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copied!<p>A <em>class</em> consists basically of data (attributes) and a collection of functions (methods) to operate on that data. Each <em>instance</em> of a class has its own copy of the data, but shares the methods.</p> <p>Thus, every<sup>*</sup> method of a class operates on some instance of the class, so the method needs to know <em>what</em> instance it is operating on. That's what <code>self</code> is for -- <code>self</code> just means the instance that a method is working with.</p> <p>So, for example:</p> <pre><code>class House: def renovate(self): ''' Renovate a house, increasing its value. ''' self.value += 10000 oldhouse = House() # make a House instance that has no attributes oldhouse.value = 100000 # set an attribute of the House mansion = House() mansion.value = 10000000 oldhouse.renovate() # self will be set to oldhouse, so that oldhouse's value goes up print oldhouse.value # prints 110000 print mansion.value # prints 10000000: it hasn't changed because renovate knew what it was operating on </code></pre> <p>Now, in this example, note that I set <code>.value</code> outside the class. You can do that (Python lets you set any attributes on almost any instance), but it's kind of cumbersome to do that for every class.</p> <p>This is where <code>__init__</code> comes in. It's a method with special meaning (hence the double underscores). In this case, it's a method which is called when the class gets created (when you call the class with parens to make a new instance). <code>init</code> here stands for <code>initialization</code>: it's a function that initializes the class attributes. Any arguments it takes (beyond <code>self</code>) will be expected when you create the class.</p> <p>So, we could change our example to this:</p> <pre><code>class House: def __init__(self, value): self.value = value # set attribute def renovate(self): ''' Renovate a house, increasing its value. ''' self.value += 10000 oldhouse = House(100000) # make a House instance with the value set mansion = House(10000000) # make another House instance with the value set oldhouse.renovate() print oldhouse.value # prints 110000 print mansion.value # prints 10000000 </code></pre> <p>and it would work the same, but be somewhat nicer (since we don't have to keep setting <code>.value</code> on new <code>House</code>s).</p> <hr> <p><sup>*</sup> except staticmethods and classmethods, and special cases like <code>__new__</code></p>

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