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<li class="nav-item nav-item-0"><a href="index.html">Evennia 0.9.5</a> &#187;</li>
<li class="nav-item nav-item-this"><a href="">Async Process</a></li>
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<section class="tex2jax_ignore mathjax_ignore" id="async-process">
<h1>Async Process<a class="headerlink" href="#async-process" title="Permalink to this headline"></a></h1>
<p><em>This is considered an advanced topic.</em></p>
<section id="synchronous-versus-asynchronous">
<h2>Synchronous versus Asynchronous<a class="headerlink" href="#synchronous-versus-asynchronous" title="Permalink to this headline"></a></h2>
<p>Most program code operates <em>synchronously</em>. This means that each statement in your code gets
processed and finishes before the next can begin. This makes for easy-to-understand code. It is also
a <em>requirement</em> in many cases - a subsequent piece of code often depend on something calculated or
defined in a previous statement.</p>
<p>Consider this piece of code in a traditional Python program:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;before call ...&quot;</span><span class="p">)</span>
<span class="n">long_running_function</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;after call ...&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>When run, this will print <code class="docutils literal notranslate"><span class="pre">&quot;before</span> <span class="pre">call</span> <span class="pre">...&quot;</span></code>, after which the <code class="docutils literal notranslate"><span class="pre">long_running_function</span></code> gets to work
for however long time. Only once that is done, the system prints <code class="docutils literal notranslate"><span class="pre">&quot;after</span> <span class="pre">call</span> <span class="pre">...&quot;</span></code>. Easy and
logical to follow. Most of Evennia work in this way and often it’s important that commands get
executed in the same strict order they were coded.</p>
<p>Evennia, via Twisted, is a single-process multi-user server. In simple terms this means that it
swiftly switches between dealing with player input so quickly that each player feels like they do
things at the same time. This is a clever illusion however: If one user, say, runs a command
containing that <code class="docutils literal notranslate"><span class="pre">long_running_function</span></code>, <em>all</em> other players are effectively forced to wait until it
finishes.</p>
<p>Now, it should be said that on a modern computer system this is rarely an issue. Very few commands
run so long that other users notice it. And as mentioned, most of the time you <em>want</em> to enforce
all commands to occur in strict sequence.</p>
<p>When delays do become noticeable and you don’t care in which order the command actually completes,
you can run it <em>asynchronously</em>. This makes use of the <code class="docutils literal notranslate"><span class="pre">run_async()</span></code> function in
<code class="docutils literal notranslate"><span class="pre">src/utils/utils.py</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="n">run_async</span><span class="p">(</span><span class="n">function</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>Where <code class="docutils literal notranslate"><span class="pre">function</span></code> will be called asynchronously with <code class="docutils literal notranslate"><span class="pre">*args</span></code> and <code class="docutils literal notranslate"><span class="pre">**kwargs</span></code>. Example:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">utils</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;before call ...&quot;</span><span class="p">)</span>
<span class="n">utils</span><span class="o">.</span><span class="n">run_async</span><span class="p">(</span><span class="n">long_running_function</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;after call ...&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>Now, when running this you will find that the program will not wait around for
<code class="docutils literal notranslate"><span class="pre">long_running_function</span></code> to finish. In fact you will see <code class="docutils literal notranslate"><span class="pre">&quot;before</span> <span class="pre">call</span> <span class="pre">...&quot;</span></code> and <code class="docutils literal notranslate"><span class="pre">&quot;after</span> <span class="pre">call</span> <span class="pre">...&quot;</span></code>
printed out right away. The long-running function will run in the background and you (and other
users) can go on as normal.</p>
</section>
<section id="customizing-asynchronous-operation">
<h2>Customizing asynchronous operation<a class="headerlink" href="#customizing-asynchronous-operation" title="Permalink to this headline"></a></h2>
<p>A complication with using asynchronous calls is what to do with the result from that call. What if
