Core Python Programming (2nd Edition)

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Chapter 8. Conditionals and Loops Chapter Topics ● if Statement ● else Statement ● elif Statement ● Conditional Expressions ● while Statement ● for Statement ● break Statement ● continue Statement ● pass Statement ● else Statement ... Take Two ● Iterators ● List Comprehensions ● Generator Expressions The primary focus of this chapter are <strong>Python</strong>'s conditional and looping statements, and all their related components. We will take a close look at if, while, for, and their friends else, elif, break, continue, and pass.
8.1. if Statement The if statement for <strong>Python</strong> will seem amazingly familiar. It is made up of three main components: the keyword itself, an expression that is tested for its truth value, and a code suite to execute if the expression evaluates to non-zero or true. The syntax for an if statement is: if expression: expr_true_suite The suite of the if clause, expr_true_suite, will be executed only if the above conditional expression results in a Boolean true value. Otherwise, execution resumes at the next statement following the suite. 8.4.1. Multiple Conditional Expressions The Boolean operators and, or, and not can be used to provide multiple conditional expressions or perform negation of expressions in the same if statement. if not warn and (system_load >= 10): print "WARNING: losing resources" warn += 1 8.1.2. Single Statement Suites If the suite of a compound statement, i.e., if clause, while or for loop, consists only of a single line, it may go on the same line as the header statement: if make_hard_copy: send_data_to_printer() Single line statements such as the above are valid syntax-wise; however, although it may be convenient, it may make your code more difficult to read, so we recommend you indent the suite on the next line. Another good reason is that if you must add another line to the suite, you have to move that line down anyway.
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Table of Contents | Index Core Pyth
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The Complete Developer's Guide to P
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Table of Contents | Index Copyright
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Section 6.2. Strings Section 6.3. S
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Section 11.5. Formal Arguments Sect
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Section 19.4. Brief Tour of Other G
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Copyright Many of the designations
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Praise for Core Python Programming
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Prentice Hall Core Series Core J2EE
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Another set of first edition reader
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We will continue to update the book
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Dictionaries are Python's mapping o
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and you will find it fun! We first
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Acknowledgments Acknowledgments for
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Christian Tismer (creator of Stackl
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Chapter 1. Welcome to Python! Chapt
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1.2. Origins Work on Python began i
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which you can build an application,
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often provides an extended distract
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Build It Yourself For most other pl
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Windows/DOS To add Python to your s
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It is also possible in Unix to auto
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MacOS X is very Unix-like (based on
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Figure 1-5. Starting IDLE in Window
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1.6. Python Documentation Python do
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orrow features from languages such
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1.8. Other Implementations The "sta
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1.9. Exercises 1-1. Python Installa
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abs(4) 4 >>> abs(-4) 4 We will intr
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%s means to substitute a string whi
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It's very tempting for beginners to
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2.4. Operators The standard mathema
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2.5. Variables and Assignment Rules
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All numeric types are covered in Ch
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2.8. Lists and Tuples Lists and tup
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2.10. Code Blocks Use Indentation C
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2.12. while Loop The standard while
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Using the string format operator al
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2.14. List Comprehensions These are
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case you would have to revert back
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2.17. Functions Like many other lan
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2.18. Classes Classes are a core pa
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2.19. Modules A module is a logical
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2.20. Useful Functions In this chap
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2-4. User Input with raw_input(). a
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put in your own as you write your c
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Chapter 3. Python Basics Chapter To
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3.1.3. Multiple Statement Groups as
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3.2. Variable Assignment This secti
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Another way of assigning multiple v
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[a] access keyword obsoleted as of
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three PEPs: 7 (Style Guide for C Co
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3. Module imports Import all the mo
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Tests in the main body are an easy
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As responsible programmers, we are
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● It is explicitly removed from a
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Lines 13 The Unix startup line is f
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1 #!/usr/bin/env python 2 3 'readTe
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3.7. Related Modules/Developer Tool
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3-10. Exceptions. Replace the call
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4.1. Python Objects Python uses the
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4.3. Other Built-in Types ● Type
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4.4. Internal Types ● Code ● Fr
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4.5. Standard Type Operators 4.5.1.
