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if _ _name_ _ == '_ _main_ _': doomed( )% python oops.pycaught an index error!2. Exception objects and lists. Here’s the way I extended this module for an exceptionof my own (here a string, at first):MyError = 'hello'def oops( ):raise MyError, 'world'def doomed( ):try:oops( )except IndexError:print 'caught an index error!'except MyError, data:print 'caught error:', MyError, dataelse:print 'no error caught...'if _ _name_ _ == '_ _main_ _':doomed( )% python oops.pycaught error: hello worldTo identify the exception with a class, I just changed the first part of the file tothis, and saved it as oop_oops.py:class MyError: passdef oops( ):raise MyError( )...rest unchanged...Like all class exceptions, the instance comes back as the extra data; the errormessage now shows both the class and its instance ().% python oop_oops.pycaught error: __main__.MyError Remember, to make this look nicer, you can define a _ _repr_ _ or _ _str_ _method in your class to return a custom print string. See Chapter 24 for details.3. Error handling. Here’s one way to solve this one (file safe2.py). I did my tests in afile, rather than interactively, but the results are about the same.import sys, tracebackdef safe(entry, *args):try:apply(entry, args)# Catch everything elseexcept:traceback.print_exc( )print 'Got', sys.exc_type, sys.exc_valuePart VII, Exceptions and Tools | 673
import oopssafe(oops.oops)% python safe2.pyTraceback (innermost last):File "safe2.py", line 5, in safeapply(entry, args)File "oops.py", line 4, in oopsraise MyError, 'world'hello: worldGot hello world# Catch everything elseToday, I would probably code this as follows, using the newer *args call syntaxand exc_info:def safe(entry, *args):try:entry(*args)# Catch everything elseexcept:traceback.print_exc( )print 'Got', sys.exc_info( ){0], sys.exc_info( )[1]4. Here are a few examples for you to study as time allows; for more, see follow-upbooks and the Web:# Find the largest file in a single directorydirname = r'C:\<strong>Python</strong>25\Lib'import os, globallsizes = []allpy = glob.glob(os.path.join(dirname, '*.py'))for filename in allpy:filesize = os.path.getsize(filename)allsizes.append((filesize, filename))allsizes.sort( )print allsizes[:2]print allsizes[-2:]# Find the largest file in an entire directory treeimport sysif sys.platform[:3] == 'win':dirname = r'C:\<strong>Python</strong>25\Lib'else:dirname = '/usr/lib/python'import os, globallsizes = []for (thisDir, subsHere, filesHere) in os.walk(dirname):for filename in filesHere:if filename.endswith('.py'):fullname = os.path.join(thisDir, filename)fullsize = os.path.getsize(fullname)allsizes.append((fullsize, fullname))674 | Appendix B: Solutions to End-of-Part Exercises
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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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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
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 and 680: To learn about other large-scale Py
Page 681 and 682: B RAINB UILDERPart VII ExercisesAs
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 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
Page 730 and 731: delegation, 527-528, 556descriptor
Page 732 and 733: Part I - Modules, 646Part I - Progr
Page 734 and 735: Hhandler, 578“has-a” relationsh
Page 736 and 737: Kkeyed-access files, 86KeyError (bu
Page 738 and 739: imports and reloads, 45-50example,
Page 740 and 741: overloading by call signatures, 519
Page 742 and 743: cross-file changes, minimizing, 318
Page 744 and 745: Tkinter, 9, 31environment setting,
Page 746: About the AuthorMark Lutz is the wo
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