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This is yet another instance of the idea of polymorphism introduced in Chapter 4 andrevisited in Chapter 15. Recall that polymorphism means that the meaning of an operationdepends on the object being operated on. Here, the method computeSalary islocated by inheritance search in each object before it is called. In other applications,polymorphism might also be used to hide (i.e., encapsulate) interface differences. Forexample, a program that processes data streams might be coded to expect objects withinput and output methods, without caring what those methods actually do:def processor(reader, converter, writer):while 1:data = reader.read()if not data: breakdata = converter(data)writer.write(data)By passing in instances of subclasses that specialize the required read and writemethod interfaces for various data sources, we can reuse the processor function forany data source we need to use, both now and in the future:class Reader:def read(self): ...# Default behavior and toolsdef other(self): ...class FileReader(Reader):def read(self): ...# Read from a local fileclass SocketReader(Reader):def read(self): ...# Read from a network socket...processor(FileReader(...), Converter, FileWriter(...))processor(SocketReader(...), Converter, TapeWriter(...))processor(FtpReader(...), Converter, XmlWriter(...))Moreover, because the internal implementations of those read and write methodshave been factored into single locations, they can be changed without impactingcode such as this that uses them. In fact, the processor function might itself be aclass to allow the conversion logic of converter to be filled in by inheritance, and toallow readers and writers to be embedded by composition (we’ll see how this workslater in this part of the book).Once you get used to programming this way (by software customization), you’ll findthat when it’s time to write a new program, much of your work may already bedone—your task largely becomes one of mixing together existing superclasses thatalready implement the behavior required by your program. For example, someoneelse might have written the Employee, Reader, and Writer classes in this example foruse in a completely different program. If so, you get all of that person’s code “forfree.”In fact, in many application domains, you can fetch or purchase collections of superclasses,known as frameworks, that implement common programming tasks asclasses, ready to be mixed into your applications. These frameworks might providedatabase interfaces, testing protocols, GUI toolkits, and so on. With frameworks,OOP from 30,000 Feet | 461
you often simply code a subclass that fills in an expected method or two; the frameworkclasses higher in the tree do most of the work for you. Programming in such anOOP world is just a matter of combining and specializing already debugged code bywriting subclasses of your own.Of course, it takes awhile to learn how to leverage classes to achieve such OOP utopia.In practice, object-oriented work also entails substantial design work to fullyrealize the code reuse benefits of classes—to this end, programmers have begun catalogingcommon OOP structures, known as design patterns, to help with designissues. The actual code you write to do OOP in Python, though, is so simple that itwill not in itself pose an additional obstacle to your OOP quest. To see why, you’llhave to move on to Chapter 23.Chapter SummaryWe took an abstract look at classes and OOP in this chapter, taking in the big picturebefore we dive into syntax details. As we’ve seen, OOP is mostly about lookingup attributes in trees of linked objects; we call this lookup an inheritance search.Objects at the bottom of the tree inherit attributes from objects higher up in thetree—a feature that enables us to program by customizing code, rather than changingit, or starting from scratch. When used well, this model of programming can cutdevelopment time radically.The next chapter will begin to fill in the coding details behind the picture paintedhere. As we get deeper into Python classes, though, keep in mind that the OOPmodel in Python is very simple; as I’ve already stated, it’s really just about looking upattributes in object trees. Before we move on, here’s a quick quiz to review whatwe’ve covered here.462 | Chapter 22: OOP: The Big Picture
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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.
