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Why You Will Care: OOP by the MastersWhen I teach <strong>Python</strong> classes, I invariably find that about halfway through the class,people who have used OOP in the past are following along intensely, while people whohave not are beginning to glaze over (or nod off completely). The point behind thetechnology just isn’t apparent.In a book like this, I have the luxury of including material like the new Big Picture overviewin Chapter 22—in fact, you should probably review that section if you’re startingto feel like OOP is just some computer science mumbo-jumbo.In real classes, however, to help get the newcomers on board (and keep them awake),I have been known to stop and ask the experts in the audience why they use OOP. Theanswers they’ve given might help shed some light on the purpose of OOP, if you arenew to the subject.Here, then, with only a few embellishments, are the most common reasons to useOOP, as cited by my students over the years:Code reuseThis one’s easy (and is the main reason for using OOP). By supporting inheritance,classes allow you to program by customization, instead of starting eachproject from scratch.EncapsulationWrapping up implementation details behind object interfaces insulates users of aclass from code changes.StructureClasses provide new local scopes, which minimizes name clashes. They also providea natural place to write and look for implementation code, and to manageobject state.MaintenanceClasses naturally promote code factoring, which allows us to minimize redundancy.Thanks both to the structure and code reuse support of classes, usuallyonly one copy of the code needs to be changed.ConsistencyClasses and inheritance allow you to implement common interfaces, and hence, acommon look and feel in your code; this eases debugging, comprehension, andmaintenance.PolymorphismThis is more a property of OOP than a reason for using it, but by supporting codegenerality, polymorphism makes code more flexible and widely applicable, andhence, more reusable.—continued—Part VI Exercises | 571
OtherAnd, of course, the number one reason students gave for using OOP: it looks goodon a résumé! (OK, I threw this one in as a joke, but it is important to be familiarwith OOP if you plan to work in the software field today.)Finally, keep in mind what I said at the beginning of Part VI: you won’t fully appreciateOOP until you’ve used it for awhile. Pick a project, study larger examples, workthrough the exercises—do whatever it takes to get your feet wet with OO code; it’sworth the effort.572 | Chapter 26: Advanced Class Topics
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Learning Python
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THIRD EDITIONLearning PythonMark Lu
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To Vera.You are my life.
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3. How You Run Programs . . . . . .
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5. Numbers . . . . . . . . . . . .
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8. Lists and Dictionaries . . . . .
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Statement rule special cases 208Blo
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Iterators: A First Look 258File Ite
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Scopes and Nested Functions 320Nest
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How Imports Work 3891. Find It 390T
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Brain Builder 445Chapter Quiz 445Qu
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25. Designing with Classes . . . .
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Preface1This book provides an intro
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In addition, this entire edition in
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This book is designed to be an intr
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Part I, Getting StartedWe begin wit
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Preparing for Python 3.0The first a
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• In the iterator protocol, the X
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We appreciate, but do not require,
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It’s been a long and winding road
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PART II.Getting Started
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Developer productivityPython boosts
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Is Python a “Scripting Language
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Who Uses Python Today?At this writi
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(both Classic and OS X). A free ext
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Gaming, Images, AI, XML, Robots, an
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Besides the language interpreter it
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Python Is Engineering, Not ArtWhen
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world, the truth is that Python cre
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B RAINB UILDERChapter QuizIn this e
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ChapterCHAPTER22How Python Runs Pro
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Because installation details are so
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If Python cannot write the byte cod
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Python Implementation AlternativesR
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Roughly, while your program runs, P
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Future Possibilities?Finally, note
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ChapterCHAPTER33How You Run Program
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Here, the last two lines typed are
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un it again without retyping it fro
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Using Command Lines and FilesRunnin
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In this case, you don’t need the
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The raw_input TrickUnfortunately, o
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In simple terms, every file of Pyth
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Alternatively, you can fetch (reall
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Because of these complications (and
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Figure 3-3. The main Python shell w
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start occasionally on some Windows
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Embedding CallsAt this point, we’
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Which Option Should I Use?With all
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6. IDLE can still be hung by some t
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5. Errors. Experiment with typing m
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Chapter 4CHAPTER 4Introducing Pytho
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Python’s Core Data TypesTable 4-1
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print the result—with more than 3
