Types of Attribute

There are different types of attributes: public, protected, and private attributes.

During this course, you have created public attributes that are available outside the class.

Most programming languages support the following attribute access levels:

• Public: can be changed outside the class

• Protected: can only be changed by the class or a subclass

• Private: can only be changed by the class

Attributes within Python are always public, but there is a convention of using either no, single, or double underscores before an attribute name to denote its access level. For example:

class MyClass():

    def __init__(self):

        # no underscore = public

        self.my_attribute = None

        # single underscore = protected

        self._my_attribute = None

        # double underscore = private

        self.__my_attribute = None

By describing the access level of an attribute, you are stating how it can be used.

Class and instance variables

The attributes in a class are just variables used to store data within an object of that class. Each time you create an object, that object has its own set of these variables.

So far, all of the attributes you have created within the class have been what are known as instance variables. For example, your Enemy class has this attribute:

self.weakness

This attribute stores the weakness of the enemy, represented by this instance of an enemy object. It can be set to a different value for each enemy object. Even if you were to set the same weakness for two different enemy objects, each object would have its own separate copy of the weakness. If you alter the weakness attribute of one object, the weakness attribute of another object of the same class is unaffected.

You can create two enemy objects, for example Dave and Brian, and set the weakness attribute to be different for each of them:

I like to imagine an instance variable like a notebook. My friend and I might each own the same notebook, with the same cover, but these books are separate instances. If I draw in my notebook, the drawing does not also appear in my friend’s copy of the notebook.

You may now be able to guess how a class variable is different from an instance variable. A class variable is like having a shared notebook, with its contents available to all notebook owners, i.e. all objects of that class.

You can create a class variable in Python by adding it outside of the constructor.

class MyClass():

    # class variable outside the constructor

    my_class_variable = 0

    def __init__(self):

        # instance variable inside the constructor

        self.my_instance_variable = None

In the next step you will add a class variable to the Room class.