<code class="docutils literal notranslate"><span class="pre">long_running_function</span></code> returns a value that you need? It makes no real sense to put any lines of
code after the call to try to deal with the result from <code class="docutils literal notranslate"><span class="pre">long_running_function</span></code> above - as we saw
the <code class="docutils literal notranslate"><span class="pre">&quot;after</span> <span class="pre">call</span> <span class="pre">...&quot;</span></code> got printed long before <code class="docutils literal notranslate"><span class="pre">long_running_function</span></code> was finished, making that
line quite pointless for processing any data from the function. Instead one has to use <em>callbacks</em>.</p>
<p><code class="docutils literal notranslate"><span class="pre">utils.run_async</span></code> takes reserved kwargs that won’t be passed into the long-running function:</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">at_return(r)</span></code> (the <em>callback</em>) is called when the asynchronous function (<code class="docutils literal notranslate"><span class="pre">long_running_function</span></code>
above) finishes successfully. The argument <code class="docutils literal notranslate"><span class="pre">r</span></code> will then be the return value of that function (or
<code class="docutils literal notranslate"><span class="pre">None</span></code>).</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">at_return</span><span class="p">(</span><span class="n">r</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p><code class="docutils literal notranslate"><span class="pre">at_return_kwargs</span></code> - an optional dictionary that will be fed as keyword arguments to the
<code class="docutils literal notranslate"><span class="pre">at_return</span></code> callback.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">at_err(e)</span></code> (the <em>errback</em>) is called if the asynchronous function fails and raises an exception.
This exception is passed to the errback wrapped in a <em>Failure</em> object <code class="docutils literal notranslate"><span class="pre">e</span></code>. If you do not supply an
errback of your own, Evennia will automatically add one that silently writes errors to the evennia
log. An example of an errback is found below:</p></li>
</ul>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">at_err</span><span class="p">(</span><span class="n">e</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;There was an error:&quot;</span><span class="p">,</span> <span class="nb">str</span><span class="p">(</span><span class="n">e</span><span class="p">))</span>
</pre></div>
</div>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">at_err_kwargs</span></code> - an optional dictionary that will be fed as keyword arguments to the <code class="docutils literal notranslate"><span class="pre">at_err</span></code>
errback.</p></li>
</ul>
<p>An example of making an asynchronous call from inside a <a class="reference internal" href="Commands.html"><span class="doc std std-doc">Command</span></a> definition:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="kn">from</span> <span class="nn">evennia</span> <span class="kn">import</span> <span class="n">utils</span><span class="p">,</span> <span class="n">Command</span>
<span class="k">class</span> <span class="nc">CmdAsync</span><span class="p">(</span><span class="n">Command</span><span class="p">):</span>
<span class="n">key</span> <span class="o">=</span> <span class="s2">&quot;asynccommand&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="k">def</span> <span class="nf">long_running_function</span><span class="p">():</span>
<span class="c1">#[... lots of time-consuming code ...]</span>
<span class="k">return</span> <span class="n">final_value</span>
<span class="k">def</span> <span class="nf">at_return_function</span><span class="p">(</span><span class="n">r</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;The final value is </span><span class="si">%s</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">r</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">at_err_function</span><span class="p">(</span><span class="n">e</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;There was an error: </span><span class="si">%s</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">e</span><span class="p">)</span>
<span class="c1"># do the async call, setting all callbacks</span>
<span class="n">utils</span><span class="o">.</span><span class="n">run_async</span><span class="p">(</span><span class="n">long_running_function</span><span class="p">,</span> <span class="n">at_return</span><span class="o">=</span><span class="n">at_return_function</span><span class="p">,</span>
<span class="n">at_err</span><span class="o">=</span><span class="n">at_err_function</span><span class="p">)</span>
