Is Python a

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it allows programs to scale up in sophistication as needed. Because of that, it iscommonly used for quick tactical tasks and longer-term strategic development.So, is Python a scripting language or not? It depends on whom you ask. In general,the term “scripting” is probably best used to describe the rapid and flexible mode ofdevelopment that Python supports, rather than a particular application domain.OK, but What’s the Downside?After using Python for 15 years and teaching it for 10, the only downside to PythonI’ve found is that, as currently implemented, its execution speed may not always beas fast as compiled languages such as C and C++.We’ll talk about implementation concepts in detail later in this book. In short, thestandard implementations of Python today compile (i.e., translate) source code statementsto an intermediate format known as byte code, and then interpret the bytecode. Byte code provides portability, as it is a platform-independent format. However,because Python is not compiled all the way down to binary machine code (e.g.,instructions for an Intel chip), some programs will run more slowly in Python than ina fully compiled language like C.Whether you will ever care about the execution speed difference depends on whatkinds of programs you write. Python has been optimized numerous times, and Pythoncode runs fast enough by itself in most application domains. Furthermore, wheneveryou do something “real” in a Python script, like process a file or construct a GUI, yourprogram is actually running at C speed, since such tasks are immediately dispatchedto compiled C code inside the Python interpreter. More fundamentally, Python’sspeed-of-development gain is often far more important than any speed-of-executionloss, especially given modern computer speeds.Even at today’s CPU speeds, though, there still are some domains that do requireoptimal execution speeds. Numeric programming and animation, for example, oftenneed at least their core number-crunching components to run at C speed (or better).If you work in such a domain, you can still use Python—simply split off the parts ofthe application that require optimal speed into compiled extensions, and link thoseinto your system for use in Python scripts.We won’t talk about extensions much in this text, but this is really just an instanceof the Python-as-control-language role that we discussed earlier. A prime example ofthis dual language strategy is the NumPy numeric programming extension forPython; by combining compiled and optimized numeric extension libraries with thePython language, NumPy turns Python into a numeric programming tool that is efficientand easy to use. You may never need to code such extensions in your ownPython work, but they provide a powerful optimization mechanism if you ever do.OK, but What’s the Downside? | 7
Who Uses Python Today?At this writing, in 2007, the best estimate anyone can seem to make of the size of thePython user base is that there are roughly 1 million Python users around the worldtoday (plus or minus a few). This estimate is based on various statistics, like downloadrates and developer surveys. Because Python is open source, a more exact count is difficult—thereare no license registrations to tally. Moreover, Python is automaticallyincluded with Linux distributions, Macintosh computers, and some products andhardware, further clouding the user-base picture.In general, though, Python enjoys a large user base, and a very active developer community.Because Python has been around for more than 15 years and has beenwidely used, it is also very stable and robust. Besides being employed by individualusers, Python is also being applied in real revenue-generating products by real companies.For instance:• Google makes extensive use of Python in its web search system, and employsPython’s creator.• The YouTube video sharing service is largely written in Python.• The popular BitTorrent peer-to-peer file sharing system is a Python program.• Intel, Cisco, Hewlett-Packard, Seagate, Qualcomm, and IBM use Python forhardware testing.• Industrial Light & Magic, Pixar, and others use Python in the production ofmovie animation.• JPMorgan Chase, UBS, Getco, and Citadel apply Python for financial marketforecasting.• NASA, Los Alamos, Fermilab, JPL, and others use Python for scientific programmingtasks.• iRobot uses Python to develop commercial robotic vacuum cleaners.• ESRI uses Python as an end-user customization tool for its popular GIS mappingproducts.• The NSA uses Python for cryptography and intelligence analysis.• The IronPort email server product uses more than 1 million lines of Python codeto do its job.• The One Laptop Per Child (OLPC) project builds its user interface and activitymodel in Python.And so on. Probably the only common thread amongst the companies using Pythontoday is that Python is used all over the map, in terms of application domains. Itsgeneral-purpose nature makes it applicable to almost all fields, not just one. In fact,it’s safe to say that Python is being used by virtually every substantial organization8 | Chapter 1: A Python Q&A Session
Page 2 and 3: Learning Python
Page 4 and 5: THIRD EDITIONLearning PythonMark Lu
Page 6: To Vera.You are my life.
