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Although we’ve already seen enough to handle module files in our programs, thenext chapter extends our coverage of the import model by presenting packageimports—a way for our import statements to specify part of the directory path leadingto the desired module. As we’ll see, package imports give us a hierarchy that isuseful in larger systems, and allow us to break conflicts between same-named modules.Before we move on, though, here’s a quick quiz on the concepts presented here.Chapter Summary | 413
B RAINB UILDERChapter Quiz1. How do you make a module?2. How is the from statement related to the import statement?3. How is the reload function related to imports?4. When must you use import instead of from?5. Name three potential pitfalls of the from statement.6. What is the airspeed velocity of an unladen swallow?Quiz Answers1. To create a module, you just write a text file containing <strong>Python</strong> statements; everysource code file is automatically a module, and there is no syntax for declaringone. Import operations load module files into module objects in memory. Youcan also make a module by writing code in an external language like C or Java,but such extension modules are beyond the scope of this book.2. The from statement imports an entire module, like the import statement, but asan extra step, it also copies one or more variables from the imported module intothe scope where the from appears. This enables you to use the imported namesdirectly (name) instead of having to go though the module (module.name).3. By default, a module is imported only once per process. The reload functionforces a module to be imported again. It is mostly used to pick up new versionsof a module’s source code during development, and in dynamic customizationscenarios.4. You must use import instead of from only when you need to access the samename in two different modules; because you’ll have to specify the names of theenclosing modules, the two names will be unique.5. The from statement can obscure the meaning of a variable (which module it isdefined in), can have problems with the reload call (names may reference priorversions of objects), and can corrupt namespaces (it might silently overwritenames you are using in your scope). The from * form is worse in most regards—it can seriously corrupt namespaces, and obscure the meaning of variables; it isprobably best used sparingly.6. What do you mean? An African or European swallow?414 | Chapter 19: Module Coding Basics
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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
Page 408 and 409: If you want to see what is going on
Page 410 and 411: for key in D:... print key, D[key].
Page 412 and 413: I’ve mentioned a few times that l
Page 414 and 415: For more insight, try modifying the
Page 416 and 417: seen some of these uses in earlier
Page 418 and 419: ...>>> selector( )99Remember, thoug
Page 420 and 421: Functions Without returnsIn Python
Page 422 and 423: B RAINB UILDERChapter Quiz1. What i
Page 424 and 425: B RAINB UILDERPart IV ExercisesIn t
Page 426: Package this code as a reusable fun
Page 430 and 431: Chapter 18CHAPTER 18Modules: The Bi
Page 432 and 433: How to Structure a ProgramGenerally
Page 434 and 435: in any other programs you may write
Page 436 and 437: Home directoryPython first looks fo
Page 438 and 439: Module file selectionKeep in mind t
Page 440 and 441: Because this last import step actua
Page 442 and 443: B RAINB UILDERChapter Quiz1. How do
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Page 446 and 447: As one consequence, because top-lev
Page 448 and 449: Like all assignments, the from stat
Page 450 and 451: Files Generate NamespacesSo, how do
Page 452 and 453: Qualification is really an expressi
Page 454 and 455: X = 1import mod2print X,print mod2.
Page 456 and 457: • Reloads impact all clients that
Page 460 and 461: Chapter 20CHAPTER 20Module Packages
Page 462 and 463: dir0\dir1\dir2\mod.pyand an import
Page 464 and 465: import dir1.dir2.mod # Later import
Page 466 and 467: A Tale of Three SystemsThe only tim
Page 468 and 469: Also, notice in the install hierarc
Page 472 and 473: from * copies out all names, the im
Page 474 and 475: Unit Tests with _ _name_ _In fact,
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Page 478 and 479: This defines a package named mypkg
Page 480 and 481: • Modules should rarely change ot
Page 482 and 483: We’ve also provided self-test log
Page 484 and 485: To get around this, you need to use
Page 486 and 487: from module import X. . .reload(mod
Page 488 and 489: import typesdef status(module):prin
Page 492: 5. Package imports. Import your fil
Page 496 and 497: Chapter 22CHAPTER 22OOP: The Big Pi
Page 498 and 499: Customization via inheritanceClasse
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Page 502 and 503: • Each class statement generates
Page 504 and 505: The _ _init_ _ method is known as t
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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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