</span></code> will contain
our <code class="docutils literal notranslate"><span class="pre">self.args.lstrip()</span></code>.</p>
</div></blockquote>
<p>Let’s try it:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">&gt;</span> <span class="n">test</span> <span class="n">Whatever</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s1">&#39;Whatever&#39;</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">test</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s1">&#39;&#39;</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">test</span> <span class="n">And</span> <span class="n">something</span> <span class="k">with</span> <span class="s1">&#39;?</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s2">&quot;And something with &#39;?&quot;</span><span class="o">.</span>
<span class="o">&gt;</span> <span class="n">test</span> <span class="n">And</span> <span class="n">something</span> <span class="k">with</span> <span class="n">lots</span> <span class="n">of</span> <span class="n">spaces</span>
<span class="n">You</span> <span class="n">have</span> <span class="n">entered</span><span class="p">:</span> <span class="s1">&#39;And something with lots of spaces&#39;</span><span class="o">.</span>
</pre></div>
</div>
<p>Spaces at the end of the string are kept, but all spaces at the beginning are removed:</p>
<blockquote>
<div><p><code class="docutils literal notranslate"><span class="pre">strip</span></code>, <code class="docutils literal notranslate"><span class="pre">lstrip</span></code> and <code class="docutils literal notranslate"><span class="pre">rstrip</span></code> without arguments will strip spaces, line breaks and other common
separators. You can specify one or more characters as a parameter. If you specify more than one
character, all of them will be stripped from your original string.</p>
</div></blockquote>
</section>
<section id="convert-arguments-to-numbers">
<h3>Convert arguments to numbers<a class="headerlink" href="#convert-arguments-to-numbers" title="Permalink to this headline"></a></h3>
<p>As pointed out, <code class="docutils literal notranslate"><span class="pre">self.args</span></code> is a string (of type <code class="docutils literal notranslate"><span class="pre">str</span></code>). What if we want the user to enter a
number?</p>
<p>Let’s take a very simple example: creating a command, <code class="docutils literal notranslate"><span class="pre">roll</span></code>, that allows to roll a six-sided die.
The player has to guess the number, specifying the number as argument. To win, the player has to
match the number with the die. Let’s see an example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>&gt; roll 3
You roll a die. It lands on the number 4.
You played 3, you have lost.
&gt; dice 1
You roll a die. It lands on the number 2.
You played 1, you have lost.
&gt; dice 1
You roll a die. It lands on the number 1.
You played 1, you have won!
</pre></div>
</div>
<p>If that’s your first command, it’s a good opportunity to try to write it. A command with a simple
and finite role always is a good starting choice. Here’s how we could (first) write it… but it
won’t work as is, I warn you:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">random</span> <span class="kn">import</span> <span class="n">randint</span>
<span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">Command</span>
<span class="k">class</span> <span class="nc">CmdRoll</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Play random, enter a number and try your luck.</span>
<span class="sd"> Usage:</span>
<span class="sd"> roll &lt;number&gt;</span>
<span class="sd"> Enter a valid number as argument. A random die will be rolled and you</span>
<span class="sd"> will win if you have specified the correct number.</span>
<span class="sd"> Example:</span>
<span class="sd"> roll 3</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;roll&quot;</span>
<span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;Convert the argument to a number.&quot;&quot;&quot;</span>
<span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Roll a random die</span>
<span class="n">figure</span> <span class="o">=</span> <span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span> <span class="c1"># return a pseudo-random number between 1 and 6, including both</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You roll a die. It lands on the number </span><span class="si">{</span><span class="n">figure</span><span class="si">}</span><span class="s2">.&quot;</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span> <span class="o">==</span> <span class="n">figure</span><span class="p">:</span> <span class="c1"># THAT WILL BREAK!</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="si">}</span><span class="s2">, you have won!&quot;</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="si">}</span><span class="s2">, you have lost.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>If you try this code, Python will complain that you try to compare a number with a string: <code class="docutils literal notranslate"><span class="pre">figure</span></code>