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In a slice, the left bound defaults
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S = 'spam'>>> S.upper( ) # Upper- a
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msg'\naaaaaaaaaaaaa\nbbb\'\'\'bbbbb
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M.reverse( )>>> M['cc', 'bb', 'aa']
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diag = [M[i][i] for i in [0, 1, 2]]
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ec['job'][-1] # Index the nested li
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on any object that is a sequence an
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TuplesThe tuple object (pronounced
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Other Core TypesBeyond the core typ
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def lastName(self):return self.name
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B RAINB UILDERChapter QuizWe’ll e
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Chapter 5CHAPTER 5Numbers5This chap
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Hexadecimal and octal literalsThe r
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Mixed Operators Follow Operator Pre
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Numbers in ActionProbably the best
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(2.0 + a) # Auto echo output: more
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(9 / 3), (9.0 / 3), (9 // 3), (9 //
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Hexadecimal and Octal NotationAs me
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The random module can be useful for
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Such operations are convenient when
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B RAINB UILDERChapter Quiz1. What i
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Python to use the name a as a varia
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However, that’s not really what
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Names References Objectsa=3b=aa 3bF
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changes, they can impact other part
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import sys>>> sys.getrefcount(1) #
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Chapter 7CHAPTER 7Strings7The next
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Single- and Double-Quoted Strings A
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Some escape sequences allow you to
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Triple Quotes Code Multiline Block
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Because Unicode is designed to hand
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Notice that operator overloading is
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• Indexing (S[i]) fetches compone
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Why You Will Care: SlicesThroughout
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Such conversions can be used in con
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The third example again inserts thr
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eply = """Greetings...Hello %(name)
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S = 'spammy'>>> S = S[:3] + 'xx' +
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can pull out its components by spli
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x'aspambspamcspam'To access the sam
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B RAINB UILDERChapter Quiz1. Can th
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Variable-length, heterogeneous, and
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espond to all of the general sequen
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Both index and slice assignments ar
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In some types of programs, the list
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Tables of object references (hash t
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Later in this chapter and book, you
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Dictionaries also provide a copy me
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Here, it looks as if D is a 100-ite
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Other ways to make dictionariesFina
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B RAINB UILDERChapter Quiz1. Name t
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Of the category immutable sequenceL
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T = tuple(tmp) # Make a tuple from
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Table 9-2. Common file operations (
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F.write('%s,%s,%s\n' % (X, Y, Z)) #
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We get back an equivalent dictionar
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Why You Will Care: Operator Overloa
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We studied this phenomenon in Chapt
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For instance, a comparison of list
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Python also provides a special obje
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Because types can be subclassed in
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Beware of Cyclic Data StructuresWe
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B RAINB UILDERChapter Quiz1. How ca
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c. Finally, how does Python handle
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PART IIIIII.Statements and Syntax
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Python’s StatementsTable 10-1 sum
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What Python AddsThe one new syntax
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Why Indentation Syntax?The indentat
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My point is that even when a langua
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prints, function calls, and the lik
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Armed with this information, we can
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an except part that gives the excep
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B RAINB UILDERChapter Quiz1. What t
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• Implicit assignments: import, f
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In fact, the original tuple and lis
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Notice that this code is using the
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Augmented assignments have three ad
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Reserved words are off-limitsNames
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Expression StatementsIn Python, you
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Table 11-5. print statement formsOp
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The print >> file ExtensionThis tri
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In Python 3.0, the print statement
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ChapterCHAPTER1212if Tests 12This c
