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Note the use of the word self in two locations in Listing 7 . I will explain the use of theword self in class definitions in a future module.
Listing 8 defines three more methods. Except for the use of the word self , there is nothing unusual about the code in these methods.
| Listing 8 . Three more methods. |
|---|
def scan(self,city):
return self.stations[city]def setStationNumber(self,index,station):
self.stationNumber[index-1]= station
def playStation(self,index):print("Playing " + self.stationNumber[index-1]) |
The purpose of the method named scan is to simulate pressing the scan button on the radio. This method is called on the Radio object referred to by radio01 in Listing 2 . It returns a reference to one of the nested dictionary objects in the stations dictionary object.
It is called again on a different Radio object referred to by radio02 in Listing 5 . It is important to note that even though both objects were instantiated from the same Radio class, they are different objects. The only things they share are the stations variable, their ancestry, and their overall structure.
The analogy to a physical car radio breaks down with respect to the class variable named stations . Physical car radios don't share any data. You could say that data stored in physical car radios is storedin instance variables only.
The purpose of the setStationNumber method is to simulate programming the station-selector buttons on a Radio object. It is called three times, once for each button, on the object referred to by radio01 in Listing 3 . It is also called three times on the different Radio object referred to by radio02 in Listing 5 .
The purpose of the playStation method is to simulate pressing a station-selector button on the radio to play the radio station thathas been programmed into that button. It is called three times on the Radio object referred to by radio01 in Listing 4 . It is also called three times on the different Radio object referred to by radio02 in Listing 5 .
Figure 5 shows a visualization of the class definition in Listing 9 .
Figure 5. Visualizing a class definition.
I recommend that you create a visualization for the code in Listing 9 and step through the program one instruction at a time. As you do that, pay attention tothe movements of the red and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to betterunderstand the concept of classes and objects in Python.
I also encourage you to copy the code from Listing 9 . Execute the code and confirm that you get the same results as those shown in Figure 6 . Experiment with the code, making changes, and observing the results of your changes. Make certain that youcan explain why your changes behave as they do.
A complete listing of the program is provided in Listing 9 below.
Listing 9 . Complete program listing.
# This program simulates the manufacture, programming, and use of a pair of
# three-button car radios.#
#--------------------------------------------------------------------------class Radio(object):
#This class provides the plans from which the radio objects are built.stations = {"Austin":{91.7:"KVRX",95.5:"KKMJ",98.1:"KVET",93.7:"KLBJ"},
"Dallas":{98.3:"KNON",91.7:"KKXT",97.9:"KBFB",98.7:"KLUV"}}
def __init__(self):self.stationNumber = [0,0,0]
def scan(self,city):return self.stations[city]
def setStationNumber(self,index,station):self.stationNumber[index-1] = stationdef playStation(self,index):
print("Playing " + self.stationNumber[index-1])#Manufacture a 3-button radio
radio01 = Radio()#Program the three buttons labeled 1, 2, and 3
#First scan for available stationsradio01Stations = radio01.scan("Austin")
print("Available stations in Austin")print(radio01Stations)
print("Program the buttons")radio01.setStationNumber(1,radio01Stations[91.7])radio01.setStationNumber(2,radio01Stations[95.5])radio01.setStationNumber(3,radio01Stations[98.1])print("Play the three programmed stations")
radio01.playStation(3)radio01.playStation(2)
radio01.playStation(1)#Manufacture another 3-button radio
radio02 = Radio()#Program the three buttons labeled 1, 2, and 3
#First scan for available stationsradio02Stations = radio02.scan("Dallas")
print("Available stations in Dallas")print(radio02Stations)
print("Program the buttons")radio02.setStationNumber(1,radio02Stations[91.7])radio02.setStationNumber(2,radio02Stations[97.9])radio02.setStationNumber(3,radio02Stations[98.3])print("Play the three programmed stations")
radio02.playStation(3)radio02.playStation(2)
radio02.playStation(1)
Figure 6 shows the output produced by the code in Listing 9 .
| Figure 6 . Output from the program in Listing 9. |
|---|
Available stations in Austin
{93.7: 'KLBJ', 91.7: 'KVRX', 98.1: 'KVET', 95.5: 'KKMJ'}Program the buttons
Play the three programmed stationsPlaying KVET
Playing KKMJPlaying KVRX
Available stations in Dallas{98.3: 'KNON', 98.7: 'KLUV', 91.7: 'KKXT', 97.9: 'KBFB'}
Program the buttonsPlay the three programmed stations
Playing KNONPlaying KBFB
Playing KKXT |
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer InformationTechnology at Austin Community College in Austin, TX.
-end-
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