is a number and <code class="docutils literal notranslate"><span class="pre">self.args</span></code> is a string and can’t be compared as-is in Python. Python doesn’t do
“implicit converting” as some languages do. By the way, this might be annoying sometimes, and other
times you will be glad it tries to encourage you to be explicit rather than implicit about what to
do. This is an ongoing debate between programmers. Let’s move on!</p>
<p>So we need to convert the command argument from a <code class="docutils literal notranslate"><span class="pre">str</span></code> into an <code class="docutils literal notranslate"><span class="pre">int</span></code>. There are a few ways to do
it. But the proper way is to try to convert and deal with the <code class="docutils literal notranslate"><span class="pre">ValueError</span></code> Python exception.</p>
<p>Converting a <code class="docutils literal notranslate"><span class="pre">str</span></code> into an <code class="docutils literal notranslate"><span class="pre">int</span></code> in Python is extremely simple: just use the <code class="docutils literal notranslate"><span class="pre">int</span></code> function, give it
the string and it returns an integer, if it could. If it can’t, it will raise <code class="docutils literal notranslate"><span class="pre">ValueError</span></code>. So
we’ll need to catch that. However, we also have to indicate to Evennia that, should the number be
invalid, no further parsing should be done. Here’s a new attempt at our command with this
converting:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">random</span> <span class="kn">import</span> <span class="n">randint</span>
<span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">Command</span><span class="p">,</span> <span class="n">InterruptCommand</span>
<span class="k">class</span> <span class="nc">CmdRoll</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Play random, enter a number and try your luck.</span>
<span class="sd"> Usage:</span>
<span class="sd"> roll &lt;number&gt;</span>
<span class="sd"> Enter a valid number as argument. A random die will be rolled and you</span>
<span class="sd"> will win if you have specified the correct number.</span>
<span class="sd"> Example:</span>
<span class="sd"> roll 3</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;roll&quot;</span>
<span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;Convert the argument to number if possible.&quot;&quot;&quot;</span>
<span class="n">args</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="c1"># Convert to int if possible</span>
<span class="c1"># If not, raise InterruptCommand. Evennia will catch this</span>
<span class="c1"># exception and not call the &#39;func&#39; method.</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">entered</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">args</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="si">}</span><span class="s2"> is not a valid number.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Roll a random die</span>
<span class="n">figure</span> <span class="o">=</span> <span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span> <span class="c1"># return a pseudo-random number between 1 and 6, including both</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You roll a die. It lands on the number </span><span class="si">{</span><span class="n">figure</span><span class="si">}</span><span class="s2">.&quot;</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">entered</span> <span class="o">==</span> <span class="n">figure</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">entered</span><span class="si">}</span><span class="s2">, you have won!&quot;</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">entered</span><span class="si">}</span><span class="s2">, you have lost.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>Before enjoying the result, let’s examine the <code class="docutils literal notranslate"><span class="pre">parse</span></code> method a little more: what it does is try to