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If you’ve used languages like C o
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As you’ve seen, there are no vari
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• Triple-quoted string literals c
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[ ] or 33>>> [ ] or { }{ }In the fi
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of Python 2.5. Again, though, you s
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ChapterCHAPTER1313while and for Loo
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Finally, notice that Python doesn
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Because continue jumps to the top o
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print 'Ni'breakx = x[1:]else:print
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ExamplesLet’s type a few for loop
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Note that this example is easier to
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eadlines loads a file all at once i
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Moreover, the iterator version has
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We saw the sorted function used her
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Although such range results may be
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To really change the list as we mar
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Dictionary construction with zipIn
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List Comprehensions: A First LookIn
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and add it to the result list. Agai
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B RAINB UILDERChapter Quiz1. When i
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ChapterCHAPTER1414The Documentation
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Only some of the many names are dis
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Note that you will generally want t
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FILE(built-in)DESCRIPTIONThis modul
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too). Enter the name of a module yo
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Figure 14-3. PyDoc can serve up doc
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Figure 14-4. Python’s standard ma
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• Use simple for loops instead of
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B RAINB UILDERChapter Quiz1. When s
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if found:print 'at index', ielse:pr
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Chapter 15CHAPTER 15Function Basics
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In this chapter, we will explore ho
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def Executes at RuntimeThe Python d
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Polymorphism in PythonAs we just sa
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s1 = "SPAM">>> s2 = "SCAM">>> inter
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In all cases, the scope of a variab
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lambdas, then the global (G) scope,
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have to import it in order to inspe
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Here, x, y, and z are all globals i
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Here again, the best prescription i
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Notice that the global declaration
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Retaining enclosing scopes’ state
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def makeActions( ):... acts = []...
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Python’s pass-by-assignment schem
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Avoiding Mutable Argument ChangesAr
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Defaults: specify values for argume
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DefaultsWe talked a little about de
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func(*args)1 2 3 4Similarly, the **
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if arg < res:res = argreturn resdef
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def union(*args):res = []for seq in
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For both scopes and arguments, we a
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Quiz Answers1. The output here is '
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Function objects returned by runnin
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a dictionary becomes a more general
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What wasn’t illustrated in the pr
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around this, simply build up the ar
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Moreover, map can be used in more a
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Together with map, filter and reduc
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... if x % 2 == 0:... res.append(x)
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List comprehensions are powerful to
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Why You Will Care: List Comprehensi
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If you want to see what is going on
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for key in D:... print key, D[key].
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I’ve mentioned a few times that l
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For more insight, try modifying the
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seen some of these uses in earlier
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...>>> selector( )99Remember, thoug
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Functions Without returnsIn Python
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B RAINB UILDERPart IV ExercisesIn t
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Package this code as a reusable fun
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Chapter 18CHAPTER 18Modules: The Bi
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How to Structure a ProgramGenerally
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in any other programs you may write
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Home directoryPython first looks fo
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Module file selectionKeep in mind t
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Because this last import step actua
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B RAINB UILDERChapter Quiz1. How do
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ecause if is a reserved word—when
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As one consequence, because top-lev
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Like all assignments, the from stat
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Files Generate NamespacesSo, how do
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Qualification is really an expressi
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X = 1import mod2print X,print mod2.
