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copied!<pre><code>class a: def __init__(self): self.val=1 </code></pre> <p>this creates a class (in Py2, a cruddy, legacy, old-style, <strong>don't do that!</strong> class; in Py3, the nasty old legacy classes have finally gone away so this would be a class of the one and only kind -- the **good* kind, which requires <code>class a(object):</code> in Py2) such that each instance starts out with its own reference to the integer object <code>1</code>.</p> <pre><code>class a: val=1 def __init__(self): pass </code></pre> <p>this creates a class (of the same kind) which itself has a reference to the integer object <code>1</code> (its instances start out with no per-instance reference).</p> <p>For immutables like <code>int</code> values, it's hard to see a practical difference. For example, in either case, if you later do <code>self.val = 2</code> on one instance of <code>a</code>, this will make an <em>instance</em> reference (the existing answer is badly wrong in this respect).</p> <p>The distinction is important for <em>mutable</em> objects, because they have mutator methods, so it's pretty crucial to know if a certain list is unique per-instance or shared among all instances. But for <em>immutable</em> objects, since you can never change the object itself but only assign (e.g. to <code>self.val</code>, which will always make a per-instance reference), it's pretty minor.</p> <p>Just about the only relevant difference for immutables: if you later assign <code>a.val = 3</code>, in the first case this will affect what's seen as <code>self.val</code> by each instance (except for instances that had their own <code>self.val</code> <em>assigned</em> to, or equivalent actions); in the second case, it will not affect what's seen as <code>self.val</code> by any instance (except for instances for which you had performed <code>del self.val</code> or equivalent actions).</p>

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