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IronPythonA third (and, at this writing, still somewhat new) implementation of Python,IronPython is designed to allow Python programs to integrate with applicationscoded to work with Microsoft’s .NET Framework for Windows, as well as the Monoopen source equivalent for Linux. .NET and its C# programming language runtimesystem are designed to be a language-neutral object communication layer, in thespirit of Microsoft’s earlier COM model. IronPython allows Python programs to actas both client and server components, accessible from other .NET languages.By implementation, IronPython is very much like Jython (and, in fact, is beingdeveloped by the same creator)—it replaces the last two bubbles in Figure 2-2 withequivalents for execution in the .NET environment. Also, like Jython, IronPython hasa special focus—it is primarily of interest to developers integrating Python with .NETcomponents. Because it is being developed by Microsoft, though, IronPython mightalso be able to leverage some important optimization tools for better performance.IronPython’s scope is still evolving as I write this; for more details, consult thePython online resources, or search the Web. *Execution Optimization ToolsCPython, Jython, and IronPython all implement the Python language in similarways: by compiling source code to byte code, and executing the byte code on anappropriate virtual machine. Still other systems, including the Psyco just-in-timecompiler, and the Shedskin C++ translator, instead attempt to optimize the basicexecution model. These systems are not required knowledge at this point in yourPython career, but a quick look at their place in the execution model might helpdemystify the model in general.The Psyco just-in-time compilerThe Psyco system is not another Python implementation, but rather, a componentthat extends the byte code execution model to make programs run faster. In terms ofFigure 2-2, Psyco is an enhancement to the PVM that collects and uses type informationwhile the program runs to translate portions of the program’s byte code all theway down to real binary machine code for faster execution. Psyco accomplishes thistranslation without requiring changes to the code or a separate compilation step duringdevelopment.* Jython and IronPython are completely independent implementations of Python that compile Python sourcefor different runtime architectures. It is also possible to access Java and .NET software from standardCPython programs: the JPype and Python for .NET systems, for example, allow CPython code to call out toJava and .NET components.Execution Model Variations | 29
Roughly, while your program runs, Psyco collects information about the kinds ofobjects being passed around; that information can be used to generate highly efficientmachine code tailored for those object types. Once generated, the machinecode then replaces the corresponding part of the original byte code to speed yourprogram’s overall execution. The net effect is that, with Psyco, your programbecomes much quicker over time, and as it is running. In ideal cases, some Pythoncode may become as fast as compiled C code under Psyco.Because this translation from byte code happens at program runtime, Psyco is generallyknown as a just-in-time (JIT) compiler. Psyco is actually a bit different from theJIT compilers some readers may have seen for the Java language, though. Really,Psyco is a specializing JIT compiler—it generates machine code tailored to the datatypes that your program actually uses. For example, if a part of your program usesdifferent data types at different times, Psyco may generate a different version ofmachine code to support each different type combination.Psyco has been shown to speed Python code dramatically. According to its web page,Psyco provides “2x to 100x speed-ups, typically 4x, with an unmodified Pythoninterpreter and unmodified source code, just a dynamically loadable C extensionmodule.” Of equal significance, the largest speedups are realized for algorithmiccode written in pure Python—exactly the sort of code you might normally migrate toC to optimize. With Psyco, such migrations become even less important.Psyco is not yet a standard part of Python; you will have to fetch and install it separately.It is also still something of a research project, so you’ll have to track its evolutiononline. In fact, at this writing, although Psyco can still be fetched and installed by itself,it appears that much of the system may eventually be absorbed into the newer “PyPy”project—an attempt to reimplement Python’s PVM in Python code, to better supportoptimizations like Psyco.Perhaps the largest downside of Psyco is that it currently only generates machinecode for Intel x86 architecture chips, though this includes Windows, Linux, andrecent Macs. For more details on the Psyco extension, and other JIT efforts that mayarise, consult http://www.python.org; you can also check out Psyco’s home page,which currently resides at http://psyco.sourceforge.net.The Shedskin C++ translatorShedskin is an emerging system that takes a different approach to Python programexecution—it attempts to translate Python source code to C++ code, which yourcomputer’s C++ compiler then compiles to machine code. As such, it represents aplatform-neutral approach to running Python code. Shedskin is still somewhatexperimental as I write these words, and it limits Python programs to an implicitstatically typed constraint that is technically not normal Python, so we won’t go into30 | Chapter 2: How Python Runs Programs
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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
Page 24 and 25: Brain Builder 445Chapter Quiz 445Qu
Page 26: 25. Designing with Classes . . . .
