3.6 Itse1359-1440-class variables  (Page 2/8)

Listings

• Listing 1 . A simple class definition.
• Listing 2 . Instantiate and display two objects.
• Listing 3 . Add a class variable directly.
• Listing 4 . Add a class variable via an object.
• Listing 5 . Display both class variables via one object.
• Listing 6 . Display both class variables via the other object.
• Listing 7 . The class definition.
• Listing 8 . Instantiate two objects from the class.
• Listing 9 . Modify class variable using object.
• Listing 10 . Modify the class variable using the class name.
• Listing 11 . Class definition with a shadowing method.
• Listing 12 . Instantiate and display classVar in two objects.
• Listing 13 . Modify classVar in one object.
• Listing 14 . Modify the contents of the class variable.
• Listing 15 . Complete program listing.
• Listing 16 . Complete program listing.
• Listing 17 . Complete program listing.

General background information

In Java and C++, once you define and compile a class, that class, (which is the blueprint for an object) , can only be modified by recompiling the class. In other words, once compiled, a class is intended to be stable and cannot be modified at runtime. (At lease I don't know how to modify a Java class definition at runtime.)

Also in Java and C++, once you instantiate an object from a class, you cannot modify the structure of the object. You can modify the values of the data storedin the object but the structure of the object is stable and cannot be modified at runtime. (At least I don't know how to modify the structure of a Java object at runtime.) Among other things, you cannot add new variables to an existing object.

Along that line, the structure of an object in Java and C++ is explicitly tied to the blueprint provided by the class from which it was instantiated.There is a fixed relationship between the object and the class from which it was instantiated.

None of that is true in Python. You learned in an earlier module that you can modify the structure of a Python object at runtime after it is instantiated. Bythat I mean that you can add new instance variables to an object such that the structure of the object no longer matches the blueprint provided by the classfrom which it was instantiated.

You will learn in this module that you can modify the blueprint provided by the class at runtime after the class has been used to instantiate one or moreobjects.

Discussion and sample code

I will discuss three different programs in this module. A completelisting of the first program is provided in Listing 15 . The output from that program is shown in Figure 10 .

I will break each program down and discuss it in fragments. The first fragment is shown in Listing 1 .

Late modification of class

Let me begin by saying that this is probably not how the designers of Python intended for classes and objects to be used. However, if you are going toprogram using classes and objects in Python, you need to understand them.

A simple class definition

Listing 1 shows a simple Python class definition for a class named TestClass . This class definition does not define any class variables and does not define anyinstance variables. It does define a method named addClassVar , which I will discuss in more detail later.