</pre></div>
</div>
<p>That’s it - from here on we can forget about <code class="docutils literal notranslate"><span class="pre">long_running_function</span></code> and go on with what else need
to be done. <em>Whenever</em> it finishes, the <code class="docutils literal notranslate"><span class="pre">at_return_function</span></code> function will be called and the final
value will
pop up for us to see. If not we will see an error message.</p>
</section>
<section id="delay">
<h2>delay<a class="headerlink" href="#delay" title="Permalink to this headline"></a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">delay</span></code> function is a much simpler sibling to <code class="docutils literal notranslate"><span class="pre">run_async</span></code>. It is in fact just a way to delay the
execution of a command until a future time. This is equivalent to something like <code class="docutils literal notranslate"><span class="pre">time.sleep()</span></code>
except delay is asynchronous while <code class="docutils literal notranslate"><span class="pre">sleep</span></code> would lock the entire server for the duration of the
sleep.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="kn">from</span> <span class="nn">evennia.utils</span> <span class="kn">import</span> <span class="n">delay</span>
<span class="c1"># [...]</span>
<span class="c1"># e.g. inside a Command, where `self.caller` is available</span>
<span class="k">def</span> <span class="nf">callback</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>
<span class="n">obj</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;Returning!&quot;</span><span class="p">)</span>
<span class="n">delay</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">caller</span><span class="p">)</span>
</pre></div>
</div>
<p>This will delay the execution of the callback for 10 seconds. This function is explored much more in
the <a class="reference internal" href="Command-Duration.html"><span class="doc std std-doc">Command Duration Tutorial</span></a>.</p>
<p>You can also try the following snippet just see how it works:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>@py from evennia.utils import delay; delay(10, lambda who: who.msg(&quot;Test!&quot;), self)
</pre></div>
</div>
<p>Wait 10 seconds and ‘Test!’ should be echoed back to you.</p>
</section>
<section id="the-interactive-decorator">
<h2>The &#64;interactive decorator<a class="headerlink" href="#the-interactive-decorator" title="Permalink to this headline"></a></h2>
<p>As of Evennia 0.9, the <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code> <a class="reference external" href="https://realpython.com/primer-on-python-decorators/">decorator</a>
is available. This makes any function or method possible to ‘pause’ and/or await player input
in an interactive way.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="kn">from</span> <span class="nn">evennia.utils</span> <span class="kn">import</span> <span class="n">interactive</span>
<span class="nd">@interactive</span>
<span class="k">def</span> <span class="nf">myfunc</span><span class="p">(</span><span class="n">caller</span><span class="p">):</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="n">caller</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;Getting ready to wait ...&quot;</span><span class="p">)</span>
<span class="k">yield</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="n">caller</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="s2">&quot;Now 5 seconds have passed.&quot;</span><span class="p">)</span>
<span class="n">response</span> <span class="o">=</span> <span class="k">yield</span><span class="p">(</span><span class="s2">&quot;Do you want to wait another 5 secs?&quot;</span><span class="p">)</span>
<span class="k">if</span> <span class="n">response</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span> <span class="ow">not</span> <span class="ow">in</span> <span class="p">(</span><span class="s2">&quot;yes&quot;</span><span class="p">,</span> <span class="s2">&quot;y&quot;</span><span class="p">):</span>
<span class="k">break</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code> decorator gives the function the ability to pause. The use
of <code class="docutils literal notranslate"><span class="pre">yield(seconds)</span></code> will do just that - it will asynchronously pause for the
number of seconds given before continuing. This is technically equivalent to
using <code class="docutils literal notranslate"><span class="pre">call_async</span></code> with a callback that continues after 5 secs. But the code
with <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code> is a little easier to follow.</p>
<p>Within the <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code> function, the <code class="docutils literal notranslate"><span class="pre">response</span> <span class="pre">=</span> <span class="pre">yield(&quot;question&quot;)</span></code> question