convert the entered argument from a <code class="docutils literal notranslate"><span class="pre">str</span></code> to an <code class="docutils literal notranslate"><span class="pre">int</span></code>. This might fail (if a user enters <code class="docutils literal notranslate"><span class="pre">roll</span> <span class="pre">something</span></code>). In such a case, Python raises a <code class="docutils literal notranslate"><span class="pre">ValueError</span></code> exception. We catch it in our
<code class="docutils literal notranslate"><span class="pre">try/except</span></code> block, send a message to the user and raise the <code class="docutils literal notranslate"><span class="pre">InterruptCommand</span></code> exception in
response to tell Evennia to not run <code class="docutils literal notranslate"><span class="pre">func()</span></code>, since we have no valid number to give it.</p>
<p>In the <code class="docutils literal notranslate"><span class="pre">func</span></code> method, instead of using <code class="docutils literal notranslate"><span class="pre">self.args</span></code>, we use <code class="docutils literal notranslate"><span class="pre">self.entered</span></code> which we have defined in
our <code class="docutils literal notranslate"><span class="pre">parse</span></code> method. You can expect that, if <code class="docutils literal notranslate"><span class="pre">func()</span></code> is run, then <code class="docutils literal notranslate"><span class="pre">self.entered</span></code> contains a valid
number.</p>
<p>If you try this command, it will work as expected this time: the number is converted as it should
and compared to the die roll. You might spend some minutes playing this game. Time out!</p>
<p>Something else we could want to address: in our small example, we only want the user to enter a
positive number between 1 and 6. And the user can enter <code class="docutils literal notranslate"><span class="pre">roll</span> <span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">roll</span> <span class="pre">-8</span></code> or <code class="docutils literal notranslate"><span class="pre">roll</span> <span class="pre">208</span></code> for
that matter, the game still works. It might be worth addressing. Again, you could write a
condition to do that, but since we’re catching an exception, we might end up with something cleaner
by grouping:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">random</span> <span class="kn">import</span> <span class="n">randint</span>
<span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">Command</span><span class="p">,</span> <span class="n">InterruptCommand</span>
<span class="k">class</span> <span class="nc">CmdRoll</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Play random, enter a number and try your luck.</span>
<span class="sd"> Usage:</span>
<span class="sd"> roll &lt;number&gt;</span>
<span class="sd"> Enter a valid number as argument. A random die will be rolled and you</span>
<span class="sd"> will win if you have specified the correct number.</span>
<span class="sd"> Example:</span>
<span class="sd"> roll 3</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;roll&quot;</span>
<span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;Convert the argument to number if possible.&quot;&quot;&quot;</span>
<span class="n">args</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="c1"># Convert to int if possible</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">entered</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">args</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="mi">1</span> <span class="o">&lt;=</span> <span class="bp">self</span><span class="o">.</span><span class="n">entered</span> <span class="o">&lt;=</span> <span class="mi">6</span><span class="p">:</span>
<span class="c1"># self.entered is not between 1 and 6 (including both)</span>
<span class="k">raise</span> <span class="ne">ValueError</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">args</span><span class="si">}</span><span class="s2"> is not a valid number.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Roll a random die</span>
<span class="n">figure</span> <span class="o">=</span> <span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span> <span class="c1"># return a pseudo-random number between 1 and 6, including both</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You roll a die. It lands on the number </span><span class="si">{</span><span class="n">figure</span><span class="si">}</span><span class="s2">.&quot;</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">entered</span> <span class="o">==</span> <span class="n">figure</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">entered</span><span class="si">}</span><span class="s2">, you have won!&quot;</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">entered</span><span class="si">}</span><span class="s2">, you have lost.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>Using grouped exceptions like that makes our code easier to read, but if you feel more comfortable