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Example 2: foo1 and foo2 reference
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4.5.3. Boolean In the above example
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4.6. Standard Type Built-in Functio
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str(2e10) '20000000000.0' >>> >>> s
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>>> type(foo) # assumes foo instant
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then we do not even need to check b
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4.7. Type Factory Functions Since P
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Another way of categorizing the sta
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Sequence types are those whose elem
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4.10. Exercises 4-1. Python Objects
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Chapter 5. Numbers Chapter Topics
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Okay, now that you have a good idea
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Core Style: Use uppercase "L" with
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5.3. Double Precision Floating Poin
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aComplex.real -8.333 >>> aComplex.i
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Automatic numeric coercion makes li
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True Division This is where divisio
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see that the unary operator binds m
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Bitwise Operator Function ~num (una
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In addition, because of the unifica
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divmod(10,2.5) (4.0, 0.0) >>> divmo
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floor(-1.2) -2.0 round(-1.2) -1.0 -
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5.7. Other Numeric Types 5.7.1. Boo
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cannot "be continued." If we were t
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uniform() Does almost the same thin
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5.9. Exercises The exercises in thi
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Why do we get 134L and not 1342 in
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Chapter 6. Sequences: Strings, List
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obj not in seq Tests if obj is not
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if not provided, or if None is used
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Extended Slicing with Stride Indice
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above. 6.1.3. Built-in Functions (B
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[a] New in Python 2.3. [b] key argu
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considered a substring. To access s
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6.3. Strings and Operators 6.3.1. S
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Notice how the omission of both ind
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numeric identifier strings that we
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method (see example below). There i
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6.4. String-Only Operators 6.4.1. F
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'6c' >>> >>> "%X" % 108 '6C' >>> >>
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>>> print s.substitute(lang='Python
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6.5. Built-in Functions 6.5.1. Stan
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isinstance() calls to verify type:
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string.isalpha() [a] , [b] [c] Retu
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>>> quest.endswith('color?') True >
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\t 011 9 0x09 HT Horizontal tab \n
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string was created when the operati
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convert the numeric value of the by
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Example 6.2. Simple Unicode String
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Mistake #2: Using the string module
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6.9. Related Modules Table 6.10 lis
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6.10. Summary of String Highlights
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How to Update Lists You can update
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str_list[2] 'over' >>> str_list[:2]
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Traceback (innermost last): File ""
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3. 4. 5. If elements are of the sam
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sum(a, 5) 20 >>> a = [6., 4., 5.] >
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[a] Removes and returns obj at give
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original list object is left as is,
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28 while True: 29 while True: 30 tr
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(Q)uit Enter choice: v You picked:
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Line-by-Line Explanation Because of
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6.16. Tuples Tuples are another con
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6.17. Tuple Operators and Built-in
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6.18. Special Features of Tuples 6.
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(4, True, 5) >>> (4, 2) < (3, 5) #
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6.19. Related Modules Table 6.12 li
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Page 274 and 275: 6.21. Summary of Sequences Sequence
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Page 278 and 279: d. Take a string and append a backw
Page 280 and 281: 6-12. Strings. a. b. c. Create a fu
Page 282 and 283: 6-19. Multi-Column Output. Given an
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Page 288 and 289: dict2['arch'] = 'sunos5' # add new
Page 290 and 291: Beginning with Python 2.2, programm
Page 292 and 293: size, their keys match, and so do t
Page 294 and 295: dict(zip(('x', 'y'), (1, 2))) {'y':
Page 296 and 297: 7.4. Mapping Type Built-in Methods
Page 298 and 299: dict2= {'host':'earth', 'port':80}
Page 300 and 301: 7.5. Dictionary Keys Dictionary val
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Page 304 and 305: 7.6. Set Types In mathematics, a se
Page 306 and 307: ... print i ... c e h o p s How to
Page 308 and 309: superset of the second set (is a su
Page 310 and 311: s &= set('shop') >>> s set(['h', 's
Page 312 and 313: 7.9. Set Type Built-in Methods 7.9.
