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Although it would be technically possible to define an empty class and instantiate objects from that class in Java and C++, such objects probablywouldn't be very useful in their own right.
In Python, you can define an empty class and instantiate objects from that class. You can add instance variables to the object at runtime after itis instantiated, which might be useful in some situations. As you will see in a future module, you can even add class variables to a class after it has beenused to instantiate objects. That might also be useful in some situations.
The code in Listing 2 uses the variable named ref01 to add a new variable with a value of 1234 to the object referred to by ref01 .
| Listing 2 . Add an instance variable to one object. |
|---|
print("Add an instance variable to one object and display it")
ref01.iVar = 1234print(ref01.iVar) |
Listing 2 also uses the reference variable named ref01 to print the value of the new variable belonging to the object referred toby ref01 . Figure 2 shows the output produced by the code in Listing 2 .
| Figure 2 . Output from the code in Listing 2. |
|---|
Add an instance variable to one object and display it
1234 |
The code in Listing 3 uses the variable named ref02 in an attempt to display the same instance variable on the object referred to by ref02 .
| Listing 3 . Try to display the same instance variable in the other object. |
|---|
print("Try to display the same instance variable in the other object")
print(ref02.iVar) |
However, the code that added a new instance variable to one object in Listing 2 did not add the same instance variable to both objects that were originally instantiated from the same class. Therefore, at this point, thetwo objects have different structures. The code in Listing 3 produced the error message shown in Figure 3 .
| Figure 3 . Output from the code in Listing 3. |
|---|
Try to display the same instance variable in the other object
Traceback (most recent call last):File "1359-1430-13.py", line 18, in<module>print(ref02.iVar)
AttributeError: 'TestClass' object has no attribute 'iVar' |
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 behavior of empty objects in Python.
The next program that I will discuss is a minor modification to the previous program but ends up with the same result. A complete listing of the program isprovided in Listing 10 and the output from the program is shown in Figure 6 . I will only discuss those things that are different between this program and theprevious program.
The first difference is shown in Listing 4 , which shows the definition of a class named TestClass . Instead of being empty as in the previous program, this class provides a method named addInstanceVar by which an object can add and initialize a new instance variable named iVar to itself at runtime.
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