Itse1359-1410-overview of python classes  (Page 6/9)

A python class definition is a set of plans

The same is true for software objects. In order to create a software object in Python, it is necessary for someone to first create a plan.In Python, we refer to that plan as a class . The class is defined by a Python programmer. Once the class definition is available, that programmer (or other programmers) , can use it to produce large numbers of nearly identical objects.

An instance of a class

If we were standing at the output end of the factory that produces car radios, we might pick up a new radio and say that it is an instance of the plans used to produce the radio. (Unless they were object-oriented programmers, the people around us might think we were a little odd when they hear us say that.) However, it is common jargon to refer to a software object as an instance of a class.

To instantiate an object

Furthermore, somewhere along the way, someone turned the word instance into a verb, and it is also common jargon to say that when creating a new object, we are instantiating an object.

Program code

As mentioned above, I will explain a program that uses the analogy of a car radio to illustrate several aspects of Python classes and objects.

I will explain this program in fragments. A complete listing of the program is provided in Listing 9 near the end of the module. The output from running the program is shown in Figure 6 . Because this is an overview module, this explanation will gloss over various details. I willrevisit and explain many of those details in future modules.

Manufacture a three-button radio

As you will see if you examine Listing 9 , this program defines a class named Radio . That class can be used to create objects that simulate car radios havingthree station-selector buttons and a scan button. I will explain that class later. First I will explain how to use the class.

Listing 1 shows the code necessary to create (instantiate) a single instance of the Radio class and to store that object's reference in a variable named radio01 . (In OOP jargon we say that an object is an instance of a class.) This simulates the manufacturing of the radio and the installation of the radio in a car.

As a practical matter, the code required to instantiate a Python object looks just like the code required to call a function having the same name asthe class from which the object is being instantiated. In my opinion, this causes Python to be a little less readable than Java, which uses a specialsyntax to create an object.

Scan for available stations

Next we want to program the station-selector buttons to cause our favorite radio stations to be played when we press a button. First, however, we willsimulate pressing the scan button on the radio to learn about the stations that are available in the area.

Listing 2 uses the reference variable named radio01 to call a method named scan belonging to the object referred to by radio01 . As you will see later when we examine the class definition, the scan method requires the name of the city as an incoming parameter and returns a reference to a dictionary that relatesfrequencies to radio station call signs in that city. The returned reference is stored in the variable named radio01Stations .