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In Java, C#, 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 in generalcannot be modified 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.
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 can modify the structure of a Python object at runtime after it is instantiated. By that I mean that you can add newinstance variables to an object such that the structure of the object is different from the blueprint provided by the class from which it wasinstantiated. Further, you can modify the blueprint provided by the class at runtime after the class has been used to instantiate one or more objects.
I'm not suggesting that this is either good or bad. I'm simply stating that itis true. If you are coming at Python from a Java or C++ background, you must significantly modify the way that you think about classes and objects to makethe transition. If you plan to progress from Python to either Java or C++, your will need to significantly modify the way that you think about classes andobjects when you make that transition.
I will discuss three different short programs in this module. A complete listing of the first program is provided in Listing 9 . The output from that program is shown in Figure 5 .
I will break each program down and discuss it in fragments. The first fragment is shown in Listing 1 .
Listing 1 shows the definition of a class named TestClass that contains no variables and no methods. In other words, the class isempty.
| Listing 1 . Empty objects. |
|---|
class TestClass(object):
passprint("Instantiate and display two objects of TestClass")
ref01 = TestClass()print(ref01)
ref02 = TestClass()print(ref02) |
Listing 1 also shows the instantiation of two objects from the empty class named TestClass . Those object's references are stored in the variables named ref01 and ref02 .
The variables are also printed in Listing 1 producing the output shown in Figure 1 .
| Figure 1 . Print the object's references. |
|---|
Instantiate and display two objects of TestClass<__main__.TestClass object at 0x00274910><__main__.TestClass object at 0x005A25F0> |
I told you in an earlier module that the variables named ref01 and ref02 contain information that somehow identifies the chunks of memory occupied by the objects. Thisis indicated by the two hexadecimal values at the right end of the last two lines of text in Figure 1 . This is apparently one thing that can't be changed about an object after it is instantiated. Even after modifying thecontents of an object, the hexadecimal identifier seems to remain constant.
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