Deitel - Python, How To Program.pdf

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Chapter 7 Object-Based Programming 231
One of the fundamental principles of good software engineering is that a client should
not need to know how a class is implemented to use that class. Python’s use of modules
facilitates this data abstraction—the program in Fig. 7.2 simply imports the Time definition
and uses class Time without knowing how the class is implemented.
Software Engineering Observation 7.3
Clients of a class do nost need access to the class’s source code to use the class. 7.3
To create an object of class Time, simply “call” the class name as if it were a function
(line 6). This call invokes the constructor for class Time. Even though the class definition
stipulates that the constructor (__init__) takes one argument, line 6 does not pass any
arguments to the constructor. Python inserts the first (object reference) argument into every
method call, including a class’s constructor call. The constructor initializes the object’s
attributes. Once the constructor exits, Python assigns the newly created object to time1.
Client code must access an object’s attributes through a reference to that object. Lines
10–12 demonstrate how a program can access an object’s attributes through the dot (.)
access operator. The name of the object appears to the left of the dot, and the attribute
appears to the right of the dot. The output demonstrates the initial values that the constructor
assigned to attributes hour, minute and second.
Client code can access an object’s methods in a similar manner. Line 16 calls time1’s
printMilitary method. Notice again that the method call passes no arguments, even
though the method definition specifies one parameter called self. Python passes a reference
to time1 in the printMilitary call, so the method may access the object’s
attributes.
Line 23 modifies the value assigned to attribute time1.hour. The output from lines
24–25 shows a problem that often arises when a client indiscriminately accesses an object’s
data. The meaning of attribute hour is unclear, because that data member now has a value
of 25. We say that the data member is in an inconsistent state (it contains an invalid value).
Some other programming languages provide ways to prevent a client from accessing an
object’s data. Python, on the other hand, does not provide such strict programming constructs.
Later in this chapter, we discuss the various ways Python programmers ensure that
an object’s data remains in a consistent state.
Common Programming Error 7.5
Directly accessing an object’s attributes may cause the data to enter an inconsistent state. 7.4
7.3 Special Attributes
Classes and objects of classes both have special attributes that can be manipulated. These
attributes, which Python creates when a class is defined or when an object of a class is created,
provide information about the class or object of a class to which they belong.
Figure 7.3 lists the special attributes that all classes contain. The interactive session in
Fig. 7.4 prints the value of each of these attributes for class Time.
Additionally, all objects of classes have attributes in common. Figure 7.5 lists these
attributes, and the interactive session in Fig. 7.6 prints the attributes’ values for an object
of class Time. Notice that objects can access the __doc__ and __module__ attributes
that belong to the object’s class.