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copied!<p>Sounds like you want to know if your class should be an <a href="http://en.wikipedia.org/wiki/Immutable_object" rel="nofollow">immutable object</a>. The idea is that, once created, an immutable object can't/should't/would't be changed.</p> <p>On <a href="http://en.wikipedia.org/wiki/Immutable_object#Python" rel="nofollow">Python</a>, built-in types like <code>int</code> or <code>tuple</code> instances are immutable, enforced by the language:</p> <pre><code>>>> a=(1, 2, 3, 1, 2, 3) >>> a[0] = 9 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment </code></pre> <p>As another example, every time you add two integers a new instance is created:</p> <pre><code>>>> a=5000 >>> b=7000 >>> d=a+b >>> d 12000 >>> id(d) 42882584 >>> d=a+b >>> id(d) 42215680 </code></pre> <p>The <a href="http://docs.python.org/library/functions.html#id" rel="nofollow"><code>id()</code> function</a> returns the address of the <code>int</code> object <code>12000</code>. And every time we add <code>a+b</code> a new <code>12000</code> object instance is created.</p> <p>User defined immutable classes must be enforced manually, or simply done as a convention with a source code comment:</p> <pre><code>class X(object): """Immutable class. Don't change instance variables values!""" def __init__(self, *args): self._some_internal_value = ... def some_operation(self, arg0): new_instance = X(arg0 + ...) new_instance._some_internal_operation(self._some_internal_value, 42) return new_instance def _some_internal_operation(self, a, b): """...""" </code></pre> <p>Either way, it's OK to create a new instance for every operation.</p>

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