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    <p>Functions are very different from classes. It looks like you took a function and just changed the <code>def</code> to <code>class</code>. I guess that <em>mostly</em> works in your case, but it's not how classes are supposed to go.</p> <p>Classes contain functions (methods) and data. For example, you have a ball:</p> <pre><code>class Ball(object): # __init__ is a special method called whenever you try to make # an instance of a class. As you heard, it initializes the object. # Here, we'll initialize some of the data. def __init__(self): # Let's add some data to the [instance of the] class. self.position = (100, 100) self.velocity = (0, 0) # We can also add our own functions. When our ball bounces, # its vertical velocity will be negated. (no gravity here!) def bounce(self): self.velocity = (self.velocity[0], -self.velocity[1]) </code></pre> <p>Now we have a <code>Ball</code> class. How can we use it?</p> <pre><code>&gt;&gt;&gt; ball1 = Ball() &gt;&gt;&gt; ball1 &lt;Ball object at ...&gt; </code></pre> <p>It doesn't look very useful. The data is where it could be useful:</p> <pre><code>&gt;&gt;&gt; ball1.position (100, 100) &gt;&gt;&gt; ball1.velocity (0, 0) &gt;&gt;&gt; ball1.position = (200, 100) &gt;&gt;&gt; ball1.position (200, 100) </code></pre> <p>Alright, cool, but what's the advantage over a global variable? If you have another <code>Ball</code> instance, it will remain independent:</p> <pre><code>&gt;&gt;&gt; ball2 = Ball() &gt;&gt;&gt; ball2.velocity = (5, 10) &gt;&gt;&gt; ball2.position (100, 100) &gt;&gt;&gt; ball2.velocity (5, 10) </code></pre> <p>And <code>ball1</code> remains independent:</p> <pre><code>&gt;&gt;&gt; ball1.velocity (0, 0) </code></pre> <p>Now what about that <code>bounce</code> method (function in a class) we defined?</p> <pre><code>&gt;&gt;&gt; ball2.bounce() &gt;&gt;&gt; ball2.velocity (5, -10) </code></pre> <p>The <code>bounce</code> method caused it to modify the <code>velocity</code> data of itself. Again, <code>ball1</code> was not touched:</p> <pre><code>&gt;&gt;&gt; ball1.velocity </code></pre> <h1>Application</h1> <p>A ball is neat and all, but most people aren't simulating that. You're making a game. Let's think of what kinds of things we have:</p> <ul> <li><strong>A room</strong> is the most obvious thing we could have.</li> </ul> <p>So let's make a room. Rooms have names, so we'll have some data to store that:</p> <pre><code>class Room(object): # Note that we're taking an argument besides self, here. def __init__(self, name): self.name = name # Set the room's name to the name we got. </code></pre> <p>And let's make an instance of it:</p> <pre><code>&gt;&gt;&gt; white_room = Room("White Room") &gt;&gt;&gt; white_room.name 'White Room' </code></pre> <p>Spiffy. This turns out not to be all that useful if you want different rooms to have different functionality, though, so let's make a <em>subclass</em>. A <em>subclass</em> inherits all functionality from its <em>superclass</em>, but you can add more functionality or override the superclass's functionality.</p> <p>Let's think about what we want to do with rooms:</p> <blockquote class="spoiler"> <p> We want to interact with rooms.</p> </blockquote> <p>And how do we do that?</p> <blockquote class="spoiler"> <p> The user types in a line of text that gets responded to.</p> </blockquote> <p>How it's responded do depends on the room, so let's make the room handle that with a method called <code>interact</code>:</p> <pre><code>class WhiteRoom(Room): # A white room is a kind of room. def __init__(self): # All white rooms have names of 'White Room'. self.name = 'White Room' def interact(self, line): if 'test' in line: print "'Test' to you, too!" </code></pre> <p>Now let's try interacting with it:</p> <pre><code>&gt;&gt;&gt; white_room = WhiteRoom() # WhiteRoom's __init__ doesn't take an argument (even though its superclass's __init__ does; we overrode the superclass's __init__) &gt;&gt;&gt; white_room.interact('test') 'Test' to you, too! </code></pre> <p>Your original example featured moving between rooms. Let's use a global variable called <code>current_room</code> to track which room we're in.<sup>1</sup> Let's also make a red room.</p> <p><sup>1. There's better options besides global variables here, but I'm going to use one for simplicity.</sup></p> <pre><code>class RedRoom(Room): # A red room is also a kind of room. def __init__(self): self.name = 'Red Room' def interact(self, line): global current_room, white_room if 'white' in line: # We could create a new WhiteRoom, but then it # would lose its data (if it had any) after moving # out of it and into it again. current_room = white_room </code></pre> <p>Now let's try that:</p> <pre><code>&gt;&gt;&gt; red_room = RedRoom() &gt;&gt;&gt; current_room = red_room &gt;&gt;&gt; current_room.name 'Red Room' &gt;&gt;&gt; current_room.interact('go to white room') &gt;&gt;&gt; current_room.name 'White Room' </code></pre> <p><strong>Exercise for the reader:</strong> Add code to <code>WhiteRoom</code>'s <code>interact</code> that allows you to go back to the red room.</p> <p>Now that we have everything working, let's put it all together. With our new <code>name</code> data on all rooms, we can also show the current room in the prompt!</p> <pre><code>def play_game(): global current_room while True: line = raw_input(current_room.name + '&gt; ') current_room.interact(line) </code></pre> <p>You might also want to make a function to reset the game:</p> <pre><code>def reset_game(): global current_room, white_room, red_room white_room = WhiteRoom() red_room = RedRoom() current_room = white_room </code></pre> <p>Put all of the class definitions and these functions into a file and you can play it at the prompt like this (assuming they're in <code>mygame.py</code>):</p> <pre><code>&gt;&gt;&gt; import mygame &gt;&gt;&gt; mygame.reset_game() &gt;&gt;&gt; mygame.play_game() White Room&gt; test 'Test' to you, too! White Room&gt; go to red room Red Room&gt; go to white room White Room&gt; </code></pre> <p>To be able to play the game just by running the Python script, you can add this at the bottom:</p> <pre><code>def main(): reset_game() play_game() if __name__ == '__main__': # If we're running as a script... main() </code></pre> <p>And that's a basic introduction to classes and how to apply it to your situation.</p>
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    1. COThat `Ball` shouldn't be a class. Rule of thumb: If it has only two methods, one of which is `__init__`, you should use a function and `functools.partial` to bind the function to some data. Same for the other classes presented here. Python is not Java! The only thing I see here that *should* be a class is the game itself, for which you use module-global state instead?
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      1. USNiklas B.
    2. COIn the user exercise portion I tried to make it possible to go from white room to red room. But I cannot. I added "if "x" in line: current_room = red_room" The current room variable does not change. However if I do something like: "if "x" in line: print "this"" it works. Thank you for your post. It was super helpful!
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    3. CONevermind I figured it out! Thanks again for your help :)
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