checking, afterward, that the number the user entered is in the right range, you can do so in a
latter condition.</p>
<blockquote>
<div><p>Notice that we have updated our <code class="docutils literal notranslate"><span class="pre">parse</span></code> method only in this last attempt, not our <code class="docutils literal notranslate"><span class="pre">func()</span></code> method
which remains the same. This is one goal of separating argument parsing from command processing,
these two actions are best kept isolated.</p>
</div></blockquote>
</section>
<section id="working-with-several-arguments">
<h3>Working with several arguments<a class="headerlink" href="#working-with-several-arguments" title="Permalink to this headline"></a></h3>
<p>Often a command expects several arguments. So far, in our example with the “roll” command, we only
expect one argument: a number and just a number. What if we want the user to specify several
numbers? First the number of dice to roll, then the guess?</p>
<blockquote>
<div><p>You won’t win often if you roll 5 dice but that’s for the example.</p>
</div></blockquote>
<p>So we would like to interpret a command like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; roll 3 12
</pre></div>
</div>
<p>(To be understood: roll 3 dice, my guess is the total number will be 12.)</p>
<p>What we need is to cut our command argument, which is a <code class="docutils literal notranslate"><span class="pre">str</span></code>, break it at the space (we use the
space as a delimiter). Python provides the <code class="docutils literal notranslate"><span class="pre">str.split</span></code> method which we’ll use. Again, here are
some examples from the Python interpreter:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt;&gt;&gt; args = &quot;3 12&quot;
&gt;&gt;&gt; args.split(&quot; &quot;)
[&#39;3&#39;, &#39;12&#39;]
&gt;&gt;&gt; args = &quot;a command with several arguments&quot;
&gt;&gt;&gt; args.split(&quot; &quot;)
[&#39;a&#39;, &#39;command&#39;, &#39;with&#39;, &#39;several&#39;, &#39;arguments&#39;]
&gt;&gt;&gt;
</pre></div>
</div>
<p>As you can see, <code class="docutils literal notranslate"><span class="pre">str.split</span></code> will “convert” our strings into a list of strings. The specified
argument (<code class="docutils literal notranslate"><span class="pre">&quot;</span> <span class="pre">&quot;</span></code> in our case) is used as delimiter. So Python browses our original string. When it
sees a delimiter, it takes whatever is before this delimiter and append it to a list.</p>
<p>The point here is that <code class="docutils literal notranslate"><span class="pre">str.split</span></code> will be used to split our argument. But, as you can see from the
above output, we can never be sure of the length of the list at this point:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt;&gt;&gt; args = &quot;something&quot;
&gt;&gt;&gt; args.split(&quot; &quot;)
[&#39;something&#39;]
&gt;&gt;&gt; args = &quot;&quot;
&gt;&gt;&gt; args.split(&quot; &quot;)
[&#39;&#39;]
&gt;&gt;&gt;
</pre></div>
</div>
<p>Again we could use a condition to check the number of split arguments, but Python offers a better
approach, making use of its exception mechanism. We’ll give a second argument to <code class="docutils literal notranslate"><span class="pre">str.split</span></code>, the
maximum number of splits to do. Let’s see an example, this feature might be confusing at first
glance:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt;&gt;&gt; args = &quot;that is something great&quot;
&gt;&gt;&gt; args.split(&quot; &quot;, 1) # one split, that is a list with two elements (before, after)
</pre></div>
</div>
<p>[‘that’, ‘is something great’]</p>
<blockquote>
<div><blockquote>
<div></div></blockquote>
</div></blockquote>
<p>Read this example as many times as needed to understand it. The second argument we give to
<code class="docutils literal notranslate"><span class="pre">str.split</span></code> is not the length of the list that should be returned, but the number of times we have
to split. Therefore, we specify 1 here, but we get a list of two elements (before the separator,
after the separator).</p>
<blockquote>
<div><p>What will happen if Python can’t split the number of times we ask?</p>
</div></blockquote>
<p>It won’t:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt;&gt;&gt; args = &quot;whatever&quot;
&gt;&gt;&gt; args.split(&quot; &quot;, 1) # there isn&#39;t even a space here...
