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(2.0 + a) # Auto echo output: more digits0.80000000000000004>>> print b / (2.0 + a) # Print rounds off digits0.8The full story behind this odd result has to do with the limitations of floating-pointhardware, and its inability to exactly represent some values. Because computer architectureis well beyond this book’s scope, though, we’ll finesse this by saying that allof the digits in the first output are really there, in your computer’s floating-pointhardware—it’s just that you’re not normally accustomed to seeing them. I’ve usedthis example to demonstrate the difference in output formatting—the interactiveprompt’s automatic result echo shows more digits than the print statement. If youdon’t want to see all the digits, include the print.Note, however, that not all values have so many digits to display:>>> 1 / 2.00.5and that there are more ways to display the bits of a number inside your computerthan prints and automatic echoes:>>> num = 1 / 3.0>>> num # Echoes0.33333333333333331>>> print num # Print rounds0.333333333333>>> "%e" % num # String formatting'3.333333e-001'>>> "%2.2f" % num # String formatting'0.33'The last two of these employ string formatting, an expression that allows for formatflexibility, which we will explore in the upcoming chapter on strings (Chapter 7).str and repr Display FormatsTechnically, the difference between default interactive echoes and prints correspondsto the difference between the built-in repr and str functions:>>> repr(num) # Used by echoes: as-code form'0.33333333333333331'>>> str(num) # Used by print: user-friendly form'0.333333333333'Both of these convert arbitrary objects to their string representations: repr (and thedefault interactive echo) produces results that look as though they were code; str (andthe print statement) converts to a typically more user-friendly format. This notion willresurface when we study strings, and you’ll find more on these built-ins in general laterin the book.Numbers in Action | 101
Division: Classic, Floor, and TrueNow that you’ve seen how division works, you should know that it is scheduled for aslight change in a future Python release (currently, 3.0, scheduled to appear sometime after this edition is released). In Python 2.5, things work as just described, butthere are actually two different division operators (one of which will change):X/YClassic division. In Python 2.5 and earlier, this operator truncates results forintegers, and keeps remainders for floating-point numbers, as described here.This operator will be changed to true division—always keeping remaindersregardless of types—in a future Python release (3.0).X// YFloor division. Added in Python 2.2, this operator always truncates fractionalremainders down to their floor, regardless of types.Floor division was added to address the fact that the results of the current classicdivision model are dependent on operand types, and so can be difficult to anticipatein a dynamically typed language like Python.Due to possible backward-compatibility issues, division in Python is in a state of fluxtoday. To recap, in version 2.5, / division works as described by default, and // floordivision can be used to truncate result remainders to their floor regardless of theirtypes:>>> (5 / 2), (5 / 2.0), (5 / -2.0), (5 / -2)(2, 2.5, -2.5, -3)>>> (5 // 2), (5 // 2.0), (5 // -2.0), (5 // -2)(2, 2.0, -3.0, -3)>>> (9 / 3), (9.0 / 3), (9 // 3), (9 // 3.0)(3, 3.0, 3, 3.0)In a future Python release, / division is slated to be changed to return a true divisionresult that always retains remainders, even for integers—for example, 1/2will be 0.5,not 0, while 1//2 will still be 0.Until this change is incorporated completely, you can see the way that the / operatorwill likely work in the future by using a special import of the form: from _ _future_ _import division. This turns the / operator into a true division operator (keepingremainders), but leaves // as is. Here’s how / will eventually behave:>>> from _ _future_ _ import division>>> (5 / 2), (5 / 2.0), (5 / -2.0), (5 / -2)(2.5, 2.5, -2.5, -2.5)>>> (5 // 2), (5 // 2.0), (5 // -2.0), (5 // -2)(2, 2.0, -3.0, -3)102 | Chapter 5: Numbers
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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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Page 97 and 98: Figure 3-3. The main Python shell w
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Page 101 and 102: Embedding CallsAt this point, we’
Page 103 and 104: Which Option Should I Use?With all
Page 105 and 106: 6. IDLE can still be hung by some t
Page 107 and 108: 5. Errors. Experiment with typing m
Page 110 and 111: Chapter 4CHAPTER 4Introducing Pytho
Page 112 and 113: Python’s Core Data TypesTable 4-1
Page 114 and 115: print the result—with more than 3
Page 116 and 117: In a slice, the left bound defaults
Page 118 and 119: S = 'spam'>>> S.upper( ) # Upper- a
Page 120 and 121: msg'\naaaaaaaaaaaaa\nbbb\'\'\'bbbbb
Page 122 and 123: M.reverse( )>>> M['cc', 'bb', 'aa']
Page 124 and 125: diag = [M[i][i] for i in [0, 1, 2]]
Page 126 and 127: ec['job'][-1] # Index the nested li
Page 128 and 129: on any object that is a sequence an
Page 130 and 131: TuplesThe tuple object (pronounced
Page 132 and 133: Other Core TypesBeyond the core typ
Page 134 and 135: def lastName(self):return self.name
Page 136 and 137: B RAINB UILDERChapter QuizWe’ll e
Page 138 and 139: Chapter 5CHAPTER 5Numbers5This chap
Page 140 and 141: Hexadecimal and octal literalsThe r
Page 142 and 143: Mixed Operators Follow Operator Pre
Page 144 and 145: Numbers in ActionProbably the best
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Page 150 and 151: Hexadecimal and Octal NotationAs me
Page 152 and 153: The random module can be useful for
Page 154 and 155: Such operations are convenient when
Page 156 and 157: B RAINB UILDERChapter Quiz1. What i
Page 158 and 159: Python to use the name a as a varia
Page 160 and 161: However, that’s not really what
Page 162 and 163: Names References Objectsa=3b=aa 3bF
Page 164 and 165: changes, they can impact other part
Page 166 and 167: import sys>>> sys.getrefcount(1) #
Page 168 and 169: Chapter 7CHAPTER 7Strings7The next
Page 170 and 171: Single- and Double-Quoted Strings A
Page 172 and 173: Some escape sequences allow you to
Page 174 and 175: Triple Quotes Code Multiline Block
Page 176 and 177: Because Unicode is designed to hand
Page 178 and 179: Notice that operator overloading is
Page 180 and 181: • Indexing (S[i]) fetches compone
Page 182 and 183: Why You Will Care: SlicesThroughout
Page 184 and 185: Such conversions can be used in con
Page 186 and 187: The third example again inserts thr
Page 188 and 189: eply = """Greetings...Hello %(name)
Page 190 and 191: S = 'spammy'>>> S = S[:3] + 'xx' +
Page 192 and 193: can pull out its components by spli
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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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B RAINB UILDERChapter Quiz1. What i
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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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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
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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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