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<section id="game-object-classes">
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<section id="makegames-4">
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<span id="id1"></span><h2>4. Game object classes<a class="headerlink" href="#makegames-4" title="Permalink to this headline">¶</a></h2>
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<p>Once you've loaded your modules, and written your resource handling functions, you'll want to get on to writing some game objects.
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The way this is done is fairly simple, though it can seem complex at first. You write a class for each type of object in the game,
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and then create an instance of those classes for the objects. You can then use those classes' methods to manipulate the objects,
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giving objects some motion and interactive capabilities. So your game, in pseudo-code, will look like this:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/python</span>
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<span class="c1"># [load modules here]</span>
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<span class="c1"># [resource handling functions here]</span>
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<span class="k">class</span> <span class="nc">Ball</span><span class="p">:</span>
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<span class="c1"># [ball functions (methods) here]</span>
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<span class="c1"># [e.g. a function to calculate new position]</span>
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<span class="c1"># [and a function to check if it hits the side]</span>
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<span class="k">def</span> <span class="nf">main</span><span class="p">:</span>
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<span class="c1"># [initiate game environment here]</span>
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<span class="c1"># [create new object as instance of ball class]</span>
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<span class="n">ball</span> <span class="o">=</span> <span class="n">Ball</span><span class="p">()</span>
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<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
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<span class="c1"># [check for user input]</span>
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<span class="c1"># [call ball's update function]</span>
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<span class="n">ball</span><span class="o">.</span><span class="n">update</span><span class="p">()</span>
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</pre></div>
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</div>
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<p>This is, of course, a very simple example, and you'd need to put in all the code, instead of those little bracketed comments. But
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you should get the basic idea. You create a class, into which you put all the functions for a ball, including <code class="docutils literal notranslate"><span class="pre">__init__</span></code>,
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which would create all the ball's attributes, and <code class="docutils literal notranslate"><span class="pre">update</span></code>, which would move the ball to its new position, before blitting
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it onto the screen in this position.</p>
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<p>You can then create more classes for all of your other game objects, and then create instances of them so that you can handle them
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easily in the <code class="docutils literal notranslate"><span class="pre">main</span></code> function and the main program loop. Contrast this with initiating the ball in the <code class="docutils literal notranslate"><span class="pre">main</span></code>
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function, and then having lots of classless functions to manipulate a set ball object, and you'll hopefully see why using classes is
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an advantage: It allows you to put all of the code for each object in one place; it makes using objects easier; it makes adding new
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objects, and manipulating them, more flexible. Rather than adding more code for each new ball object, you could simply create new
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instances of the <code class="docutils literal notranslate"><span class="pre">Ball</span></code> class for each new ball object. Magic!</p>
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<section id="a-simple-ball-class">
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<span id="makegames-4-1"></span><h3>4.1. A simple ball class<a class="headerlink" href="#a-simple-ball-class" title="Permalink to this headline">¶</a></h3>
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<p>Here is a simple class with the functions necessary for creating a ball object that will, if the <code class="docutils literal notranslate"><span class="pre">update</span></code> function is called
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in the main loop, move across the screen:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Ball</span><span class="p">(</span><span class="n">pygame</span><span class="o">.</span><span class="n">sprite</span><span class="o">.</span><span class="n">Sprite</span><span class="p">):</span>
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<span class="w"> </span><span class="sd">"""A ball that will move across the screen</span>
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<span class="sd"> Returns: ball object</span>
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<span class="sd"> Functions: update, calcnewpos</span>
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<span class="sd"> Attributes: area, vector"""</span>
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<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">vector</span><span class="p">):</span>
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<span class="n">pygame</span><span class="o">.</span><span class="n">sprite</span><span class="o">.</span><span class="n">Sprite</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">image</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">rect</span> <span class="o">=</span> <span class="n">load_png</span><span class="p">(</span><span class="s1">'ball.png'</span><span class="p">)</span>
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<span class="n">screen</span> <span class="o">=</span> <span class="n">pygame</span><span class="o">.</span><span class="n">display</span><span class="o">.</span><span class="n">get_surface</span><span class="p">()</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">area</span> <span class="o">=</span> <span class="n">screen</span><span class="o">.</span><span class="n">get_rect</span><span class="p">()</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">vector</span> <span class="o">=</span> <span class="n">vector</span>
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<span class="k">def</span> <span class="nf">update</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
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<span class="n">newpos</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">calcnewpos</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">rect</span><span class="p">,</span><span class="bp">self</span><span class="o">.</span><span class="n">vector</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">rect</span> <span class="o">=</span> <span class="n">newpos</span>
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<span class="k">def</span> <span class="nf">calcnewpos</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span><span class="n">rect</span><span class="p">,</span><span class="n">vector</span><span class="p">):</span>
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<span class="p">(</span><span class="n">angle</span><span class="p">,</span><span class="n">z</span><span class="p">)</span> <span class="o">=</span> <span class="n">vector</span>
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<span class="p">(</span><span class="n">dx</span><span class="p">,</span><span class="n">dy</span><span class="p">)</span> <span class="o">=</span> <span class="p">(</span><span class="n">z</span><span class="o">*</span><span class="n">math</span><span class="o">.</span><span class="n">cos</span><span class="p">(</span><span class="n">angle</span><span class="p">),</span><span class="n">z</span><span class="o">*</span><span class="n">math</span><span class="o">.</span><span class="n">sin</span><span class="p">(</span><span class="n">angle</span><span class="p">))</span>
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<span class="k">return</span> <span class="n">rect</span><span class="o">.</span><span class="n">move</span><span class="p">(</span><span class="n">dx</span><span class="p">,</span><span class="n">dy</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>Here we have the <code class="docutils literal notranslate"><span class="pre">Ball</span></code> class, with an <code class="docutils literal notranslate"><span class="pre">__init__</span></code> function that sets the ball up, an <code class="docutils literal notranslate"><span class="pre">update</span></code>
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function that changes the ball's rectangle to be in the new position, and a <code class="docutils literal notranslate"><span class="pre">calcnewpos</span></code> function to calculate the ball's
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new position based on its current position, and the vector by which it is moving. I'll explain the physics in a moment. The one other
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thing to note is the documentation string, which is a little bit longer this time, and explains the basics of the class. These strings
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are handy not only to yourself and other programmers looking at the code, but also for tools to parse your code and document it. They
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won't make much of a difference in small programs, but with large ones they're invaluable, so it's a good habit to get into.</p>
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<section id="diversion-1-sprites">
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<span id="makegames-4-1-1"></span><h4>4.1.1. Diversion 1: Sprites<a class="headerlink" href="#diversion-1-sprites" title="Permalink to this headline">¶</a></h4>
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<p>The other reason for creating a class for each object is sprites. Each image you render in your game will be a sprite object, and so
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to begin with, the class for each object should inherit the <a class="reference internal" href="../ref/sprite.html#pygame.sprite.Sprite" title="pygame.sprite.Sprite"><code class="xref py py-class docutils literal notranslate"><span class="pre">Sprite</span></code></a> class.
