Classes

A class in Python 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:

>>> 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:

>>> 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:

>>> 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:

>>> 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:

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