Page 10 and 11: 3. How You Run Programs . . . . . .
Page 12 and 13: 5. Numbers . . . . . . . . . . . .
Page 14 and 15: 8. Lists and Dictionaries . . . . .
Page 16 and 17: Statement rule special cases 208Blo
Page 18 and 19: 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: Is Python a “Scripting Language
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 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
Page 99 and 100: start occasionally on some Windows
Page 101 and 102: Embedding CallsAt this point, we’
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Which Option Should I Use?With all
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6. IDLE can still be hung by some t
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5. Errors. Experiment with typing m
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Chapter 4CHAPTER 4Introducing Pytho
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Python’s Core Data TypesTable 4-1
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print the result—with more than 3
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In a slice, the left bound defaults
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S = 'spam'>>> S.upper( ) # Upper- a
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msg'\naaaaaaaaaaaaa\nbbb\'\'\'bbbbb
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M.reverse( )>>> M['cc', 'bb', 'aa']
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diag = [M[i][i] for i in [0, 1, 2]]
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ec['job'][-1] # Index the nested li
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on any object that is a sequence an
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TuplesThe tuple object (pronounced
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Other Core TypesBeyond the core typ
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def lastName(self):return self.name
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B RAINB UILDERChapter QuizWe’ll e
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Chapter 5CHAPTER 5Numbers5This chap
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Hexadecimal and octal literalsThe r
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Mixed Operators Follow Operator Pre
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Numbers in ActionProbably the best
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(2.0 + a) # Auto echo output: more
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(9 / 3), (9.0 / 3), (9 // 3), (9 //
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Hexadecimal and Octal NotationAs me
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The random module can be useful for
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Such operations are convenient when
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B RAINB UILDERChapter Quiz1. What i
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Python to use the name a as a varia
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However, that’s not really what
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Names References Objectsa=3b=aa 3bF
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changes, they can impact other part
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import sys>>> sys.getrefcount(1) #
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Chapter 7CHAPTER 7Strings7The next
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Single- and Double-Quoted Strings A
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Some escape sequences allow you to
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Triple Quotes Code Multiline Block
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Because Unicode is designed to hand
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Notice that operator overloading is
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• Indexing (S[i]) fetches compone
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Why You Will Care: SlicesThroughout
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Such conversions can be used in con
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The third example again inserts thr
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eply = """Greetings...Hello %(name)
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S = 'spammy'>>> S = S[:3] + 'xx' +
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can pull out its components by spli
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x'aspambspamcspam'To access the sam
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B RAINB UILDERChapter Quiz1. Can th
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Variable-length, heterogeneous, and
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espond to all of the general sequen
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Both index and slice assignments ar
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In some types of programs, the list
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Tables of object references (hash t
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Later in this chapter and book, you
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Dictionaries also provide a copy me
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Here, it looks as if D is a 100-ite
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Other ways to make dictionariesFina
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B RAINB UILDERChapter Quiz1. Name t
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Of the category immutable sequenceL
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T = tuple(tmp) # Make a tuple from
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Table 9-2. Common file operations (
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F.write('%s,%s,%s\n' % (X, Y, Z)) #
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We get back an equivalent dictionar
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Why You Will Care: Operator Overloa
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We studied this phenomenon in Chapt
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For instance, a comparison of list
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Python also provides a special obje
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Because types can be subclassed in
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Beware of Cyclic Data StructuresWe
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B RAINB UILDERChapter Quiz1. How ca
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c. Finally, how does Python handle
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PART IIIIII.Statements and Syntax
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Python’s StatementsTable 10-1 sum
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What Python AddsThe one new syntax
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Why Indentation Syntax?The indentat
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My point is that even when a langua
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prints, function calls, and the lik
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Armed with this information, we can
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an except part that gives the excep
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B RAINB UILDERChapter Quiz1. What t
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• Implicit assignments: import, f
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In fact, the original tuple and lis
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Notice that this code is using the
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Augmented assignments have three ad
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Reserved words are off-limitsNames
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Expression StatementsIn Python, you
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Table 11-5. print statement formsOp
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The print >> file ExtensionThis tri
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In Python 3.0, the print statement
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ChapterCHAPTER1212if Tests 12This c
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If you’ve used languages like C o
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As you’ve seen, there are no vari
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• Triple-quoted string literals c
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[ ] or 33>>> [ ] or { }{ }In the fi
