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We’ve just run a Python script that prints a string and a number. We probably won’twin any programming awards with this code, but it’s enough to capture the basics ofprogram execution.Python’s ViewThe brief description in the prior section is fairly standard for scripting languages,and it’s usually all that most Python programmers need to know. You type code intotext files, and you run those files through the interpreter. Under the hood, though, abit more happens when you tell Python to “go.” Although knowledge of Pythoninternals is not strictly required for Python programming, a basic understanding ofthe runtime structure of Python can help you grasp the bigger picture of programexecution.When you instruct Python to run your script, there are a few steps that Pythoncarries out before your code actually starts crunching away. Specifically, it’s firstcompiled to something called “byte code” and then routed to something called a“virtual machine.”Byte code compilationInternally, and almost completely hidden from you, when you execute a program,Python first compiles your source code (the statements in your file) into a formatknown as byte code. Compilation is simply a translation step, and byte code is alower-level, platform-independent representation of your source code. Roughly,Python translates each of your source statements into a group of byte code instructionsby decomposing them into individual steps. This byte code translation isperformed to speed execution—byte code can be run much more quickly than theoriginal source code statements in your text file.You’ll notice that the prior paragraph said that this is almost completely hidden fromyou. If the Python process has write access on your machine, it will store the bytecode of your program in files that end with a .pyc extension (“.pyc” means compiled“.py” source). You will see these files show up on your computer after you’ve run afew programs alongside the corresponding source code files (that is, in the samedirectories).Python saves byte code like this as a startup speed optimization. The next time yourun your program, Python will load the .pyc and skip the compilation step, as long asyou haven’t changed your source code since the byte code was last saved. Pythonautomatically checks the timestamps of source and byte code files to know when itmust recompile—if you resave your source code, byte code is automatically re-createdthe next time your program is run.Program Execution | 25
If Python cannot write the byte code files to your machine, your program stillworks—the byte code is generated in memory and simply discarded on program exit. *However, because .pyc files speed startup time, you’ll want to make sure they arewritten for larger programs. Byte code files are also one way to ship Python programs—Pythonis happy to run a program if all it can find are .pyc files, even if theoriginal .py source files are absent. (See “Frozen Binaries” later in this chapter foranother shipping option.)The Python Virtual Machine (PVM)Once your program has been compiled to byte code (or the byte code has beenloaded from existing .pyc files), it is shipped off for execution to something generallyknown as the Python Virtual Machine (PVM, for the more acronym-inclined amongyou). The PVM sounds more impressive than it is; really, it’s not a separate program,and it need not be installed by itself. In fact, the PVM is just a big loop thatiterates through your byte code instructions, one by one, to carry out their operations.The PVM is the runtime engine of Python; it’s always present as part of thePython system, and it’s the component that truly runs your scripts. Technically, it’sjust the last step of what is called the “Python interpreter.”Figure 2-2 illustrates the runtime structure described here. Keep in mind that all of thiscomplexity is deliberately hidden from Python programmers. Byte code compilation isautomatic, and the PVM is just part of the Python system that you have installed onyour machine. Again, programmers simply code and run files of statements.SourceByte codeRuntimem.pym.pycPVMFigure 2-2. Python’s traditional runtime execution model: source code you type is translated to bytecode, which is then run by the Python Virtual Machine. Your code is automatically compiled, butthen it is interpreted.Performance implicationsReaders with a background in fully compiled languages such as C and C++ mightnotice a few differences in the Python model. For one thing, there is usually no buildor “make” step in Python work: code runs immediately after it is written. Foranother, Python byte code is not binary machine code (e.g., instructions for an Intelchip). Byte code is a Python-specific representation.* And, strictly speaking, byte code is saved only for files that are imported, not for the top-level file of a program.We’ll explore imports in Chapter 3, and again in Part V. Byte code is also never saved for code typedat the interactive prompt, which is described in Chapter 3.26 | 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
Page 20 and 21: Scopes and Nested Functions 320Nest
Page 22 and 23: 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: Because installation details are so
Page 73 and 74: Python Implementation AlternativesR
Page 75 and 76: Roughly, while your program runs, P
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
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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
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msg'\naaaaaaaaaaaaa\nbbb\'\'\'bbbbb
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M.reverse( )>>> M['cc', 'bb', 'aa']
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diag = [M[i][i] for i in [0, 1, 2]]
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ec['job'][-1] # Index the nested li
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on any object that is a sequence an
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TuplesThe tuple object (pronounced
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Other Core TypesBeyond the core typ
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def lastName(self):return self.name
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B RAINB UILDERChapter QuizWe’ll e
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Chapter 5CHAPTER 5Numbers5This chap
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Hexadecimal and octal literalsThe r
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Mixed Operators Follow Operator Pre
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Numbers in ActionProbably the best
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(2.0 + a) # Auto echo output: more
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(9 / 3), (9.0 / 3), (9 // 3), (9 //
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Hexadecimal and Octal NotationAs me
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The random module can be useful for
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Such operations are convenient when
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B RAINB UILDERChapter Quiz1. What i
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Python to use the name a as a varia
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However, that’s not really what
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Names References Objectsa=3b=aa 3bF
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changes, they can impact other part
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import sys>>> sys.getrefcount(1) #
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Chapter 7CHAPTER 7Strings7The next
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Single- and Double-Quoted Strings A
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Some escape sequences allow you to
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Triple Quotes Code Multiline Block
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Because Unicode is designed to hand
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Notice that operator overloading is
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• Indexing (S[i]) fetches compone