[&#39;whatever&#39;]
&gt;&gt;&gt;
</pre></div>
</div>
<p>This is one moment I would have hoped for an exception and didn’t get one. But there’s another way
which will raise an exception if there is an error: variable unpacking.</p>
<p>We won’t talk about this feature in details here. It would be complicated. But the code is really
straightforward to use. Let’s take our example of the roll command but let’s add a first argument:
the number of dice to roll.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">random</span> <span class="kn">import</span> <span class="n">randint</span>
<span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">Command</span><span class="p">,</span> <span class="n">InterruptCommand</span>
<span class="k">class</span> <span class="nc">CmdRoll</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd"> Play random, enter a number and try your luck.</span>
<span class="sd"> Specify two numbers separated by a space. The first number is the</span>
<span class="sd"> number of dice to roll (1, 2, 3) and the second is the expected sum</span>
<span class="sd"> of the roll.</span>
<span class="sd"> Usage:</span>
<span class="sd"> roll &lt;dice&gt; &lt;number&gt;</span>
<span class="sd"> For instance, to roll two 6-figure dice, enter 2 as first argument.</span>
<span class="sd"> If you think the sum of these two dice roll will be 10, you could enter:</span>
<span class="sd"> roll 2 10</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;roll&quot;</span>
<span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="w"> </span><span class="sd">&quot;&quot;&quot;Split the arguments and convert them.&quot;&quot;&quot;</span>
<span class="n">args</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="c1"># Split: we expect two arguments separated by a space</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">number</span><span class="p">,</span> <span class="n">guess</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s2">&quot; &quot;</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;Invalid usage. Enter two numbers separated by a space.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
<span class="c1"># Convert the entered number (first argument)</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">number</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">&lt;=</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">ValueError</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">number</span><span class="si">}</span><span class="s2"> is not a valid number of dice.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
<span class="c1"># Convert the entered guess (second argument)</span>
<span class="k">try</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">guess</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">guess</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="mi">1</span> <span class="o">&lt;=</span> <span class="bp">self</span><span class="o">.</span><span class="n">guess</span> <span class="o">&lt;=</span> <span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">*</span> <span class="mi">6</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">ValueError</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</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">guess</span><span class="si">}</span><span class="s2"> is not a valid guess.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
<span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Roll a random die X times (X being self.number)</span>
<span class="n">figure</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">number</span><span class="p">):</span>
<span class="n">figure</span> <span class="o">+=</span> <span class="n">randint</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You roll </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">number</span><span class="si">}</span><span class="s2"> dice and obtain the sum </span><span class="si">{</span><span class="n">figure</span><span class="si">}</span><span class="s2">.&quot;</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">guess</span> <span class="o">==</span> <span class="n">figure</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">guess</span><span class="si">}</span><span class="s2">, you have won!&quot;</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;You played </span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">guess</span><span class="si">}</span><span class="s2">, you have lost.&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>The beginning of the <code class="docutils literal notranslate"><span class="pre">parse()</span></code> method is what interests us most:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
<span class="n">number</span><span class="p">,</span> <span class="n">guess</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s2">&quot; &quot;</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;Invalid usage. Enter two numbers separated by a space.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
</pre></div>
</div>
<p>We split the argument using <code class="docutils literal notranslate"><span class="pre">str.split</span></code> but we capture the result in two variables. Python is smart
enough to know that we want what’s left of the space in the first variable, what’s right of the
space in the second variable. If there is not even a space in the string, Python will raise a
<code class="docutils literal notranslate"><span class="pre">ValueError</span></code> exception.</p>
<p>This code is much easier to read than browsing through the returned strings of <code class="docutils literal notranslate"><span class="pre">str.split</span></code>. We can
convert both variables the way we did previously. Actually there are not so many changes in this
version and the previous one, most of it is due to name changes for clarity.</p>
<blockquote>
<div><p>Splitting a string with a maximum of splits is a common occurrence while parsing command
arguments. You can also see the <code class="docutils literal notranslate"><span class="pre">str.rspli8t</span></code> method that does the same thing but from the right of
the string. Therefore, it will attempt to find delimiters at the end of the string and work toward
the beginning of it.</p>
</div></blockquote>
<p>We have used a space as a delimiter. This is absolutely not necessary. You might remember that
most default Evennia commands can take an <code class="docutils literal notranslate"><span class="pre">=</span></code> sign as a delimiter. Now you know how to parse them
as well:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt;&gt;&gt; cmd_key = &quot;tel&quot;
&gt;&gt;&gt; cmd_args = &quot;book = chest&quot;
&gt;&gt;&gt; left, right = cmd_args.split(&quot;=&quot;) # mighht raise ValueError!
