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B RAINB UILDERChapter Quiz1. (This question is a repeat from your first quiz in Chapter 1—see, I told you thiswould be easy :-)). Why did spam appear in so many examples in this book?Quiz Answers1. Because Python is named after the British comedy group Monty Python (basedon surveys I’ve conducted in classes, this is a much-too-well-kept secret in thePython world!). The spam reference comes from a Monty Python skit, where acouple who are trying to order food in a cafeteria keep getting drowned out by achorus of Vikings singing a song about spam. And if I could insert an audio clipof that song here as the theme song for this book’s closing credits, I would....Chapter Quiz | 635
B RAINB UILDERPart VII ExercisesAs we’ve reached the end of this part of the book, it’s time for a few exception exercisesto give you a chance to practice the basics. Exceptions really are simple tools; ifyou get these, you’ve got them mastered.See“Part VII, Exceptions and Tools” in Appendix B for the solutions.1. try/except. Write a function called oops that explicitly raises an IndexError exceptionwhen called. Then, write another function that calls oops inside a try/exceptstatement to catch the error. What happens if you change oops to raise a KeyErrorinstead of an IndexError? Where do the names KeyError and IndexError comefrom? (Hint: recall that all unqualified names come from one of four scopes, bythe LEGB rule.)2. Exception objects and lists. Change the oops function you just wrote to raise anexception you define yourself, called MyError, and pass an extra data item alongwith the exception. You may identify your exception with a string or a class.Then, extend the try statement in the catcher function to catch this exception,and its data in addition to IndexError, and print the extra data item. Finally, ifyou used a string for your exception, go back and change it to a class instance;what now comes back as the extra data to the handler?3. Error handling. Write a function called safe(func, *args) that runs any functionusing apply (or the newer *name call syntax), catches any exception raised whilethe function runs, and prints the exception using the exc_type and exc_valueattributes in the sys module (or the newer sys.exc_info call result). Then, useyour safe function to run your oops function from exercise 1 or 2. Put safe in amodule file called tools.py, and pass it the oops function interactively. What sortof error messages do you get? Finally, expand safe to also print a Python stacktrace when an error occurs by calling the built-in print_exc function in the standardtraceback module (see the Python library reference manual for details).4. Self-study examples. At the end of Appendix B, I’ve included a handful of examplescripts developed as group exercises in live Python classes for you to studyand run on your own in conjunction with Python’s standard manual set. Theseare not described, and they use tools in the Python standard library that you’llhave to look up. But, for many readers, it helps to see how the concepts we’vediscussed in this book come together in real programs. If these whet your appetitefor more, you can find a wealth of larger and more realistic application-levelPython program examples in follow-up books like Programming Python, and onthe Web in your favorite browser.636 | Chapter 29: Designing with Exceptions
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Learning Python
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THIRD EDITIONLearning PythonMark Lu
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To Vera.You are my life.
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3. How You Run Programs . . . . . .
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5. Numbers . . . . . . . . . . . .
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8. Lists and Dictionaries . . . . .
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Statement rule special cases 208Blo
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Iterators: A First Look 258File Ite
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Scopes and Nested Functions 320Nest
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How Imports Work 3891. Find It 390T
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Brain Builder 445Chapter Quiz 445Qu
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25. Designing with Classes . . . .
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Preface1This book provides an intro
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In addition, this entire edition in
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This book is designed to be an intr
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Part I, Getting StartedWe begin wit
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Preparing for Python 3.0The first a
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• In the iterator protocol, the X
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We appreciate, but do not require,
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It’s been a long and winding road
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PART II.Getting Started
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Developer productivityPython boosts
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Is Python a “Scripting Language
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Who Uses Python Today?At this writi
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(both Classic and OS X). A free ext
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Gaming, Images, AI, XML, Robots, an
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Besides the language interpreter it
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Python Is Engineering, Not ArtWhen
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world, the truth is that Python cre
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B RAINB UILDERChapter QuizIn this e
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ChapterCHAPTER22How Python Runs Pro
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Because installation details are so
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If Python cannot write the byte cod
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Python Implementation AlternativesR
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Roughly, while your program runs, P
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Future Possibilities?Finally, note
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ChapterCHAPTER33How You Run Program
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Here, the last two lines typed are
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un it again without retyping it fro
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Using Command Lines and FilesRunnin
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In this case, you don’t need the
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The raw_input TrickUnfortunately, o
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In simple terms, every file of Pyth
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Alternatively, you can fetch (reall
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Because of these complications (and
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Figure 3-3. The main Python shell w
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start occasionally on some Windows
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Embedding CallsAt this point, we’
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Which Option Should I Use?With all
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6. IDLE can still be hung by some t
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5. Errors. Experiment with typing m
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Chapter 4CHAPTER 4Introducing Pytho
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Python’s Core Data TypesTable 4-1
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print the result—with more than 3
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In a slice, the left bound defaults
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S = 'spam'>>> S.upper( ) # Upper- a
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msg'\naaaaaaaaaaaaa\nbbb\'\'\'bbbbb
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M.reverse( )>>> M['cc', 'bb', 'aa']
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diag = [M[i][i] for i in [0, 1, 2]]
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ec['job'][-1] # Index the nested li
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on any object that is a sequence an
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TuplesThe tuple object (pronounced
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Other Core TypesBeyond the core typ
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def lastName(self):return self.name
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B RAINB UILDERChapter QuizWe’ll e
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Chapter 5CHAPTER 5Numbers5This chap
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Hexadecimal and octal literalsThe r
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Mixed Operators Follow Operator Pre
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Numbers in ActionProbably the best
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(2.0 + a) # Auto echo output: more
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(9 / 3), (9.0 / 3), (9 // 3), (9 //
