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>>> False == 0 True >>> True + False + 42 43 So now, if you see a logical expression returning 1 or 0 (probably in an older version of Python), you will know that what is really meant is True and False. The Boolean values True and False belong to the type bool, which can be used (just like, for example, list, str, and tuple) to convert other values: >>> bool('I think, therefore I am') True >>> bool(42) True >>> bool('') False >>> bool(0) False Because any value can be used as Boolean values, you will most likely rarely (if ever) need such an explicit conversion.

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Encapsulation is the ability for an object to have certain methods and attributes available for use publicly (from any section of code), but for others to be visible only within the class itself or by other objects of the same class.

Many of the sketch possibilities you have reviewed so far in this chapter can also be a good t with the rest of the Explanation slides. For example, simply sketch a few words on an Explanation slide, as shown on the upper-right slide in Figure 7-21, to note that you ll include the names of every major rm on the slide. Sketch a personal digital assistant, as shown on the lower-left slide, to illustrate only a text message away, or sketch several photographs, including a phone and a computer screen, to illustrate communications according to your style, as shown on the lower right slide.

Note At a more technical level, encapsulation refers to the ability of objects to hide their constituent data

The rationale for encapsulation is that you should make as few methods available from your classes as possible, so that even if you choose to rewrite the internals of your classes, you can maintain a few methods that interface between other elements of your system and your classes and objects. Encapsulation helps you keep a lot of functionality within your classes, but gives you the security of only having a few ways for the outside world to manipulate your object s data. This can allow you to extend and change your classes without the worry that other elements of your application will break. Here s an example class that represents a person:

Truth values can be combined (which you ll see in a while), but let s first see what you can use them for. Try running the following script: name = raw_input('What is your name ') if name.endswith('Gumby'): print 'Hello, Mr. Gumby' This is the if statement, which lets you do conditional execution. That means that if the condition (the expression after if but before the colon) evaluates to true (as defined previously), the following block (in this case, a single print statement) is executed. If the condition is false, then the block is not executed (but you guessed that, didn t you ).

FIGURE 7-21 Sketches of a Key Point slide and Explanation slides that show photographs, text, and a screen capture.

class Person def initialize(name) set_name(name) end def name @first_name + ' ' + @last_name end def set_name(name) first_name, last_name = name.split(/\s+/) set_first_name(first_name) set_last_name(last_name) end def set_first_name(name) @first_name = name end def set_last_name(name) @last_name = name end end

In previous examples, you would have written this with a single attr_accessor :name and simply assigned the name to an object variable. Unfortunately, though, real-life constraints often require a different approach.

In this case, the first name and last name are stored separately within each Person object, in object variables called @first_name and @last_name. When a Person object is created, the name is split into two halves and each is assigned to the correct object variable by set_first_name and set_last_name, respectively. One possible reason for such a construction could be that although you want to work with complete names in your application, the database design might demand you have first names and last names in separate columns. Therefore, you need to hide this difference by handling it in the class code, as in the preceding code.