Page 30 and 31: Preface1This book provides an intro
Page 32 and 33: In addition, this entire edition in
Page 34 and 35: This book is designed to be an intr
Page 36 and 37: Part I, Getting StartedWe begin wit
Page 38 and 39: Preparing for Python 3.0The first a
Page 40 and 41: • In the iterator protocol, the X
Page 42 and 43: We appreciate, but do not require,
Page 44 and 45: It’s been a long and winding road
Page 46: PART II.Getting Started
Page 49 and 50: Developer productivityPython boosts
Page 51 and 52: Is Python a “Scripting Language
Page 53 and 54: Who Uses Python Today?At this writi
Page 55 and 56: (both Classic and OS X). A free ext
Page 57 and 58: Gaming, Images, AI, XML, Robots, an
Page 59 and 60: Besides the language interpreter it
Page 61 and 62: Python Is Engineering, Not ArtWhen
Page 63 and 64: world, the truth is that Python cre
Page 65 and 66: B RAINB UILDERChapter QuizIn this e
Page 67 and 68: ChapterCHAPTER22How Python Runs Pro
Page 69 and 70: Because installation details are so
Page 71 and 72: If Python cannot write the byte cod
Page 73: Python Implementation AlternativesR
Page 77 and 78: Future Possibilities?Finally, note
Page 79 and 80: ChapterCHAPTER33How You Run Program
Page 81 and 82: Here, the last two lines typed are
Page 83 and 84: un it again without retyping it fro
Page 85 and 86: Using Command Lines and FilesRunnin
Page 87 and 88: In this case, you don’t need the
Page 89 and 90: The raw_input TrickUnfortunately, o
Page 91 and 92: In simple terms, every file of Pyth
Page 93 and 94: Alternatively, you can fetch (reall
Page 95 and 96: Because of these complications (and
Page 97 and 98: Figure 3-3. The main Python shell w
Page 99 and 100: start occasionally on some Windows
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']
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diag = [M[i][i] for i in [0, 1, 2]]
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ec['job'][-1] # Index the nested li
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on any object that is a sequence an
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TuplesThe tuple object (pronounced
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Other Core TypesBeyond the core typ
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def lastName(self):return self.name
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B RAINB UILDERChapter QuizWe’ll e
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Chapter 5CHAPTER 5Numbers5This chap
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Hexadecimal and octal literalsThe r
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Mixed Operators Follow Operator Pre
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Numbers in ActionProbably the best
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(2.0 + a) # Auto echo output: more
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(9 / 3), (9.0 / 3), (9 // 3), (9 //
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Hexadecimal and Octal NotationAs me
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The random module can be useful for
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Such operations are convenient when
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B RAINB UILDERChapter Quiz1. What i
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Python to use the name a as a varia
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However, that’s not really what
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Names References Objectsa=3b=aa 3bF
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changes, they can impact other part
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import sys>>> sys.getrefcount(1) #
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Chapter 7CHAPTER 7Strings7The next
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Single- and Double-Quoted Strings A
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Some escape sequences allow you to
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Triple Quotes Code Multiline Block
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Because Unicode is designed to hand
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Notice that operator overloading is
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• Indexing (S[i]) fetches compone
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Why You Will Care: SlicesThroughout
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Such conversions can be used in con
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The third example again inserts thr
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eply = """Greetings...Hello %(name)
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S = 'spammy'>>> S = S[:3] + 'xx' +
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can pull out its components by spli
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x'aspambspamcspam'To access the sam
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B RAINB UILDERChapter Quiz1. Can th
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Variable-length, heterogeneous, and
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espond to all of the general sequen
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Both index and slice assignments ar
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In some types of programs, the list
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Tables of object references (hash t
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Later in this chapter and book, you
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Dictionaries also provide a copy me
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Here, it looks as if D is a 100-ite
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Other ways to make dictionariesFina
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B RAINB UILDERChapter Quiz1. Name t
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Of the category immutable sequenceL
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T = tuple(tmp) # Make a tuple from
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Table 9-2. Common file operations (
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F.write('%s,%s,%s\n' % (X, Y, Z)) #
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We get back an equivalent dictionar
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Why You Will Care: Operator Overloa
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We studied this phenomenon in Chapt
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For instance, a comparison of list
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Python also provides a special obje
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Because types can be subclassed in
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Beware of Cyclic Data StructuresWe
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B RAINB UILDERChapter Quiz1. How ca
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c. Finally, how does Python handle
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PART IIIIII.Statements and Syntax
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Python’s StatementsTable 10-1 sum
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What Python AddsThe one new syntax
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Why Indentation Syntax?The indentat
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My point is that even when a langua
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prints, function calls, and the lik
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Armed with this information, we can
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an except part that gives the excep
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B RAINB UILDERChapter Quiz1. What t
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• Implicit assignments: import, f
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In fact, the original tuple and lis
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Notice that this code is using the
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Augmented assignments have three ad
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Reserved words are off-limitsNames
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Expression StatementsIn Python, you
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Table 11-5. print statement formsOp
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The print >> file ExtensionThis tri
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In Python 3.0, the print statement
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ChapterCHAPTER1212if Tests 12This c
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If you’ve used languages like C o
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As you’ve seen, there are no vari
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• Triple-quoted string literals c
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[ ] or 33>>> [ ] or { }{ }In the fi
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of Python 2.5. Again, though, you s
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ChapterCHAPTER1313while and for Loo
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Finally, notice that Python doesn
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Because continue jumps to the top o
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print 'Ni'breakx = x[1:]else:print
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ExamplesLet’s type a few for loop
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Note that this example is easier to
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eadlines loads a file all at once i
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Moreover, the iterator version has
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We saw the sorted function used her
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Although such range results may be
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To really change the list as we mar
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Dictionary construction with zipIn
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List Comprehensions: A First LookIn
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and add it to the result list. Agai
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B RAINB UILDERChapter Quiz1. When i
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ChapterCHAPTER1414The Documentation
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Only some of the many names are dis
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Note that you will generally want t
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FILE(built-in)DESCRIPTIONThis modul
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too). Enter the name of a module yo
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Figure 14-3. PyDoc can serve up doc
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Figure 14-4. Python’s standard ma
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• Use simple for loops instead of
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B RAINB UILDERChapter Quiz1. When s
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if found:print 'at index', ielse:pr
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Chapter 15CHAPTER 15Function Basics
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In this chapter, we will explore ho
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def Executes at RuntimeThe Python d
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Polymorphism in PythonAs we just sa
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s1 = "SPAM">>> s2 = "SCAM">>> inter
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In all cases, the scope of a variab
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lambdas, then the global (G) scope,
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have to import it in order to inspe
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Here, x, y, and z are all globals i
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Here again, the best prescription i
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Notice that the global declaration
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Retaining enclosing scopes’ state
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def makeActions( ):... acts = []...