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Hexadecimal and Octal NotationAs me
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The random module can be useful for
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Such operations are convenient when
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B RAINB UILDERChapter Quiz1. What i
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Python to use the name a as a varia
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However, that’s not really what
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Names References Objectsa=3b=aa 3bF
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changes, they can impact other part
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import sys>>> sys.getrefcount(1) #
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Chapter 7CHAPTER 7Strings7The next
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Single- and Double-Quoted Strings A
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Some escape sequences allow you to
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Triple Quotes Code Multiline Block
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Because Unicode is designed to hand
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Notice that operator overloading is
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• Indexing (S[i]) fetches compone
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Why You Will Care: SlicesThroughout
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Such conversions can be used in con
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The third example again inserts thr
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eply = """Greetings...Hello %(name)
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S = 'spammy'>>> S = S[:3] + 'xx' +
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can pull out its components by spli
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x'aspambspamcspam'To access the sam
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B RAINB UILDERChapter Quiz1. Can th
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Variable-length, heterogeneous, and
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espond to all of the general sequen
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Both index and slice assignments ar
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In some types of programs, the list
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Tables of object references (hash t
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Later in this chapter and book, you
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Dictionaries also provide a copy me
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Here, it looks as if D is a 100-ite
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Other ways to make dictionariesFina
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B RAINB UILDERChapter Quiz1. Name t
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Of the category immutable sequenceL
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T = tuple(tmp) # Make a tuple from
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Table 9-2. Common file operations (
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F.write('%s,%s,%s\n' % (X, Y, Z)) #
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We get back an equivalent dictionar
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Why You Will Care: Operator Overloa
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We studied this phenomenon in Chapt
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For instance, a comparison of list
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Python also provides a special obje
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Because types can be subclassed in
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Beware of Cyclic Data StructuresWe
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B RAINB UILDERChapter Quiz1. How ca
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c. Finally, how does Python handle
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PART IIIIII.Statements and Syntax
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Python’s StatementsTable 10-1 sum
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What Python AddsThe one new syntax
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Why Indentation Syntax?The indentat
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My point is that even when a langua
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prints, function calls, and the lik
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Armed with this information, we can
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an except part that gives the excep
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B RAINB UILDERChapter Quiz1. What t
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• Implicit assignments: import, f
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In fact, the original tuple and lis
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Notice that this code is using the
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Augmented assignments have three ad
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Reserved words are off-limitsNames
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Expression StatementsIn Python, you
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Table 11-5. print statement formsOp
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The print >> file ExtensionThis tri
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In Python 3.0, the print statement
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ChapterCHAPTER1212if Tests 12This c
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If you’ve used languages like C o
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As you’ve seen, there are no vari
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• Triple-quoted string literals c
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[ ] or 33>>> [ ] or { }{ }In the fi
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of Python 2.5. Again, though, you s
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ChapterCHAPTER1313while and for Loo
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Finally, notice that Python doesn
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Because continue jumps to the top o
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print 'Ni'breakx = x[1:]else:print
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ExamplesLet’s type a few for loop
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Note that this example is easier to
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eadlines loads a file all at once i
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Moreover, the iterator version has
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We saw the sorted function used her
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Although such range results may be
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To really change the list as we mar
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Dictionary construction with zipIn
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List Comprehensions: A First LookIn
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and add it to the result list. Agai
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B RAINB UILDERChapter Quiz1. When i
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ChapterCHAPTER1414The Documentation
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Only some of the many names are dis
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Note that you will generally want t
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FILE(built-in)DESCRIPTIONThis modul
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too). Enter the name of a module yo
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Figure 14-3. PyDoc can serve up doc
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Figure 14-4. Python’s standard ma
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• Use simple for loops instead of
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B RAINB UILDERChapter Quiz1. When s
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if found:print 'at index', ielse:pr
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Chapter 15CHAPTER 15Function Basics
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In this chapter, we will explore ho
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def Executes at RuntimeThe Python d
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Polymorphism in PythonAs we just sa
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s1 = "SPAM">>> s2 = "SCAM">>> inter
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In all cases, the scope of a variab
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lambdas, then the global (G) scope,
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have to import it in order to inspe
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Here, x, y, and z are all globals i
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Here again, the best prescription i
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Notice that the global declaration
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Retaining enclosing scopes’ state
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def makeActions( ):... acts = []...