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of Python 2.5. Again, though, you s
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ChapterCHAPTER1313while and for Loo
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Finally, notice that Python doesn
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Because continue jumps to the top o
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print 'Ni'breakx = x[1:]else:print
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ExamplesLet’s type a few for loop
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Note that this example is easier to
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eadlines loads a file all at once i
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Moreover, the iterator version has
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We saw the sorted function used her
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Although such range results may be
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To really change the list as we mar
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Dictionary construction with zipIn
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List Comprehensions: A First LookIn
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and add it to the result list. Agai
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B RAINB UILDERChapter Quiz1. When i
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ChapterCHAPTER1414The Documentation
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Only some of the many names are dis
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Note that you will generally want t
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FILE(built-in)DESCRIPTIONThis modul
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too). Enter the name of a module yo
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Figure 14-3. PyDoc can serve up doc
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Figure 14-4. Python’s standard ma
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• Use simple for loops instead of
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B RAINB UILDERChapter Quiz1. When s
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if found:print 'at index', ielse:pr
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Chapter 15CHAPTER 15Function Basics
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In this chapter, we will explore ho
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def Executes at RuntimeThe Python d
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Polymorphism in PythonAs we just sa
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s1 = "SPAM">>> s2 = "SCAM">>> inter
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In all cases, the scope of a variab
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lambdas, then the global (G) scope,
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have to import it in order to inspe
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Here, x, y, and z are all globals i
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Here again, the best prescription i
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Notice that the global declaration
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Retaining enclosing scopes’ state
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def makeActions( ):... acts = []...
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Python’s pass-by-assignment schem
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Avoiding Mutable Argument ChangesAr
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Defaults: specify values for argume
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DefaultsWe talked a little about de
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func(*args)1 2 3 4Similarly, the **
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if arg < res:res = argreturn resdef
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def union(*args):res = []for seq in
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For both scopes and arguments, we a
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Quiz Answers1. The output here is '
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Function objects returned by runnin
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a dictionary becomes a more general
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What wasn’t illustrated in the pr
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around this, simply build up the ar
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Moreover, map can be used in more a
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Together with map, filter and reduc
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... if x % 2 == 0:... res.append(x)
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List comprehensions are powerful to
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Why You Will Care: List Comprehensi
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If you want to see what is going on
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for key in D:... print key, D[key].
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I’ve mentioned a few times that l
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For more insight, try modifying the
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seen some of these uses in earlier
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...>>> selector( )99Remember, thoug
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Functions Without returnsIn Python
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B RAINB UILDERPart IV ExercisesIn t
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Package this code as a reusable fun
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Chapter 18CHAPTER 18Modules: The Bi
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How to Structure a ProgramGenerally
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in any other programs you may write
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Home directoryPython first looks fo
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Module file selectionKeep in mind t
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Because this last import step actua
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B RAINB UILDERChapter Quiz1. How do
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ecause if is a reserved word—when
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As one consequence, because top-lev
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Like all assignments, the from stat
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Files Generate NamespacesSo, how do
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Qualification is really an expressi
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X = 1import mod2print X,print mod2.
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• Reloads impact all clients that
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Although we’ve already seen enoug
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Chapter 20CHAPTER 20Module Packages
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dir0\dir1\dir2\mod.pyand an import
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import dir1.dir2.mod # Later import
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A Tale of Three SystemsThe only tim
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Also, notice in the install hierarc
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from * copies out all names, the im
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Unit Tests with _ _name_ _In fact,
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of a running Python program, and so
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This defines a package named mypkg
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• Modules should rarely change ot
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We’ve also provided self-test log
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To get around this, you need to use
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from module import X. . .reload(mod
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import typesdef status(module):prin
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5. Package imports. Import your fil
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Chapter 22CHAPTER 22OOP: The Big Pi
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Customization via inheritanceClasse
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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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