Introduction and the Reactor

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 <h2>Why I'm writing this</h2><a href="https://twistedmatrix.com/">Twisted</a> has enabled me to do some amazing things -- things I hadn't thought possible. Others haven't had the same delightful experience. Their descriptions of Twisted include less glowing adjectives. My intent with this and related posts is to ameliorate some of those adjectives.I'm borrowing conceptually from JP Calderone's excellent <a href="https://twistedmatrix.com/documents/current/web/howto/web-in-60/index.html">Twisted Web in 60 Seconds</a> series by keeping these posts relatively short. Also, there's <a href="https://krondo.com/?page_id=1327">KRONDO blog's Twisted Introduction</a>, another great series for learning Twisted (and general Asynchronous Programming).<h2>Bomberman</h2>Bomberman is great! So, let's make it. By the end (or maybe somewhere in the middle) of these Bite-sized Twisted posts, I'm hoping that some semblance of a Bomberman clone emerges. That's the goal, at least.Also, am I the only one who thought Super Smash Bros should have included Bomberman? And MegaMan?<h2>The Reactor</h2>The reactor is central to Twisted. You may not always interact with it directly, but it'll almost always be there, reacting away. To start the reactor, call its <code>run</code> method:<div class="code python"><pre>from twisted.internet import reactor reactor.run() </pre></div><div class="caption">snippet1.py</div>Run this code and you'll see it hang:<div class="code bash"><pre> </pre></div><h2>Nothing != Something</h2>You might be tempted to say, "nothing happened." Don't say that. Something happened; in fact it's still happening, and will continue to happen until you stop it (press <code>Control-C</code>). The something happening is the reactor running. To prove it, run this code:<div class="code python"><pre>from twisted.internet import reactor print &#39;Hello,&#39; reactor.run() print &#39;world!&#39; </pre></div><div class="caption">snippet2.py</div>You should see the <code>Hello,</code> but won't see the <code>world!</code> until you stop the reactor:<div class="code text"><pre>Hello, ^Cworld! </pre></div>(The <code>^C</code> is what my terminal prints when I press <code>Control-C</code>)<h2>What does the reactor do?</h2>The reactor reacts to events. One kind of event it can react to is the passage of time (scheduled events):<div class="code python"><pre>from twisted.internet import reactor reactor.callLater(2, reactor.stop) reactor.run() </pre></div><div class="caption">snippet3.py</div>snippet3.py tells the reactor to call <code>reactor.stop</code> after 2 seconds, then it starts the reactor with <code>reactor.run()</code>. After 2 seconds, <code>reactor.stop</code> is called, stopping the reactor, and the process exits.The first argument to <code>reactor.callLater</code> is the number of seconds to wait before calling the function. The second argument is the function to call. It's important that it's <code>reactor.stop</code>, not <code>reactor.stop()</code>. <code>reactor.callLater</code> expects the reference to the function, not the result of the function. Passing function references is a common idiom in the Twisted API.Also, you can <a href="https://twistedmatrix.com/documents/current/api/twisted.internet.base.ReactorBase.html#callLater">read the <code>callLater</code> docs for yourself.</a> (See twisted.internet.interfaces.IReactorTime.callLater as it says)<h2>Make a bomb</h2>If we're going to make Bomberman, we'll need bombs. Bombs have fuses. Let's light some fuses:<div class="code python"><pre>from twisted.internet import reactor def explode(message): print &#39;BOOM! &#39; + message reactor.callLater(1, explode, &#39;first one&#39;) reactor.callLater(2, explode, &#39;second one&#39;) reactor.callLater(3, reactor.stop) reactor.run() </pre></div><div class="caption">bomb.py</div>Run it. After 1 second, the first bomb explodes. After 1 more second, the second explodes. After 1 more, the process exits:<div class="code text"><pre>BOOM! first one BOOM! second one </pre></div>In bomb.py, note two things:First, when scheduling <code>explode</code> we also pass a string (<code>'first one'</code>) as an extra argument to <code>reactor.callLater</code>. This argument is eventually passed to <code>explode</code> as it's first argument. <code>callLater</code> takes any number of arguments and keyword arguments, which it will use to call the scheduled method.Second, do the explosions happen when you expect them to? Consider this synchronous code, which has the same timed output as bomb.py:<div class="code python"><pre>import time def explode(message): print &#39;BOOM! &#39; + message time.sleep(1) explode(&#39;first one&#39;) time.sleep(1) explode(&#39;second one&#39;) time.sleep(1) </pre></div><div class="caption">synchronous.py</div>In the synchronous code, since commands happen sequentially, you specify the timing offset from one command to the other. In the asynchronous code, scheduled commands are independent of each other: you specify the timing offset from the time that you schedule the command.synchronous.py has the same output as bomb.py, but it does not do the same thing. This will become more apparent when we start adding input/output to programs.
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