Page 314 and 315: 7.10. Operator, Function/Method Sum
Page 316 and 317: 7.11. Related Modules The sets modu
Page 318 and 319: 7-5. userpw2.py. The following prob
Page 320 and 321: characters in srcstr that are beyon
Page 324 and 325: 8.2. else Statement Like other lang
Page 326 and 327: 8.3. elif (aka else-if) Statement e
Page 328: 8.4. Conditional Expressions (aka "
Page 331 and 332: One must use caution when using whi
Page 333 and 334: Iterating by Sequence Item >>> name
Page 335 and 336: ange(start, end, step=1) range() wi
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Page 339 and 340: 8.8. continue Statement Core Note:
Page 341 and 342: 8.10. else Statement ... Take Two I
Page 343 and 344: 8.11. Iterators and the iter() Func
Page 345 and 346: Dictionaries Dictionaries and files
Page 347 and 348: 8.12. List Comprehensions List comp
Page 349 and 350: and split each line up into words,
Page 351 and 352: of building up this long list, it w
Page 353 and 354: lines)? That way, we can use the ma
Page 355 and 356: 8.15. Exercises 8-1. Conditionals.
Page 357 and 358: 8-9. Fibonacci Numbers. The Fibonac
Page 360 and 361: Chapter 9. Files and Input/Output C
Page 362 and 363: 9.2. File Built-in Functions [open(
Page 364 and 365: you see references to open(), you c
Page 366 and 367: Note that there is no "writeline()"
Page 368 and 369: We originally described how this pr
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9.4. File Built-in Attributes File
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9.6. Command-Line Arguments The sys
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9.7. File System Access to your fil
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Separation basename() Remove direct
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53 print '*** deleting test file' 5
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to confirm that the directory is in
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9.8. File Execution Whether we want
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other external extension modules th
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9.10. Related Modules There are ple
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Some other Python modules that gene
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9-10. Home Finances. Create a home
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9-15. Copying Files. Prompt for two
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9-24. File Transfer Between Archive
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10.1. What Are Exceptions? 10.1.1.
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10.2. Exceptions in Python As you w
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our case with the foo (non-)attribu
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As you can see, our code now runs s
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etval = None return retval Bear in
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except (Exc1[, Exc2[, ... ExcN]])[,
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except Exception, e: # handle real
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try: ... float(['float() does not',
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13 log = open('cardlog.txt', 'w') 1
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determine whether there were defect
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log.write('no txns this month\n') f
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10.4. Context Management 10.4.1. wi
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Now the with_suite executes. When e
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10.6. Raising Exceptions The interp
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aise string, args, tb Same as above
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try: assert 1 == 0, 'One does not e
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[d] Error in FloatingPointError flo
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[h] Unicode- UnicodeError related e
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10.9. *Creating Exceptions Although
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79 f.close() 80 81 for eachTest in
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the host's name and the port number
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10.10. Why Exceptions (Now)? There
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10.12. Exceptions and the sys Modul
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10.14. Exercises 10-1. Raising Exce
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10-9. Improving math.sqrt(). The ma
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11.1. What Are Functions? Functions
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Stated Number of Objects to Return
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Keyword arguments may also be used
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Line-by-Line Explanation Lines 14 O
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11.3. Creating Functions 11.3.1. de
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modules foo and bar, but can use th
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The main motivation behind decorato
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func = deco1(deco_arg)(deco2(func))
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11.4. Passing Functions The concept
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11.5. Formal Arguments A Python fun
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mandatory, whereas default argument
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11.6. Variable-Length Arguments The
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formal arg1: 1220 formal arg2: 740.
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We will leave a timeit() function a
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11.7. Functional Programming Python
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a(0,9) 9 >>> >>> b = lambda *z: z >
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In Figure 11-1, we observe our orig
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consisting of all the return values
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sequences together. This idiom was
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print 'the total is:', reduce((lamb
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some of those arguments, such as ma
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11.8. Variable Scope The scope of a
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print is_this_global def 11.8.3. Nu
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count2 = counter(100) >>> print cou
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For example, let us assume that fun
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name, e.g., post_logged() will log
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variable. Beginning in 2.1, the ent
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11.9. *Recursion A function is recu
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Now that we have our generator func
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count.next() 5 >>> count.next() 6 >
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11-3. Functions. In this exercise,
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11-14. *Recursion. We also looked a
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12.1. What Are Modules? A module al
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definitely vary as you go from syst
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Notice that each of the namespaces
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You will see just how useful they a
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It is possible to import specific m
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12.5. Features of Module Import 12.