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Python’s pass-by-assignment schem
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Avoiding Mutable Argument ChangesAr
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Defaults: specify values for argume
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DefaultsWe talked a little about de
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func(*args)1 2 3 4Similarly, the **
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if arg < res:res = argreturn resdef
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def union(*args):res = []for seq in
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For both scopes and arguments, we a
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Quiz Answers1. The output here is '
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Function objects returned by runnin
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a dictionary becomes a more general
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What wasn’t illustrated in the pr
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around this, simply build up the ar
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Moreover, map can be used in more a
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Together with map, filter and reduc
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... if x % 2 == 0:... res.append(x)
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List comprehensions are powerful to
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Why You Will Care: List Comprehensi
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If you want to see what is going on
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for key in D:... print key, D[key].
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I’ve mentioned a few times that l
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For more insight, try modifying the
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seen some of these uses in earlier
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...>>> selector( )99Remember, thoug
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Functions Without returnsIn Python
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B RAINB UILDERPart IV ExercisesIn t
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Package this code as a reusable fun
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Chapter 18CHAPTER 18Modules: The Bi
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How to Structure a ProgramGenerally
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in any other programs you may write
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Home directoryPython first looks fo
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Module file selectionKeep in mind t
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Because this last import step actua
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B RAINB UILDERChapter Quiz1. How do
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ecause if is a reserved word—when
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As one consequence, because top-lev
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Like all assignments, the from stat
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Files Generate NamespacesSo, how do
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Qualification is really an expressi
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X = 1import mod2print X,print mod2.
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• Reloads impact all clients that
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Although we’ve already seen enoug
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Chapter 20CHAPTER 20Module Packages
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dir0\dir1\dir2\mod.pyand an import
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import dir1.dir2.mod # Later import
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A Tale of Three SystemsThe only tim
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Also, notice in the install hierarc
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from * copies out all names, the im
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Unit Tests with _ _name_ _In fact,
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of a running Python program, and so
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This defines a package named mypkg
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• Modules should rarely change ot
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We’ve also provided self-test log
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To get around this, you need to use
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from module import X. . .reload(mod
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import typesdef status(module):prin
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5. Package imports. Import your fil
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Chapter 22CHAPTER 22OOP: The Big Pi
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Customization via inheritanceClasse
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ecause the search proceeds bottom-u
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• Each class statement generates
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The _ _init_ _ method is known as t
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This is yet another instance of the
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Chapter 23CHAPTER 23Class Coding Ba
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A First ExampleLet’s turn to a re
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Although less common, we could even
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Z (instance)- datais-aSecondClass-
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foster object interfaces that are m
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Specially named methods such as _ _
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Really, these instances have no att
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B RAINB UILDERChapter Quiz1. How ar
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Chapter 24CHAPTER 24Class Coding De
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Here, because the name spam is assi
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always go through self to fetch or
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In Python, inheritance happens when
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This extension coding pattern is al
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def delegate(self):self.action( )de
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All operator overloading methods ar
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def _ _iter_ _(self): # Get iterato
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class SkipIterator:def _ _init_ _(s
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makes age look like a real attribut
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class addrepr(adder): # Inherit _ _
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class Prod:... def _ _init_ _(self,
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You’ll also see another _ _call_
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principle translates to code, and s
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To help you understand how attribut
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X._ _dict_ _{'data1': 'spam', 'data
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every attribute in the instance. It
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JonesTrace through the code in this
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6. Printing sue ultimately runs the
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Overloading by Call Signatures (or
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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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