Coherent example
================

The points mentioned so far are the basics of using classes and objects in
Python. I will now illustrate these basics in a coherent example:
:download:`form.py`.

#. First, we create a base class:

   .. literalinclude:: form.py
      :language: python
      :linenos:
      :lines: 1-12
      :lineno-start: 1

   Line 7
       The ``__init__`` method requires one instance (``self``) and two
       parameters.
   Lines 8 and 9
       The two instance variables ``x`` and ``y``, which are accessed via
       ``self``.
   Line 10
       The ``move`` method requires one instance (``self``) and two parameters.
   Lines 11 and 12
       Instance variables that are set in the ``move`` method.

#. Next, create a subclass that inherits from the base class ``Form``:

   .. literalinclude:: form.py
      :language: python
      :linenos:
      :lines: 16-21
      :lineno-start: 16

   Line 16
       The class ``Square`` inherits from the class ``Form``.
   Line 19
       ``Square``’s ``__init__`` takes one instance (``self``) and three
       parameters, all with defaults.
   Line 20
       ``Square``’s ``__init__`` uses ``super()`` to call ``Form``’s
       ``__init__``.

#. Finally, we create another subclass that also contains a static method:

   .. literalinclude:: form.py
      :language: python
      :linenos:
      :lines: 27-43
      :lineno-start: 27

   Lines 29 and 30
       ``pi`` and ``circles`` are class variables for ``Circle``.
   Line 34
       In the ``__init__`` method, the instance inserts itself into the
       ``circles`` list.
   Lines 37 and 38
       ``circumferences`` is a class method and takes the class itself
       (``cls``) as a parameter.
   Line 41
       uses the parameter ``cls`` to access the class variable ``circles``.

Now you can create some instances of the class ``Circle`` and analyse them.
Since the ``__init__`` method of ``Circle`` has default parameters, you can
create a circle without specifying any parameters:

.. code-block:: pycon

   >>> import form
   >>> c1 = form.Circle()
   >>> c1.diameter, c1.x, c1.y
   (1, 0, 0)

If you specify parameters, they are used to set the values of the instance:

.. code-block:: pycon

   >>> c2 = form.Circle(2, 3, 4)
   >>> c2.diameter, c2.x, c2.y
   (2, 3, 4)

When you call the ``move()`` method, Python does not find a ``move()`` method in
the ``Circle`` class, so it goes up the inheritance hierarchy and uses the
``move()`` method of ``Form``:

.. code-block:: pycon

   >>> c2.move(5, 6)
   >>> c2.diameter, c2.x, c2.y
   (2, 8, 10)

You can also call the class method ``circumferences()`` of the class ``Circle``,
either through the class itself or through an instance:

.. code-block:: pycon

   >>> form.Circle.circumferences()
   9.424769999999999
   >>> c2.circumferences()
   9.424769999999999