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Why You Will Care: SlicesThroughout
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Such conversions can be used in con
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The third example again inserts thr
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eply = """Greetings...Hello %(name)
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S = 'spammy'>>> S = S[:3] + 'xx' +
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can pull out its components by spli
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x'aspambspamcspam'To access the sam
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B RAINB UILDERChapter Quiz1. Can th
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Variable-length, heterogeneous, and
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espond to all of the general sequen
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Both index and slice assignments ar
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In some types of programs, the list
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Tables of object references (hash t
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Later in this chapter and book, you
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Dictionaries also provide a copy me
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Here, it looks as if D is a 100-ite
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Other ways to make dictionariesFina
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B RAINB UILDERChapter Quiz1. Name t
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Of the category immutable sequenceL
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T = tuple(tmp) # Make a tuple from
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Table 9-2. Common file operations (
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F.write('%s,%s,%s\n' % (X, Y, Z)) #
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We get back an equivalent dictionar
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Why You Will Care: Operator Overloa
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We studied this phenomenon in Chapt
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For instance, a comparison of list
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Python also provides a special obje
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Because types can be subclassed in
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Beware of Cyclic Data StructuresWe
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B RAINB UILDERChapter Quiz1. How ca
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c. Finally, how does Python handle
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PART IIIIII.Statements and Syntax
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Python’s StatementsTable 10-1 sum
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What Python AddsThe one new syntax
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Why Indentation Syntax?The indentat
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My point is that even when a langua
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prints, function calls, and the lik
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Armed with this information, we can
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an except part that gives the excep
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B RAINB UILDERChapter Quiz1. What t
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• Implicit assignments: import, f
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In fact, the original tuple and lis
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Notice that this code is using the
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Augmented assignments have three ad
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Reserved words are off-limitsNames
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Expression StatementsIn Python, you
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Table 11-5. print statement formsOp
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The print >> file ExtensionThis tri
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In Python 3.0, the print statement
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ChapterCHAPTER1212if Tests 12This c
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If you’ve used languages like C o
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As you’ve seen, there are no vari
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• Triple-quoted string literals c
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[ ] or 33>>> [ ] or { }{ }In the fi
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of Python 2.5. Again, though, you s
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ChapterCHAPTER1313while and for Loo
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Finally, notice that Python doesn
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Because continue jumps to the top o
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print 'Ni'breakx = x[1:]else:print
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ExamplesLet’s type a few for loop
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Note that this example is easier to
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eadlines loads a file all at once i
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Moreover, the iterator version has
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We saw the sorted function used her
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Although such range results may be
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To really change the list as we mar
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Dictionary construction with zipIn
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List Comprehensions: A First LookIn
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and add it to the result list. Agai
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B RAINB UILDERChapter Quiz1. When i
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ChapterCHAPTER1414The Documentation
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Only some of the many names are dis
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Note that you will generally want t
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FILE(built-in)DESCRIPTIONThis modul
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too). Enter the name of a module yo
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Figure 14-3. PyDoc can serve up doc
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Figure 14-4. Python’s standard ma
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• Use simple for loops instead of
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B RAINB UILDERChapter Quiz1. When s
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if found:print 'at index', ielse:pr
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Chapter 15CHAPTER 15Function Basics
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In this chapter, we will explore ho
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def Executes at RuntimeThe Python d
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Polymorphism in PythonAs we just sa
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s1 = "SPAM">>> s2 = "SCAM">>> inter
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In all cases, the scope of a variab
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lambdas, then the global (G) scope,
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have to import it in order to inspe
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Here, x, y, and z are all globals i
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Here again, the best prescription i
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Notice that the global declaration
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Retaining enclosing scopes’ state
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def makeActions( ):... acts = []...
