Classes
=======

A :doc:`class in Python <python3:tutorial/classes>` is actually a data type. All
of Python’s built-in data types are classes, and Python provides you with
powerful tools to manipulate every aspect of a class’s behaviour. You can define
a class with the ``class`` statement:

.. code-block:: pycon

    >>> class MyClass:
    ...     STATEMENTS
    ...

``MyClass``
    Class identifiers are usually written in capital letters, that mean the
    first letter of each word is capitalised to emphasise the identifiers.
``STATEMENTS``
    is a list of Python statements – usually variable assignments and function
    definitions. However, no assignments or function definitions are required;
    it can just be a single ``pass`` statement.

After you have defined the class, you can create a new object of the class type
(an instance of the class) by calling the class name as a function:

.. code-block:: pycon

    >>> instance = MyClass()

Class instances can be used as structures or data sets. However, unlike C
structures or Java classes, the data fields of an instance do not have to be
declared in advance. The following short example defines a class called
``Square``, creates a ``Square`` instance, assigns a value to the edge length
and then uses this value to calculate the total edge length:

.. code-block:: pycon

    >>> my_square = Square()
    >>> my_square.length = 3
    >>> print(f"The perimeter of the square is {4 * my_square.length}.")
    The perimeter of the square is 12.

As in Java and many other languages, the fields of an instance are addressed
using dot notation.

You can initialise fields of an instance automatically by including an
``__init__`` initialisation method in the class. This function is executed each
time an instance of the class is created with this new instance as the first
argument ``self``. Unlike in Java and C++, Python classes can also have only one
``__init__`` method. In the following example, squares with an edge length of
``1`` are created by default:

.. code-block:: pycon
    :linenos:

    >>> class Square:
    ...     def __init__(self):
    ...         self.length = 1
    ...
    >>> my_square = Square()
    >>> print(f"The perimeter of the square is {4 * my_square.length}.")
    The perimeter of the square is 4.

Line 2
    By convention, ``self`` is always the name of the first argument of
    ``__init__``. ``self`` is set to the newly created ``Square`` instance when
    ``__init__`` is executed.
Line 5
    Next, the code uses the class definition. You first create a ``Square``
    instance object.
Line 6
    This line takes advantage of the fact that the ``length`` field is already
    initialised.

    You can also overwrite the ``length`` field so that the last line gives a
    different result than the previous ``print`` statement:

    .. code-block:: pycon

        >>> my_square.length = 3
        >>> print(f"The perimeter of the square is {4 * my_square.length}.")
        The perimeter of the square is 12.

Checks
------

* Write a :class:`Triangle` class that can also calculate the area.