&gt;&gt;&gt; left
&#39;book &#39;
&gt;&gt;&gt; right
&#39; chest&#39;
&gt;&gt;&gt;
</pre></div>
</div>
</section>
<section id="optional-arguments">
<h3>Optional arguments<a class="headerlink" href="#optional-arguments" title="Permalink to this headline"></a></h3>
<p>Sometimes, you’ll come across commands that have optional arguments. These arguments are not
necessary but they can be set if more information is needed. I will not provide the entire command
code here but just enough code to show the mechanism in Python:</p>
<p>Again, we’ll use <code class="docutils literal notranslate"><span class="pre">str.split</span></code>, knowing that we might not have any delimiter at all. For instance,
the player could enter the “tel” command like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; tel book
&gt; tell book = chest
</pre></div>
</div>
<p>The equal sign is optional along with whatever is specified after it. A possible solution in our
<code class="docutils literal notranslate"><span class="pre">parse</span></code> method would be:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">args</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="c1"># = is optional</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">obj</span><span class="p">,</span> <span class="n">destination</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s2">&quot;=&quot;</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">args</span>
<span class="n">destination</span> <span class="o">=</span> <span class="kc">None</span>
</pre></div>
</div>
<p>This code would place everything the user entered in <code class="docutils literal notranslate"><span class="pre">obj</span></code> if she didn’t specify any equal sign.
Otherwise, what’s before the equal sign will go in <code class="docutils literal notranslate"><span class="pre">obj</span></code>, what’s after the equal sign will go in
<code class="docutils literal notranslate"><span class="pre">destination</span></code>. This makes for quick testing after that, more robust code with less conditions that
might too easily break your code if you’re not careful.</p>
<blockquote>
<div><p>Again, here we specified a maximum numbers of splits. If the users enters:</p>
</div></blockquote>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; tel book = chest = chair
</pre></div>
</div>
<p>Then <code class="docutils literal notranslate"><span class="pre">destination</span></code> will contain: <code class="docutils literal notranslate"><span class="pre">&quot;</span> <span class="pre">chest</span> <span class="pre">=</span> <span class="pre">chair&quot;</span></code>. This is often desired, but it’s up to you to
set parsing however you like.</p>
</section>
</section>
<section id="evennia-searches">
<h2>Evennia searches<a class="headerlink" href="#evennia-searches" title="Permalink to this headline"></a></h2>
<p>After this quick tour of some <code class="docutils literal notranslate"><span class="pre">str</span></code> methods, we’ll take a look at some Evennia-specific features
that you won’t find in standard Python.</p>
<p>One very common task is to convert a <code class="docutils literal notranslate"><span class="pre">str</span></code> into an Evennia object. Take the previous example:
having <code class="docutils literal notranslate"><span class="pre">&quot;book&quot;</span></code> in a variable is great, but we would prefer to know what the user is talking
about… what is this <code class="docutils literal notranslate"><span class="pre">&quot;book&quot;</span></code>?</p>
<p>To get an object from a string, we perform an Evennia search. Evennia provides a <code class="docutils literal notranslate"><span class="pre">search</span></code> method on
all typeclassed objects (you will most likely use the one on characters or accounts). This method
supports a very wide array of arguments and has <a class="reference internal" href="Beginner-Tutorial/Part1/Beginner-Tutorial-Searching-Things.html"><span class="doc std std-doc">its own tutorial</span></a>.