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Python’s pass-by-assignment schem
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Avoiding Mutable Argument ChangesAr
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Defaults: specify values for argume
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DefaultsWe talked a little about de
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func(*args)1 2 3 4Similarly, the **
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if arg < res:res = argreturn resdef
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def union(*args):res = []for seq in
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For both scopes and arguments, we a
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Quiz Answers1. The output here is '
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Function objects returned by runnin
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a dictionary becomes a more general
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What wasn’t illustrated in the pr
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around this, simply build up the ar
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Moreover, map can be used in more a
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Together with map, filter and reduc
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... if x % 2 == 0:... res.append(x)
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List comprehensions are powerful to
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Why You Will Care: List Comprehensi
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If you want to see what is going on
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for key in D:... print key, D[key].
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I’ve mentioned a few times that l
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For more insight, try modifying the
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seen some of these uses in earlier
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...>>> selector( )99Remember, thoug
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Functions Without returnsIn Python
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B RAINB UILDERPart IV ExercisesIn t
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Package this code as a reusable fun
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Chapter 18CHAPTER 18Modules: The Bi
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How to Structure a ProgramGenerally
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in any other programs you may write
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Home directoryPython first looks fo
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Module file selectionKeep in mind t
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Because this last import step actua
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B RAINB UILDERChapter Quiz1. How do
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ecause if is a reserved word—when
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As one consequence, because top-lev
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Like all assignments, the from stat
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Files Generate NamespacesSo, how do
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Qualification is really an expressi
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X = 1import mod2print X,print mod2.
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• Reloads impact all clients that
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Although we’ve already seen enoug
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Chapter 20CHAPTER 20Module Packages
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dir0\dir1\dir2\mod.pyand an import
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import dir1.dir2.mod # Later import
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A Tale of Three SystemsThe only tim
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Also, notice in the install hierarc
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from * copies out all names, the im
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Unit Tests with _ _name_ _In fact,
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of a running Python program, and so
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This defines a package named mypkg
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• Modules should rarely change ot
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We’ve also provided self-test log
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To get around this, you need to use
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from module import X. . .reload(mod
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import typesdef status(module):prin
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5. Package imports. Import your fil
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Chapter 22CHAPTER 22OOP: The Big Pi
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Customization via inheritanceClasse
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ecause the search proceeds bottom-u
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• Each class statement generates
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The _ _init_ _ method is known as t
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This is yet another instance of the
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Chapter 23CHAPTER 23Class Coding Ba
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A First ExampleLet’s turn to a re
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Although less common, we could even
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Z (instance)- datais-aSecondClass-
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foster object interfaces that are m
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Specially named methods such as _ _
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Really, these instances have no att
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B RAINB UILDERChapter Quiz1. How ar
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Chapter 24CHAPTER 24Class Coding De
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Here, because the name spam is assi
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always go through self to fetch or
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In Python, inheritance happens when
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This extension coding pattern is al
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def delegate(self):self.action( )de
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All operator overloading methods ar
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def _ _iter_ _(self): # Get iterato
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class SkipIterator:def _ _init_ _(s
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makes age look like a real attribut
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class addrepr(adder): # Inherit _ _
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class Prod:... def _ _init_ _(self,
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You’ll also see another _ _call_
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principle translates to code, and s
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To help you understand how attribut
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X._ _dict_ _{'data1': 'spam', 'data
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every attribute in the instance. It
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JonesTrace through the code in this
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6. Printing sue ultimately runs the
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Overloading by Call Signatures (or
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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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