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# impter.py # ############# import
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getting them "registered" (or rathe
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foo() We are going to ask for the d
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Furthermore, we can go down one mor
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12.8. Other Features of Modules 12.
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# overly massive handlers for the c
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12.9. Related Modules The following
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12-6. Extended Import. Create a new
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13.1. Introduction Before we get in
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class MyNewObjectType: 'define MyNe
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(We will add print statements to ou
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Each subclass must define its own c
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13.2. Object-Oriented Programming T
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interfacing with components of the
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derive; and class_suite consists of
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100 >>> C.foo = C.foo + 1 >>> print
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As you can tell, dir() returns a li
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ascertain the location of a class s
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In contrast for those of you using
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id(c1), id(c2), id(c3) # all refer
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13.6. Instance Attributes Instances
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wasWkEnd = HotelRoomCalc(119, 0.045
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The __dict__ attribute consists of
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However, we can only update the val
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As we have seen above, it is perilo
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override a parent method where you
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tcm = TestClassMethod() >>> TestCla
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13.10. Subclassing and Derivation C
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13.11. Inheritance Inheritance desc
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Now let us create the child class C
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class C(P): def __init__(self): sup
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def keys(self): return sorted(self.
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gc = GC() >>> gc.foo() # GC C1 P1 c
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class B(object): pass class C(objec
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isinstance(c2, C2) True >>> isinsta
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The super() function was added in 2
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13.13. Customizing Classes with Spe
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C.__*rshift__(self, obj) Right shif
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The Numeric Types set of special me
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We still have a few problems ... on
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The output is very nice, exactly wh
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1 #!/usr/bin/env python 2 3 class T
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from randseq import RandSeq >>> for
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Addition We define the addition ope
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[9 :: 'foofoofoo'] >>> b + e [3 ::
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which we raise in such cases. Lines
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13.15. *Delegation 13.15.1. Wrappin
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wrappedComplex.imag # imaginary att
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Creation time, modification time, a
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Wrapping a Specific Object with Enh
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13.16. Advanced Features of New-Sty
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preventing users from adding instan
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A descriptor is a class attribute,
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hide such a descriptor, just as you
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43 FileDescr.saved.remove(self.name
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If we try it out, we see that it sa
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def fget(self): return ~self.__x de
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print '*** Created class %r at: %s'
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44 __metaclass__ = ReqStrSugRepr 45
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... print '%s(%d, %d) = %d' % \ ...
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13.18. Exercises 13-1. Programming.
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cash.update(100000.4567) >>> cash 1
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13-9. Queue Class. A queue is a dat
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13-17. Subclassing Numeric Types. T
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Chapter 14. Execution Environment C
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Internally, BIFs are represented as
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__dict__ object. Later on in this c
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>>> c.foo # bound method object >>
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14.2. Code Objects Callables are a
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14.3.2. compile() compile() is a fu
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not a string representation of an i
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The above was performed with raw_in
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-----------------------------------
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A sequence was chosen; enter the se
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them into real Python objects. Othe
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import import2 And here are the con
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$ python /usr/local/lib/python2x/CG
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wait() Wait for child process to co
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As you can see, popen() returns a f
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14.5.5. subprocess Module After Pyt
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14.6. Restricted Execution At one t
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Calling sys.exit() causes the Pytho
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14.8. Miscellaneous Operating Syste
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14.9. Related Modules In Table 14.9
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14-9. Shells. Create a shell (opera
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Chapter 15. Regular Expressions Cha
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Python supports REs through the sta
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15.2. Special Symbols and Character
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"\.". 15.2.3. Matching from the Beg
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We will now introduce the most comm
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\d+(\.\d*)? Strings representing si
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match(pattern, string, flags=0) Att
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m = re.match('foo', 'foo') # patter
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15.3.7. Matching Any Single Charact
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... >>> In the above code, we creat
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Subgroup searches result in a more
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saving the output to a whodata.txt
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15.4. Regular Expressions Example W
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domain. After the final double-colo