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Python’s pass-by-assignment schem
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Avoiding Mutable Argument ChangesAr
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Defaults: specify values for argume
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DefaultsWe talked a little about de
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func(*args)1 2 3 4Similarly, the **
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if arg < res:res = argreturn resdef
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def union(*args):res = []for seq in
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For both scopes and arguments, we a
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Quiz Answers1. The output here is '
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Function objects returned by runnin
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a dictionary becomes a more general
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What wasn’t illustrated in the pr
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around this, simply build up the ar
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Moreover, map can be used in more a
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Together with map, filter and reduc
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... if x % 2 == 0:... res.append(x)
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List comprehensions are powerful to
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Why You Will Care: List Comprehensi
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If you want to see what is going on
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for key in D:... print key, D[key].
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I’ve mentioned a few times that l
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For more insight, try modifying the
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seen some of these uses in earlier
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...>>> selector( )99Remember, thoug
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Functions Without returnsIn Python
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B RAINB UILDERPart IV ExercisesIn t
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Package this code as a reusable fun
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Chapter 18CHAPTER 18Modules: The Bi
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How to Structure a ProgramGenerally
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in any other programs you may write
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Home directoryPython first looks fo
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Module file selectionKeep in mind t
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Because this last import step actua
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B RAINB UILDERChapter Quiz1. How do
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ecause if is a reserved word—when
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As one consequence, because top-lev
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Like all assignments, the from stat
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Files Generate NamespacesSo, how do
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Qualification is really an expressi
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X = 1import mod2print X,print mod2.
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• Reloads impact all clients that
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Although we’ve already seen enoug
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Chapter 20CHAPTER 20Module Packages
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dir0\dir1\dir2\mod.pyand an import
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import dir1.dir2.mod # Later import
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A Tale of Three SystemsThe only tim
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Also, notice in the install hierarc
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from * copies out all names, the im
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Unit Tests with _ _name_ _In fact,
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of a running Python program, and so
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This defines a package named mypkg
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• Modules should rarely change ot
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We’ve also provided self-test log
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To get around this, you need to use
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from module import X. . .reload(mod
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import typesdef status(module):prin
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5. Package imports. Import your fil
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Chapter 22CHAPTER 22OOP: The Big Pi
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Customization via inheritanceClasse
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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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Page 631 and 632: def gosouth(x):print gobad(x, 0)gos
Page 633 and 634: • If an exception does occur duri
Page 635 and 636: The code in this statement’s main
Page 637 and 638: When this code is run, the followin
Page 639 and 640: the globals reside in. Passing extr
Page 641 and 642: traps programming errors itself, th
Page 643 and 644: The Context Management ProtocolThe
Page 645 and 646: Notice that this class’ _ _exit_
Page 647 and 648: ChapterCHAPTER2828Exception Objects
Page 649 and 650: Class-Based ExceptionsStrings are a
Page 651 and 652: Why Class Exceptions?Because there
Page 653 and 654: But this workaround might catch mor
Page 655 and 656: except StandardError:...handle Pyth
Page 657 and 658: def parser( ):... # when error foun
Page 659 and 660: aise instanceraise class, instance#
Page 661 and 662: B RAINB UILDERChapter Quiz1. How ar
Page 663 and 664: try:func1()except E:...def func1():
Page 665 and 666: Really, this code just sets up the
Page 667 and 668: Functions Signal Conditions with ra
Page 669 and 670: At the end of this chapter, we’ll
Page 671 and 672: For example, scripts normally exit
Page 673 and 674: String Exceptions Match by Identity
Page 675 and 676: The Python ToolsetFrom this point f
Page 677 and 678: DoctestThe doctest standard library
Page 679: To learn about other large-scale Py
Page 684 and 685: Appendix AAPPENDIX AInstallation an
Page 686 and 687: When installed on Windows, Python a
Page 688 and 689: These variables are straightforward
Page 690 and 691: eboot your machine for the changes
Page 692 and 693: Python’s search path. The .pyc fi
Page 694 and 695: S = "ham">>> "eggs " + S'eggs ham'>
Page 696 and 697: 4. Tuple assignment. The values of
Page 698 and 699: 9. Immutable types. Either of the f
Page 700 and 701: a => 1b => 2c => 3d => 4e => 5f =>
Page 702 and 703: 3. varargs. Two alternative adder f
Page 704 and 705: 7. More argument-matching examples.
Page 706 and 707: es[1.4142135623730951, 2.0, 3.0, 4.
Page 708 and 709: % python myclient.py13 346As for th
Page 710 and 711: % python>>> from adder import *>>>
Page 712 and 713: Note that it’s important to copy
Page 714 and 715: My solution to the multiple-operand
Page 716 and 717: class Customer:def _ _init_ _(self)
Page 718 and 719: if _ _name_ _ == '_ _main_ _': doom
Page 720 and 721: allsizes.sort( )print allsizes[:2]p
Page 722 and 723: win = Tk( )L = Label(win, text='Spa
Page 724 and 725: # CGI client-side script to interac
Page 726 and 727: IndexSymbols# comments, 279% (remai
Page 728 and 729: 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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