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As you have probably already figured out, each object gets and prints the contents of the class variable named classVar causing matching output values to be seen in Figure 7 .
| Figure 7 . Output from the coded in Listing 12. |
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1 Instantiate two objects
2 Display classVar using objectsTestClass object at 0x02201DD0: 1234
TestClass object at 0x022FC710: 1234 |
The code in Listing 13 calls the shadowClassVariable method on the object referred to by the variable named ref01 . As I described earlier , this call will add a new instance variable named classVar to the object.From that point forward, the class variable named classVar won't be visible insofar as that object is concerned unless it accesses theclass variable using the name of the class, TestClass .
| Listing 13 . Modify classVar in one object. |
|---|
print("3 Shadow the classVar in one object")
ref01.shadowClassVariable()print("4 Display classVar using objects")
ref01.printClassVar()ref02.printClassVar() |
Once again, The code in Listing 13 also uses the objects' references to call the method named printClassVar on each object. This causes the contents of classVar to be printed as seen by each object at this point in the program. The results are shown in Figure 8 .
| Figure 8 . Output from the code in Listing 13. |
|---|
3 Shadow the classVar in one object
4 Display classVar using objectsTestClass object at 0x02201DD0: ABCD
TestClass object at 0x022FC710: 1234 |
The two output values in Figure 8 no longer match. This is because the object referred to by ref01 no longer sees the class variable named classVar . Instead, it sees the new instance variable named classVar , which contains a different value.
The code in Listing 14 uses the name of the class, TestClass , to change the value stored in its class variable named classVar . Then the objects' reference are used to once again print the contents of classVar as seen by each object.
| Listing 14 . Modify the contents of the class variable. |
|---|
print("5 Modify classVar using class")
TestClass.classVar = Trueprint("6 Display classVar using objects")
ref01.printClassVar()ref02.printClassVar() |
The results are shown in Figure 9 . The change in the value of the class variable is reflected in the output produced by the object referred to by ref02 . However, the object referred to by ref01 is blind to that change because the class variable is shadowed or hidden by aninstance variable having the same name in that object.
| Figure 9 . Output from the code in Listing 14. |
|---|
6 Display classVar using objects
TestClass object at 0x02201DD0: ABCDTestClass object at 0x022FC710: True |
I encourage you to copy the code from Listing 15 , Listing 16 , and Listing 17 . Execute the code and confirm that you get the same results as those shown. Experiment with the code,making changes, and observing the results of your changes. Make certain that you can explain why your changes behave as they do.
I also encourage you to create visualizations for the code in Listing 15 , Listing 16 , and Listing 17 . Step through the programs one instruction at a time. As you do that, pay attention to the movements of the redand green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to better understand the use of classvariables in Python.
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