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You’ll also see another _ _call_ _ example in Chapter 26, where we will use it toimplement something known as a function decorator—a callable object that adds alayer of logic on top of an embedded function. Because _ _call_ _ allows us to attachstate information to a callable object, it’s a natural implementation technique for afunction that must remember and call another function._ _del_ _ Is a DestructorThe _ _init_ _ constructor is called whenever an instance is generated. Its counterpart,the destructor method _ _del_ _, is run automatically when an instance’s spaceis being reclaimed (i.e., at “garbage collection” time):>>> class Life:... def _ _init_ _(self, name='unknown'):... print 'Hello', name... self.name = name... def _ _del_ _(self):... print 'Goodbye', self.name...>>> brian = Life('Brian')Hello Brian>>> brian = 'loretta'Goodbye BrianHere, when brian is assigned a string, we lose the last reference to the Life instance,and so trigger its destructor method. This works, and it may be useful for implementingsome cleanup activities (such as terminating server connections). However,destructors are not as commonly used in Python as in some OOP languages, for anumber of reasons.For one thing, because Python automatically reclaims all space held by an instancewhen the instance is reclaimed, destructors are not necessary for space management. *For another, because you cannot always easily predict when an instance will bereclaimed, it’s often better to code termination activities in an explicitly calledmethod (or try/finally statement, described in the next part of the book); in somecases, there may be lingering references to your objects in system tables that preventdestructors from running.That’s as many overloading examples as we have space for here. Most of the otheroperator overloading methods work similarly to the ones we’ve explored, and all arejust hooks for intercepting built-in type operations; some overloading methods, forexample, have unique argument lists or return values. You’ll see a few others inaction later in the book, but for complete coverage, I’ll defer to other documentationsources.* In the current C implementation of Python, you also don’t need to close file objects held by the instance indestructors because they are automatically closed when reclaimed. However, as mentioned in Chapter 9, it’sbetter to explicitly call file close methods because auto-close-on-reclaim is a feature of the implementation,not of the language itself (this behavior can vary under Jython).Operator Overloading | 505
Namespaces: The Whole StoryNow that we’ve examined class and instance objects, the Python namespace story iscomplete. For reference, I’ll quickly summarize all the rules used to resolve nameshere. The first things you need to remember are that qualified and unqualified namesare treated differently, and that some scopes serve to initialize object namespaces:• Unqualified names (e.g., X) deal with scopes.• Qualified attribute names (e.g., object.X) use object namespaces.• Some scopes initialize object namespaces (for modules and classes).Simple Names: Global Unless AssignedUnqualified simple names follow the LEGB lexical scoping rule outlined for functionsin Chapter 16:Assignment (X= value)Makes names local: creates or changes the name X in the current local scope,unless declared global.Reference (X)Looks for the name X in the current local scope, then any and all enclosing functions,then the current global scope, then the built-in scope.Attribute Names: Object NamespacesQualified attribute names refer to attributes of specific objects, and obey the rules formodules and classes. For class and instance objects, the reference rules are augmentedto include the inheritance search procedure:Assignment (object.X= value)Creates or alters the attribute name X in the namespace of the object being qualified,and none other. Inheritance-tree climbing happens only on attributereference, not on attribute assignment.Reference (object.X)For class-based objects, searches for the attribute name X in object, then in allaccessible classes above it, using the inheritance search procedure. For nonclassobjects such as modules, fetches X from object directly.The “Zen” of Python Namespaces: Assignments Classify NamesWith distinct search procedures for qualified and unqualified names, and multiplelookup layers for both, it can sometimes be difficult to tell where a name will wind upgoing. In Python, the place where you assign a name is crucial—it fully determines thescope or object in which a name will reside. The file manynames.py illustrates how this506 | Chapter 24: Class Coding Details
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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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Page 502 and 503: • Each class statement generates
Page 504 and 505: The _ _init_ _ method is known as t
Page 506 and 507: This is yet another instance of the
Page 508 and 509: B RAINB UILDERChapter Quiz1. What i
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 552 and 553: principle translates to code, and s
Page 554 and 555: To help you understand how attribut
Page 556 and 557: X._ _dict_ _{'data1': 'spam', 'data
Page 558 and 559: every attribute in the instance. It
Page 560 and 561: JonesTrace through the code in this
Page 562 and 563: 6. Printing sue ultimately runs the
Page 564 and 565: Overloading by Call Signatures (or
Page 566 and 567: mark = Person('ml', 'trainer')>>> d
Page 568 and 569: Employee does stuffChef makes foodS
Page 570 and 571: def processor(reader, converter, wr
Page 572 and 573: Because composition is a design iss
Page 574 and 575: Multiple InheritanceIn a class stat
Page 576 and 577: if __name__ == "_ _main_ _":X = Sub
Page 578 and 579: Why Factories?So what good is the f
Page 580 and 581: On the other hand, if we qualify th
Page 582 and 583: docstr.func._ _doc_ _'I am: docstr.
Page 584 and 585: Chapter 26CHAPTER 26Advanced Class
Page 586 and 587: This change allows you to customize
Page 588 and 589: explored in Chapter 5; this type pr
Page 590 and 591: class C2:def metha(self): self._ _X
Page 592 and 593: Diamond inheritance exampleTo illus
Page 594 and 595: ensures that your code won’t vary
Page 596 and 597: x = classic( )>>> x.age # Runs _ _g
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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
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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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