Page 456 and 457: • Reloads impact all clients that
Page 458 and 459: Although we’ve already seen enoug
Page 460 and 461: Chapter 20CHAPTER 20Module Packages
Page 462 and 463: dir0\dir1\dir2\mod.pyand an import
Page 464 and 465: import dir1.dir2.mod # Later import
Page 466 and 467: A Tale of Three SystemsThe only tim
Page 468 and 469: Also, notice in the install hierarc
Page 470 and 471: B RAINB UILDERChapter Quiz1. What i
Page 472 and 473: from * copies out all names, the im
Page 474 and 475: Unit Tests with _ _name_ _In fact,
Page 476 and 477: of a running Python program, and so
Page 478 and 479: This defines a package named mypkg
Page 480 and 481: • Modules should rarely change ot
Page 482 and 483: We’ve also provided self-test log
Page 484 and 485: To get around this, you need to use
Page 486 and 487: from module import X. . .reload(mod
Page 488 and 489: import typesdef status(module):prin
Page 492: 5. Package imports. Import your fil
Page 496 and 497: Chapter 22CHAPTER 22OOP: The Big Pi
Page 498 and 499: Customization via inheritanceClasse
Page 500 and 501: ecause the search proceeds bottom-u
Page 502 and 503: • Each class statement generates
Page 504 and 505: The _ _init_ _ method is known as t
Page 510 and 511: Chapter 23CHAPTER 23Class Coding Ba
Page 512 and 513: A First ExampleLet’s turn to a re
Page 514 and 515: Although less common, we could even
Page 516 and 517: Z (instance)- datais-aSecondClass-
Page 518 and 519: foster object interfaces that are m
Page 520 and 521: Specially named methods such as _ _
Page 522 and 523: Really, these instances have no att
Page 524 and 525: B RAINB UILDERChapter Quiz1. How ar
Page 526 and 527: Chapter 24CHAPTER 24Class Coding De
Page 528 and 529: Here, because the name spam is assi
Page 530 and 531: always go through self to fetch or
Page 532 and 533: In Python, inheritance happens when
Page 534 and 535: This extension coding pattern is al
Page 536 and 537: def delegate(self):self.action( )de
Page 538 and 539: All operator overloading methods ar
Page 540 and 541: def _ _iter_ _(self): # Get iterato
Page 542 and 543: class SkipIterator:def _ _init_ _(s
Page 544 and 545: makes age look like a real attribut
Page 546 and 547: class addrepr(adder): # Inherit _ _
Page 548 and 549: class Prod:... def _ _init_ _(self,
Page 550 and 551: You’ll also see another _ _call_
Page 552 and 553: principle translates to code, and s
Page 554 and 555: 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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mark = Person('ml', 'trainer')>>> d
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Employee does stuffChef makes foodS
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def processor(reader, converter, wr
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Because composition is a design iss
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Multiple InheritanceIn a class stat
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if __name__ == "_ _main_ _":X = Sub
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Why Factories?So what good is the f
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On the other hand, if we qualify th
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docstr.func._ _doc_ _'I am: docstr.
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Chapter 26CHAPTER 26Advanced Class
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This change allows you to customize
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explored in Chapter 5; this type pr
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class C2:def metha(self): self._ _X
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Diamond inheritance exampleTo illus
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ensures that your code won’t vary
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x = classic( )>>> x.age # Runs _ _g
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class attributes instead of instanc
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such methods, classes call the buil
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Python provides some built-in funct
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Class GotchasMost class issues can
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in which we list them in the class
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A better alternative would be to re
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B RAINB UILDERChapter Quiz1. Name t
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c. Is there a general way to route
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The order simulation works as follo
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Why You Will Care: OOP by the Maste
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PART VIIVII.Exceptions and Tools
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Why Use Exceptions?In a nutshell, e
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However, if we ask this function to
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... print 'after fetch'...'m'after
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except , :except (name3, name4):exc
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uns the statements under the first
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def gosouth(x):print gobad(x, 0)gos
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• If an exception does occur duri
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The code in this statement’s main
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When this code is run, the followin
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the globals reside in. Passing extr
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traps programming errors itself, th
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The Context Management ProtocolThe
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Notice that this class’ _ _exit_
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ChapterCHAPTER2828Exception Objects
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Class-Based ExceptionsStrings are a
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Why Class Exceptions?Because there
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But this workaround might catch mor
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except StandardError:...handle Pyth
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def parser( ):... # when error foun
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aise instanceraise class, instance#
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B RAINB UILDERChapter Quiz1. How ar
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try:func1()except E:...def func1():
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Really, this code just sets up the
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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
Page 688 and 689:
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
Page 734 and 735:
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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