Simple Class & Objects

A class creates a new type where objects are instances of the class.

You can think of class as a blueprint, it simply describes how to make something. You can create lots of objects of a particular kind from that blueprint, technically objects are called instances of that class.

The syntax of defining a class:

class MyClassName:

    data attributes...

    functions...

The first line defines the class container, which starts with the keyword class and the class name, then a colon : ends the line.

All indented statements (4 spaces) form the body of the class. The class body consists of all the component statements defining class members: data attributes(variables) and functions(also called methods).

Class names are usually capitalized, MyClassNameLikeThis, but this is only a convention, not a requirement

Everything in a class is indented, just like the code within a function, if statement, for loop. The first thing not indented is not in the class.

See a very simple class example:

Apple class

Let's make a simple class to represent an apple, this class has only one variable color:

class Apple:           
    name = 'Fuji'

The first line defines the class Apple, in the indented class body, there is only one attribute: the class variable named name, which is set to a value of 'Fuji'.

When creating a class Apple, I haven't actually created an apple object yet. Instead, what I've created is a sort of instruction manual for constructing "Apple" objects.

Class Instantiation, Object

Class instantiation means we create an object/instance of the class. this uses function notation: using the name of the class followed by a pair of parentheses.

Using the class to create an object:

x = MyClassName()

This creates a new instance of the class and assigns this object to the local variable x. For now, just pretend that a class object is a parameterless function that returns a new instance/object of the class.

x, as an instance of the class will get all the attributes defined in the class. Accessing the attributes of x using dot notation: x.attribute

Let's create an object using class Apple

a1 = Apple()         # create an object
print( a1.name )     # Fuji (access attribute)

a1.name = 'Gala'     # assign a new value to attribute
print( a1.name)      # Gala

We create an Apple object, a1, by calling the class name with an parenthesis(it's empty in this example, just pretend it's a function with no input parameters). a1 as an instance/object of Apple class, it has attribute, name, that starts at value 'Fuji', we can access it by using objectName.attribute, here it's a1.name; we can also assign new value to the attributes like a1.name = 'Gala'. This is data attribute.

Objects, created from a class, have attributes defined in the class:

data attributes, function attributes(also called methods).

Next lesson: Functions belong to an object, methods.