Deitel - Python, How To Program.pdf

pythonhtp1_07.fm Page 238 Saturday, December 8, 2001 2:29 PM
238 Object-Based Programming Chapter 7
Python 2.2b2 (#26, Nov. 16 2001, 11:44:11) [MSC 32 bit (Intel)] on
win32
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>>>
>>> raise ValueError, "This is an error message"
Traceback (most recent call last):
File "", line 1, in ?
ValueError: This is an error message
Fig. Fig. 7.8 7.8 Raising an exception.
Lines 45–58 contain the get methods for class Time. Clients use these methods
(getHour, getMinute and getSecond) to retrieve the values of an attributes _hour,
_minute and _second, respectively. The remainder of the class definition does not
differ from the previous definition we presented.
Software Engineering Observation 7.8
If a class provides access methods for its data, clients should use only access methods to retrieve/modify
data. This “agreement” between class and client helps maintain data in a consistent
state. 7.8
Software Engineering Observation 7.9
The class designer need not provide set or get methods for each data item; these capabilities
should be provided only when appropriate. If the service is appropriate for clients, that service
should be provided in the class’s interface. 7.9
Software Engineering Observation 7.10
Every method that modifies the data of an object should ensure that the data remains in a
consistent state. 7.10
Figure 7.9 contains a driver for modified class Time. A driver is a program that tests
a class’s interface. Lines 4–6 import class Time from module Time2 and create an
object of the class. Lines 9–12 call methods printMilitary and printStandard to
display the initial time values of the object.
1 # Fig. 7.9: fig07_09.py
2 # Driver to test class TimeControl.
3
4 from Time2 import Time
5
6 time1 = Time()
7
8 # print initial time
9 print "The initial military time is",
10 time1.printMilitary()
11 print "\nThe initial standard time is",
12 time1.printStandard()
Fig. Fig. 7.9 7.9 Access methods called to change data. (Part 1 of 2.)