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This is a really nice feature of Python - class
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inheritance. Now the <code class="docutils literal notranslate"><span class="pre">Ball</span></code> class has all of the functions that come with the <code class="docutils literal notranslate"><span class="pre">Sprite</span></code> class, and any object
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instances of the <code class="docutils literal notranslate"><span class="pre">Ball</span></code> class will be registered by Pygame as sprites. Whereas with text and the background, which don't
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move, it's OK to blit the object onto the background, Pygame handles sprite objects in a different manner, which you'll see when we
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look at the whole program's code.</p>
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<p>Basically, you create both a ball object, and a sprite object for that ball, and you then call the ball's update function on the
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sprite object, thus updating the sprite. Sprites also give you sophisticated ways of determining if two objects have collided.
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Normally you might just check in the main loop to see if their rectangles overlap, but that would involve a lot of code, which would
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be a waste because the <code class="docutils literal notranslate"><span class="pre">Sprite</span></code> class provides two functions (<code class="docutils literal notranslate"><span class="pre">spritecollide</span></code> and <code class="docutils literal notranslate"><span class="pre">groupcollide</span></code>)
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to do this for you.</p>
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</section>
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<section id="diversion-2-vector-physics">
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<span id="makegames-4-1-2"></span><h4>4.1.2. Diversion 2: Vector physics<a class="headerlink" href="#diversion-2-vector-physics" title="Permalink to this headline">¶</a></h4>
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<p>Other than the structure of the <code class="docutils literal notranslate"><span class="pre">Ball</span></code> class, the notable thing about this code is the vector physics, used to calculate
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the ball's movement. With any game involving angular movement, you won't get very far unless you're comfortable with trigonometry, so
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I'll just introduce the basics you need to know to make sense of the <code class="docutils literal notranslate"><span class="pre">calcnewpos</span></code> function.</p>
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<p>To begin with, you'll notice that the ball has an attribute <code class="docutils literal notranslate"><span class="pre">vector</span></code>, which is made up of <code class="docutils literal notranslate"><span class="pre">angle</span></code> and <code class="docutils literal notranslate"><span class="pre">z</span></code>.
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The angle is measured in radians, and will give you the direction in which the ball is moving. Z is the speed at which the ball
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moves. So by using this vector, we can determine the direction and speed of the ball, and therefore how much it will move on the x and
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y axes:</p>
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<img alt="../_images/tom_radians.png" src="../_images/tom_radians.png" />
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<p>The diagram above illustrates the basic maths behind vectors. In the left hand diagram, you can see the ball's projected movement
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represented by the blue line. The length of that line (z) represents its speed, and the angle is the direction in which
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it will move. The angle for the ball's movement will always be taken from the x axis on the right, and it is measured clockwise from
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that line, as shown in the diagram.</p>
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<p>From the angle and speed of the ball, we can then work out how much it has moved along the x and y axes. We need to do this because
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Pygame doesn't support vectors itself, and we can only move the ball by moving its rectangle along the two axes. So we need to
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<em class="firstterm">resolve</em> the angle and speed into its movement on the x axis (dx) and on the y axis (dy). This is a simple matter of
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trigonometry, and can be done with the formulae shown in the diagram.</p>
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<p>If you've studied elementary trigonometry before, none of this should be news to you. But just in case you're forgetful, here are some
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useful formulae to remember, that will help you visualise the angles (I find it easier to visualise angles in degrees than in radians!)</p>
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<img alt="../_images/tom_formulae.png" src="../_images/tom_formulae.png" />
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</section>
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</section>
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</section>
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</section>
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<br /><br />
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<hr />
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<a href="https://github.com/pygame/pygame/edit/main/docs/reST/tut\tom_games4.rst" rel="nofollow">Edit on GitHub</a>
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<div class="clearer"></div>
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</div>
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</div>
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<div class="clearer"></div>
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</div>
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