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Smart Properties
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$Id$

lzprop
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The `lzprop` decorator allows the declaration of lazy properties - attributes
that are calculated only on first access, and then just once. This is
extremely useful when working with objects of a limited lifetime (e.g.
adapters) that provide results from expensive calculations.

We use a simple class with one lazy property that tracks the calculation
by printing an informative message:

    >>> from cybertools.util.property import lzprop

    >>> class Demo(object):
    ...     base = 6
    ...     @lzprop
    ...     def value(self):
    ...         print 'calculating'
    ...         return self.base * 7

    >>> demo = Demo()

When we first access the `value` attribute the corresponding method will be
called:

    >>> demo.value
    calculating
    42

On subsequent accesses the previously calculated value will be returned:

    >>> demo.value
    42
    >>> demo.base = 15
    >>> demo.value
    42

Let's make sure the value is really calculated upon first access (and not
already during compilation or object creation):

    >>> demo2 = Demo()
    >>> demo2.base = 15
    >>> demo2.value
    calculating
    105
    >>> demo2.value
    105