allows you to ask the user for input. You can then process the input, just like
you would if you used the Python <code class="docutils literal notranslate"><span class="pre">input</span></code> function. There is one caveat to this
functionality though - <em>it will only work if the function/method has an
argument named exactly <code class="docutils literal notranslate"><span class="pre">caller</span></code></em>. This is because internally Evennia will look
for the <code class="docutils literal notranslate"><span class="pre">caller</span></code> argument and treat that as the source of input.</p>
<p>All of this makes the <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code> decorator very useful. But it comes with a
few caveats. Notably, decorating a function/method with <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code> turns it
into a Python <a class="reference external" href="https://wiki.python.org/moin/Generators">generator</a>. The most
common issue is that you cannot use <code class="docutils literal notranslate"><span class="pre">return</span> <span class="pre">&lt;value&gt;</span></code> from a generator (just an
empty <code class="docutils literal notranslate"><span class="pre">return</span></code> works). To return a value from a function/method you have decorated
with <code class="docutils literal notranslate"><span class="pre">&#64;interactive</span></code>, you must instead use a special Twisted function
<code class="docutils literal notranslate"><span class="pre">twisted.internet.defer.returnValue</span></code>. Evennia also makes this function
conveniently available from <code class="docutils literal notranslate"><span class="pre">evennia.utils</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span> <span class="kn">from</span> <span class="nn">evennia.utils</span> <span class="kn">import</span> <span class="n">interactive</span><span class="p">,</span> <span class="n">returnValue</span>
<span class="nd">@interactive</span>
<span class="k">def</span> <span class="nf">myfunc</span><span class="p">():</span>
<span class="c1"># ...</span>
<span class="n">result</span> <span class="o">=</span> <span class="mi">10</span>
<span class="c1"># this must be used instead of `return result`</span>
<span class="n">returnValue</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="assorted-notes">
<h2>Assorted notes<a class="headerlink" href="#assorted-notes" title="Permalink to this headline"></a></h2>
<p>Overall, be careful with choosing when to use asynchronous calls. It is mainly useful for large
administration operations that have no direct influence on the game world (imports and backup
operations come to mind). Since there is no telling exactly when an asynchronous call actually ends,
using them for in-game commands is to potentially invite confusion and inconsistencies (and very
hard-to-reproduce bugs).</p>
<p>The very first synchronous example above is not <em>really</em> correct in the case of Twisted, which is
inherently an asynchronous server. Notably you might find that you will <em>not</em> see the first <code class="docutils literal notranslate"><span class="pre">before</span> <span class="pre">call</span> <span class="pre">...</span></code> text being printed out right away. Instead all texts could end up being delayed until
after the long-running process finishes. So all commands will retain their relative order as
expected, but they may appear with delays or in groups.</p>
</section>
<section id="further-reading">
<h2>Further reading<a class="headerlink" href="#further-reading" title="Permalink to this headline"></a></h2>
<p>Technically, <code class="docutils literal notranslate"><span class="pre">run_async</span></code> is just a very thin and simplified wrapper around a
<a class="reference external" href="http://twistedmatrix.com/documents/9.0.0/core/howto/defer.html">Twisted Deferred</a> object; the
wrapper sets
up a default errback also if none is supplied. If you know what you are doing there is nothing
stopping you from bypassing the utility function, building a more sophisticated callback chain after
your own liking.</p>
</section>
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<p><h3><a href="index.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Async Process</a><ul>
<li><a class="reference internal" href="#synchronous-versus-asynchronous">Synchronous versus Asynchronous</a></li>
<li><a class="reference internal" href="#customizing-asynchronous-operation">Customizing asynchronous operation</a></li>
<li><a class="reference internal" href="#delay">delay</a></li>
<li><a class="reference internal" href="#the-interactive-decorator">The &#64;interactive decorator</a></li>
<li><a class="reference internal" href="#assorted-notes">Assorted notes</a></li>
<li><a class="reference internal" href="#further-reading">Further reading</a></li>
</ul>
</li>
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