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    <p>Variables <code>a</code>, <code>b</code>, and <code>c</code> are class variables. They're evaluated and set once, when the class is first created. Variables <code>x</code>, <code>y</code>, and <code>z</code> are instance variables, which are evaluated and set whenever an object of that class is instantiated.</p> <p>In general, you use class variables if the same values are going to be used by every instance of the class, and only needs to be calculated once. You use instance variables for variables that are going to be different for each instance of that class.</p> <p>You can access class variables through the instance variable syntax <code>self.a</code>, but the object in question is being shared between all instances of the class. This doesn't affect much when using immutable data types such as integers or strings, but when using mutable data types such as lists, appending to <code>self.a</code> would cause all instances to see the newly-appended value.</p> <p>Some examples from IDLE are probably helpful in understanding this:</p> <pre><code>&gt;&gt;&gt; class Foo(object): a = 'Hello' b = [] def __init__(self): self.y = [] &gt;&gt;&gt; instance_1 = Foo() &gt;&gt;&gt; instance_2 = Foo() &gt;&gt;&gt; instance_1.a 'Hello' &gt;&gt;&gt; instance_2.a 'Hello' &gt;&gt;&gt; instance_1.a = 'Goodbye' &gt;&gt;&gt; instance_1.a 'Goodbye' &gt;&gt;&gt; instance_2.a 'Hello' &gt;&gt;&gt; instance_1.b [] &gt;&gt;&gt; instance_2.b.append('12345') &gt;&gt;&gt; instance_1.b ['12345'] &gt;&gt;&gt; instance_2.y [] &gt;&gt;&gt; instance_2.y.append('abcde') &gt;&gt;&gt; instance_2.y ['abcde'] &gt;&gt;&gt; instance_1.y [] </code></pre>
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    1. PODefining a Python class with lots of attributes - style
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    1. CO-1: This is misleading and not how Python class attributes (which includes methods!) work at all. They're an acceptable way of providing defaults for instance attributes.
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    2. CO@millimoose, true, but not quite the whole picture. What you're saying happens to work for immutable types (such as int/str like the OP used), because when you do say `self.a = 'Byebye'` it'll be a new string and not change `Foo.a`. However, if `Foo.a` was a list, and you appended to `self.a` the original `Foo.a` would change too (it's the same object). Same "issue" with [mutable default parameters](http://effbot.org/zone/default-values.htm).
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    3. CO@benhoyt I guess that makes my downvote an overreaction so I'll nuke it. Still, this is a Python behaviour that probably warrants explanation, not dogma, especially since it ties into the more general question of inadvertently sharing mutable objects which is not always so easily avoided.
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