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patt = '\d+-\d+-\d+' >>> re.search(
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15.5. Exercises Regular Expressions
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15-19. Extract the complete timesta
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16.1. Introduction 16.1.1. What Is
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The teller is, of course, the serve
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16.2. Sockets: Communication Endpoi
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word "datagram." Because these sock
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Method Description Server Socket Me
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We do not implement this in our exa
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it can immediately make a connectio
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One way to create this "friendly" e
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cs = socket() # create client socke
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[a] Socket address families support
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16.4. *SocketServer Module SocketSe
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Lines 1115 18 print 'waiting for co
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$ And here is the server's: $ tsTse
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This is a timestamp TCP server usin
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calling the write() method of the t
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16.7. Exercises 16-1. Sockets. What
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3. Add logging capability to your n
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17.1. What Are Internet Clients? Be
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3. 4. Client logs in with username
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pwd() Current working directory cwd
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31 f.quit() 32 return 33 print '***
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after-hours, using Python is a grea
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17.3.3. Python and NNTP Based on yo
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article (id) Also similar to body()
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16 except socket.gaierror, e: 17 pr
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deltas = [ x[1]-x[0] for x in parti
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17.4. Electronic Mail Electronic ma
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4. Make service request(s) Quit As
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data: (250, 'ok ; id=20051226235837
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... p.quit() Before we take a look
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any errors. Example 17.3. SMTP and
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17.5. Related Modules One of Python
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17.6. Exercises FTP 17-1. Simple FT
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E-MAIL 17-16. Threaded Newsreader.
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Miscellaneous c. d. e. - Is the mes
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Chapter 18. Multithreaded Programmi
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18.2. Threads and Processes 18.2.1.
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Those of you interested in the sour
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18.3.5. Python Threading Modules Py
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The same loops from onethr.py are e
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25 thread.start_new_thread(loop, 26
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18.5. tHReading Module We will now
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isDaemon() Return daemon flag of th
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In this example we pass in a callab
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19 print 'start loop', nloop, 'at:'
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16 def sum(x): 17 sleep(0.1) 18 if
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We use the Queue module to provide
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for random numbers!) Seriously, tho
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18.7. Exercises 18-1. Processes ver
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Chapter 19. GUI Programming Chapter
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distribution for specific instructi
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some applications use root rather t
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either from the Tkinter topics page
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Example 19.2. Button Widget Demo (t
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in the Label widget. The greater th
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PFAs fits perfectly into a situatio
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Lines 3033 These lines represent ou
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71 def doLS(self, ev=None): 72 erro
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Line-by-Line Explanation Lines 15 T
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The last pieces of code in listdir.
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Things are different once we start
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Our second example uses the Python
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Lines 537 Here we instantiate a Fra
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our primary widgets. This is exactl
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Qt/KDE-Related Modules PyQt Python
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19-9. Multithreaded Chat Applicatio
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20.1. Introduction This introductor
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As you can see from the figure, the
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net_loc can be broken down into sev
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urlunparse(urltup) Unparses a tuple
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urlretrieve(urlstr, localfile=None,
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87 if eachLink not in self.q: 88 se
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urllib Functions Description urlope
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(). This code was inspired by Mike
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parse the newly downloaded page and
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20.4. CGI: Helping Web Servers Proc
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Finally, as you can probably guess,
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This HTML file presents a form to t
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Example 20.4. Results Screen CGI co
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Both friends.html and friends1.py a
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The other reason for creating actio
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12 ERROR 13 %s 14 16 ''' 17 18 def
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link with all the existing informat
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Now the user is able to make change
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This script outputs Unicode strings
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When more than one checkbox is subm
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Figure 20-16. Submitting our advanc
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Figure 20-17. Results page generate
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This script has one main class that
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113 MAXBYTES = 1024 114 langlist =
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all the information and sends the f
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20.8. Web (HTTP) Servers Until now,
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33 main() This server subclasses Ba
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[b] Interface to the non-validating
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20.10. Exercises 20-1. urllib Modul
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20-19. Electronic Commerce Engine.