Some examples of useful cases follow:</p>
<section id="local-searches">
<h3>Local searches<a class="headerlink" href="#local-searches" title="Permalink to this headline"></a></h3>
<p>When an account or a character enters a command, the account or character is found in the <code class="docutils literal notranslate"><span class="pre">caller</span></code>
attribute. Therefore, <code class="docutils literal notranslate"><span class="pre">self.caller</span></code> will contain an account or a character (or a session if that’s
a session command, though that’s not as frequent). The <code class="docutils literal notranslate"><span class="pre">search</span></code> method will be available on this
caller.</p>
<p>Let’s take the same example of our little “tel” command. The user can specify an object as
argument:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
</pre></div>
</div>
<p>We then need to “convert” this string into an Evennia object. The Evennia object will be searched
in the caller’s location and its contents by default (that is to say, if the command has been
entered by a character, it will search the object in the character’s room and the character’s
inventory).</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">obj</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
</pre></div>
</div>
<p>We specify only one argument to the <code class="docutils literal notranslate"><span class="pre">search</span></code> method here: the string to search. If Evennia finds a
match, it will return it and we keep it in the <code class="docutils literal notranslate"><span class="pre">obj</span></code> attribute. If it can’t find anything, it will
return <code class="docutils literal notranslate"><span class="pre">None</span></code> so we need to check for that:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">obj</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">obj</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="c1"># A proper error message has already been sent to the caller</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
</pre></div>
</div>
<p>That’s it. After this condition, you know that whatever is in <code class="docutils literal notranslate"><span class="pre">self.obj</span></code> is a valid Evennia object
(another character, an object, an exit…).</p>
</section>
<section id="quiet-searches">
<h3>Quiet searches<a class="headerlink" href="#quiet-searches" title="Permalink to this headline"></a></h3>
<p>By default, Evennia will handle the case when more than one match is found in the search. The user
will be asked to narrow down and re-enter the command. You can, however, ask to be returned the
list of matches and handle this list yourself:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="n">objs</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">quiet</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">objs</span><span class="p">:</span>
<span class="c1"># This is an empty list, so no match</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;No </span><span class="si">{</span><span class="n">name</span><span class="si">!r}</span><span class="s2"> was found.&quot;</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">InterruptCommand</span>
<span class="bp">self</span><span class="o">.</span><span class="n">obj</span> <span class="o">=</span> <span class="n">objs</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="c1"># Take the first match even if there are several</span>
</pre></div>
</div>
<p>All we have changed to obtain a list is a keyword argument in the <code class="docutils literal notranslate"><span class="pre">search</span></code> method: <code class="docutils literal notranslate"><span class="pre">quiet</span></code>. If set
to <code class="docutils literal notranslate"><span class="pre">True</span></code>, then errors are ignored and a list is always returned, so we need to handle it as such.
Notice in this example, <code class="docutils literal notranslate"><span class="pre">self.obj</span></code> will contain a valid object too, but if several matches are
found, <code class="docutils literal notranslate"><span class="pre">self.obj</span></code> will contain the first one, even if more matches are available.</p>
</section>
<section id="global-searches">
<h3>Global searches<a class="headerlink" href="#global-searches" title="Permalink to this headline"></a></h3>
<p>By default, Evennia will perform a local search, that is, a search limited by the location in which
the caller is. If you want to perform a global search (search in the entire database), just set the
<code class="docutils literal notranslate"><span class="pre">global_search</span></code> keyword argument to <code class="docutils literal notranslate"><span class="pre">True</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">parse</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">args</span><span class="o">.</span><span class="n">lstrip</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">obj</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="o">.</span><span class="n">search</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">global_search</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
</pre></div>
</div>
</section>
</section>
<section id="conclusion">
<h2>Conclusion<a class="headerlink" href="#conclusion" title="Permalink to this headline"></a></h2>
<p>Parsing command arguments is vital for most game designers. If you design “intelligent” commands,
users should be able to guess how to use them without reading the help, or with a very quick peek at
said help. Good commands are intuitive to users. Better commands do what they’re told to do. For
game designers working on MUDs, commands are the main entry point for users into your game. This is
no trivial. If commands execute correctly (if their argument is parsed, if they don’t behave in
unexpected ways and report back the right errors), you will have happier players that might stay
longer on your game. I hope this tutorial gave you some pointers on ways to improve your command
parsing. There are, of course, other ways you will discover, or ways you are already using in your
code.</p>
</section>
</section>
</div>
</div>
</div>
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