Core Python Programming (2nd Edition)

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generator, but serves as a short form, allowing for the simplistic syntax of a list comprehension. These simple examples should give you an idea of how generators work, but you may be asking, "Where can I use generators in my application?" Or perhaps, you may be asking, "What are the most appropriate places for using this powerful construct?" The "best" places to use generators are when you are iterating through a large dataset that is cumbersome to repeat or reiterate over, such as a large disk file, or a complex database query. For every row of data, you wish to perform non-elementary operations and processing, but you "do not want to lose your place" as you are cursoring or iterating over it. You want to grab a wad of data, yield it back to the caller for processing and possible insertion into a (nother) database for example, and then you want to do a next() to get the next wad of data, and so forth. The state is preserved across suspends and resumptions, so you are more comfortable that you have a safe environment in which to process your data. Without generators, you application code will likely have a very long function, with a very lengthy for loop inside of it. Of course, just because a language has a feature does not mean you have to use it. If there does not appear to be an obvious fit in your application, then do not add any more complexity! You will know when generators are the right thing to use when you come across an appropriate situation. 11.10.2. Enhanced Generator Features A few enhancements were made to generators in <strong>Python</strong> 2.5, so in addition to next() to get the next value generated, users can now send values back into generators [send()], they can raise exceptions in generators [tHRow()], and request that a generator quit [close()]. Due to the two-way action involved with code calling send() to send values to a generator (and the generator yielding values back out), the yield statement now must be an expression since you may be receiving an incoming object when resuming execution back in the generator. Below is a simple example demonstrating some of these features. Let us take our simple closure example, the counter: def counter(start_at=0): count = start_at while True: val = (yield count) if val is not None: count = val else: count += 1 This generator takes an initial value, and counts up by one for each call to continue the generator [next ()]. Users also have the option to reset this value if they so desire by calling send() with the new counter value instead of calling next(). This generator runs forever, so if you wish to terminate it, call the close() method. If we run this code interactively, we get the following output: >>> count = counter(5)
count.next() 5 >>> count.next() 6 >>> count.send(9) 9 >>> count.next() 10 >>> count.close() >>> count.next() Traceback (most recent call last): File "", line 1, in StopIteration You can read more about generators in PEPs 255 and 342, as well as in this Linux Journal article introducing readers to the new features in <strong>Python</strong> 2.2: http://www.linuxjournal.com/article/5597
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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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[id(x) for x in hubby] [9919616, 11
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6.21. Summary of Sequences Sequence
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str() • • • strip() • swapc
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d. Take a string and append a backw
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6-12. Strings. a. b. c. Create a fu
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6-19. Multi-Column Output. Given an
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7.1. Mapping Type: Dictionaries Dic
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dict2 = {'name': 'earth', 'port': 8
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dict2['arch'] = 'sunos5' # add new
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Beginning with Python 2.2, programm
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size, their keys match, and so do t
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dict(zip(('x', 'y'), (1, 2))) {'y':
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7.4. Mapping Type Built-in Methods
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dict2= {'host':'earth', 'port':80}
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7.5. Dictionary Keys Dictionary val
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39 try: 40 choice = raw_input(promp
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7.6. Set Types In mathematics, a se
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... print i ... c e h o p s How to
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superset of the second set (is a su
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s &= set('shop') >>> s set(['h', 's
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7.9. Set Type Built-in Methods 7.9.
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7.10. Operator, Function/Method Sum
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7.11. Related Modules The sets modu
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7-5. userpw2.py. The following prob
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characters in srcstr that are beyon
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Chapter 8. Conditionals and Loops C
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8.2. else Statement Like other lang
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8.3. elif (aka else-if) Statement e
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8.4. Conditional Expressions (aka "
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One must use caution when using whi
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Iterating by Sequence Item >>> name
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ange(start, end, step=1) range() wi
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are "sequence-related" is that half
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8.8. continue Statement Core Note:
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8.10. else Statement ... Take Two I
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8.11. Iterators and the iter() Func
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Dictionaries Dictionaries and files
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8.12. List Comprehensions List comp
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and split each line up into words,
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of building up this long list, it w
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lines)? That way, we can use the ma
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8.15. Exercises 8-1. Conditionals.
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8-9. Fibonacci Numbers. The Fibonac
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Chapter 9. Files and Input/Output C
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9.2. File Built-in Functions [open(
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you see references to open(), you c
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Note that there is no "writeline()"
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We originally described how this pr
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f.seek(-12, 1) # move back 12 bytes
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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
Page 456 and 457: 11.3. Creating Functions 11.3.1. de
Page 458 and 459: modules foo and bar, but can use th
Page 460 and 461: The main motivation behind decorato
Page 462 and 463: func = deco1(deco_arg)(deco2(func))
Page 464 and 465: 11.4. Passing Functions The concept
Page 466 and 467: 11.5. Formal Arguments A Python fun
Page 468 and 469: mandatory, whereas default argument
Page 471 and 472: 11.6. Variable-Length Arguments The
Page 473 and 474: formal arg1: 1220 formal arg2: 740.
Page 475 and 476: We will leave a timeit() function a
Page 477 and 478: 11.7. Functional Programming Python
Page 479 and 480: a(0,9) 9 >>> >>> b = lambda *z: z >
Page 481 and 482: In Figure 11-1, we observe our orig
Page 483 and 484: consisting of all the return values
Page 485 and 486: sequences together. This idiom was
Page 487 and 488: print 'the total is:', reduce((lamb
Page 489 and 490: some of those arguments, such as ma
Page 491 and 492: 11.8. Variable Scope The scope of a
Page 493 and 494: print is_this_global def 11.8.3. Nu
Page 495 and 496: count2 = counter(100) >>> print cou
Page 497 and 498: For example, let us assume that fun
Page 499 and 500: name, e.g., post_logged() will log
Page 501 and 502: variable. Beginning in 2.1, the ent
Page 503 and 504: 11.9. *Recursion A function is recu
Page 505: Now that we have our generator func
Page 509 and 510: 11-3. Functions. In this exercise,
Page 511 and 512: 11-14. *Recursion. We also looked a
Page 513 and 514: 12.1. What Are Modules? A module al
Page 515 and 516: definitely vary as you go from syst
Page 517 and 518: Notice that each of the namespaces
Page 519 and 520: You will see just how useful they a
Page 521 and 522: It is possible to import specific m
Page 523 and 524: 12.5. Features of Module Import 12.
Page 525 and 526: # impter.py # ############# import
Page 527 and 528: getting them "registered" (or rathe
Page 529 and 530: foo() We are going to ask for the d
Page 531 and 532: Furthermore, we can go down one mor
Page 534 and 535: 12.8. Other Features of Modules 12.
Page 536 and 537: # overly massive handlers for the c
Page 538 and 539: 12.9. Related Modules The following
Page 540 and 541: 12-6. Extended Import. Create a new
Page 542 and 543: 13.1. Introduction Before we get in
Page 544 and 545: class MyNewObjectType: 'define MyNe
Page 546 and 547: (We will add print statements to ou
Page 548 and 549: Each subclass must define its own c
Page 550 and 551: 13.2. Object-Oriented Programming T
Page 552 and 553: interfacing with components of the
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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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