================================
The Natural class implementation
================================

Now we merely have to provide an implementation for our Natural class.

Create a file ``src/transient/natural.py`` under your Grok instance and give it
the contents:

.. code-block:: python

 class Natural(object):
     """A natural number, here defined as an integer greater than zero."""

     def __init__(self, n):
         self.n = abs(int(n)) or 1

     def __str__(self):
         return '%d' % self.n

     def __repr__(self):
         return 'Natural(%d)' % self.n

     @property
     def previous(self):
         if self.n < 2:
             raise AttributeError('There is no natural number less than 1.')
         return Natural(self.n - 1)

     @property
     def next(self):
         return Natural(self.n + 1)

     @property
     def factors(self):
         if not hasattr(self, '_factors'):  # compute factors only once!
             n, i = self.n, 2
             self._factors = []
             while i <= n:
                 while n % i == 0:          # while n is divisible by i
                     self._factors.append(Natural(i))
                     n /= i
                 i += 1
         return self._factors

     @property
     def is_prime(self):
         return len(self.factors) < 2

     @property
     def is_composite(self):
	 return len(self.factors) > 1

If you try running ``./bin/test`` again after creating this file, you should
find that the entire ``natural.txt`` docfile now runs correctly!

I hope that if you are new to Python, you are not too confused by the code
above, which uses ``@property`` which may not have been covered in the Python
tutorial. But I prefer to show you “real Python” like this, that reflects how
people actually use the language, rather than artifically simple code that
hides from you the best ways to use Python. Note that it is *not* necessary to
understand ``natural.py`` to enjoy the rest of this tutorial! Everything we do
from this point on will involve building a framework to use this object on the
web; we will be doing no further development on the class itself. So all you
actually need to understand is how a ``Natural`` behaves, which was entirely
explained in the doctest.

Note that the ``Natural`` class knows nothing about Grok! This is an important
feature of the whole Zope 3 framework, that bears frequent repeating: objects
are supposed to be simple, and not have to know that they are being presented
on the web. You should be able to grab objects created anywhere, from any old
library of useful functions you happen to download, and suit them up to be
displayed and manipulated with a browser. And the ``Natural`` class is exactly
like that: it has no idea that we are about to build a framework around it that
will soon be publishing it on the web.