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Chapter 21. Database Programming Ch
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When you query a database, you can
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Python world. As a software enginee
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This string (not float) indicates t
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DatabaseError Database error DataEr
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callproc( func[, args]) Call a stor
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● http://python.org/topics/databa
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cur.execute("INSERT INTO users VALU
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'', None, '{pgsql=C*T*/pgsql}'] ['t
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23 24 if db == 'sqlite': 25 try: 26
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140 db = setup() 141 print '*** Con
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whereas Gadfly does not, so rows ha
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21.3. Object-Relational Managers (O
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54 def insert(self): 55 d = [dict(z
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44 def create(self): 45 Users = sel
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the rows from the database and pret
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The code works something like this:
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We hope that we have provided you w
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adodbapi http://adodbapi.sf.net Syb
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Chapter 22. Extending Python Chapte
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● Added/extra (non-Python) functi
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The following code represents our l
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usr/include/python2.x, where the "2
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} res = PyArg_ParseTuple(args, "i",
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PyArg_ParseTupleAndKeywords() with
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$ python setup.py build running bui
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1 #include 2 #include 3 #include
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after its reference count has decre
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22.3. Related Topics SWIG There is
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22.4. Exercises 22-1. Extending Pyt
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23.1. Web Services There are many W
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The field names are given in the or
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23.2. Programming Microsoft Office
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This script launches Excel and writ
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23.2.3. Microsoft Word The next dem
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Applying PowerPoint in an applicati
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This script launches Outlook, creat
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Then when you are trying to send a
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Lines 113 We import all of the attr
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23.3. Python and Java Programming w
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Example 23.8. Swing "Hello World" i
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23.4. Exercises Web Services 23-1.
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23-15. Excel Cell Formatting. In th
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23-21. Python and Jython. Find a Py
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Chapter 22 Chapter 23
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or print i, s[i] for i, x in enumer
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Chapter 4 1. Python objects All Pyt
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When i % 2 == 0, it's even (divisib
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Chapter 7 1. Dictionary methods dic
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Chapter 8 3. range() built-in funct
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Chapter 10 1. Raising exceptions e)
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Chapter 12 2. Importing attributes
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Chapter 14 1. Callable objects Func
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9. Python floats [0-9]+(\.[0-9]*)?
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Chapter 17 20. Identifiers pass is
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Chapter 19 1. Client/server archite
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Chapter 21 1. Extending Python ●
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Chapter 23 3. Web services and the
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Python Keywords Table B.1 lists Pyt
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[a] Boolean comparisons return eith
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% Modulo/remainder • • • •
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isnumeric() • isspace() • istit
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String Format Operator Conversion S
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- Use left justification + Use a pl
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string.isdigit() [b] [c] Returns tr
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List Type Built-in Methods In Table
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[c] New in Python 2.3. [d] New in P
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s.copy() Copy operation: return (sh
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[d] Encoding that this file useswhe
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OverflowError Calculation exceeded
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[i] Warning about FutureWarning con
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[a] Get attribute; getattr() built-
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__radd__, or __iadd__. [f] New in P
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= • • • • • • • •
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Index [SYMBOL] [A] [B] [C] [D] [E]
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| (pipe symbol) 2nd 3rd |= (pipe eq
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arguments class methods command-lin
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instance attributes vs. methods mod
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compound statements concatenation (
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copy( ) creating 2nd dict( ) exact
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sys module try statement with multi
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float( ) built-in function 2nd floa
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eturn values standard type 2nd vari
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grouping 2nd groups( ) method GTK+
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installing Python instance attribut
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through files iterators about any n
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list( ) built-in function 2nd list(
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for re module group(s) invoking 2nd
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modulus operator MRO [See method re
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and OOP classic classes vs. descrip
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intersection update ( &= ) key-look
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PyQt PyQtGPL Pyrex Python Enhanceme
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tuple elements/tuples repetition li
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for statement used with keys for 2n
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about as original Internet protocol
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ules for suites static data static
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subtraction ( - ) operator suites 2
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definition of examples exiting glob
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categorizing standard function to c
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Universal NEWLINE Support (UNS) 2nd
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wxGlade wxPython 2nd 3rd wxWidgets
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Core Python Programming (2nd Edition)

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