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• Reloads impact all clients that
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Although we’ve already seen enoug
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Chapter 20CHAPTER 20Module Packages
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dir0\dir1\dir2\mod.pyand an import
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import dir1.dir2.mod # Later import
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A Tale of Three SystemsThe only tim
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Also, notice in the install hierarc
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from * copies out all names, the im
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Unit Tests with _ _name_ _In fact,
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of a running Python program, and so
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This defines a package named mypkg
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• Modules should rarely change ot
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We’ve also provided self-test log
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To get around this, you need to use
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from module import X. . .reload(mod
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import typesdef status(module):prin
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5. Package imports. Import your fil
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Chapter 22CHAPTER 22OOP: The Big Pi
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Customization via inheritanceClasse
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ecause the search proceeds bottom-u
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• Each class statement generates
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The _ _init_ _ method is known as t
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This is yet another instance of the
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Chapter 23CHAPTER 23Class Coding Ba
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A First ExampleLet’s turn to a re
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Although less common, we could even
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Z (instance)- datais-aSecondClass-
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foster object interfaces that are m
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Specially named methods such as _ _
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Really, these instances have no att
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B RAINB UILDERChapter Quiz1. How ar
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Chapter 24CHAPTER 24Class Coding De
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Here, because the name spam is assi
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always go through self to fetch or
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In Python, inheritance happens when
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This extension coding pattern is al
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def delegate(self):self.action( )de
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All operator overloading methods ar
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def _ _iter_ _(self): # Get iterato
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class SkipIterator:def _ _init_ _(s
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makes age look like a real attribut
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class addrepr(adder): # Inherit _ _
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class Prod:... def _ _init_ _(self,
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You’ll also see another _ _call_
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principle translates to code, and s
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To help you understand how attribut
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X._ _dict_ _{'data1': 'spam', 'data
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every attribute in the instance. It
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JonesTrace through the code in this
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6. Printing sue ultimately runs the
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Overloading by Call Signatures (or
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Page 568 and 569: Employee does stuffChef makes foodS
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Page 572 and 573: Because composition is a design iss
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Page 578 and 579: Why Factories?So what good is the f
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Page 584 and 585: Chapter 26CHAPTER 26Advanced Class
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Page 596 and 597: x = classic( )>>> x.age # Runs _ _g
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Page 604 and 605: Class GotchasMost class issues can
Page 606 and 607: in which we list them in the class
Page 608 and 609: A better alternative would be to re
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Page 612 and 613: c. Is there a general way to route
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Page 618: PART VIIVII.Exceptions and Tools
Page 621 and 622: Why Use Exceptions?In a nutshell, e
Page 623 and 624: However, if we ask this function to
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Page 627 and 628: except , :except (name3, name4):exc
Page 629 and 630: uns the statements under the first
Page 631 and 632: def gosouth(x):print gobad(x, 0)gos
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Page 635 and 636: The code in this statement’s main
Page 637 and 638: When this code is run, the followin
Page 639 and 640: the globals reside in. Passing extr
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Page 643 and 644: The Context Management ProtocolThe
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Page 647 and 648: ChapterCHAPTER2828Exception Objects
Page 649 and 650: Class-Based ExceptionsStrings are a
Page 651 and 652: Why Class Exceptions?Because there
Page 653 and 654: But this workaround might catch mor
Page 655 and 656: except StandardError:...handle Pyth
Page 657 and 658: def parser( ):... # when error foun
Page 659 and 660: aise instanceraise class, instance#
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Functions Signal Conditions with ra
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At the end of this chapter, we’ll
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For example, scripts normally exit
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String Exceptions Match by Identity
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The Python ToolsetFrom this point f
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DoctestThe doctest standard library
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To learn about other large-scale Py
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B RAINB UILDERPart VII ExercisesAs
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Appendix AAPPENDIX AInstallation an
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When installed on Windows, Python a
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These variables are straightforward
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eboot your machine for the changes
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Python’s search path. The .pyc fi
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S = "ham">>> "eggs " + S'eggs ham'>
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4. Tuple assignment. The values of
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9. Immutable types. Either of the f
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a => 1b => 2c => 3d => 4e => 5f =>
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3. varargs. Two alternative adder f
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7. More argument-matching examples.
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es[1.4142135623730951, 2.0, 3.0, 4.
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% python myclient.py13 346As for th
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% python>>> from adder import *>>>
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Note that it’s important to copy
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My solution to the multiple-operand
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class Customer:def _ _init_ _(self)
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if _ _name_ _ == '_ _main_ _': doom
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allsizes.sort( )print allsizes[:2]p
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win = Tk( )L = Label(win, text='Spa
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# CGI client-side script to interac
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IndexSymbols# comments, 279% (remai
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ord function, 355pow function, 353r
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delegation, 527-528, 556descriptor
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Part I - Modules, 646Part I - Progr
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Hhandler, 578“has-a” relationsh
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Kkeyed-access files, 86KeyError (bu
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imports and reloads, 45-50example,
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overloading by call signatures, 519
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cross-file changes, minimizing, 318
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Tkinter, 9, 31environment setting,
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About the AuthorMark Lutz is the wo
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