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Appendix O: Additional Python 2.2
Features
Outline
O.1
O.2
O.3
O.4
O.5
Introduction
Iterators
Generators
Nested Scopes
Internet and World Wide Web Resources
 2002 Prentice Hall. All rights reserved.
O.1 Introduction
• Overview of new features in Python 2.2 (e.g.,
iterators, generators and nested scopes) that allow
programmers to express algorithms more clearly
and simply
 2002 Prentice Hall. All rights reserved.
O.2 Iterators
• Special objects that define operations for
progressing through sequences
• Useful for many kinds of sequences, including
list-like objects (such as built-in types list and
tuple), program-defined container objects (e.g.,
trees) or unbounded sequences whose length is
unknown (e.g., input and output streams)
• __getitem__ intended for random attribute
access (e.g., index access for lists or dictionary
key lookups), rather than iteration
• Can be more efficient for processing a sequence
 2002 Prentice Hall. All rights reserved.
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# Fig. O.1: NewRange.py
Class simulates range of
# __getitem__ used in a for loop.
Outline
integer values
class MyRange:
"""Simple class
to simulate
a range
values"""
Constructor
receives
valuesof
forinteger
parameters
start,
stop andNewRange.py
step
Constructor
calls range to generate sequence
def __init__( self, start, stop,
step ):
"""Class MyRange constructor; takes start, stop and step"""
of integers
Provides subscript access (using operator [])
self.__sequence = range( start, stop, step )
def __getitem__( self, subscript
):
Python
raises
Returns
element
"""Overridden sequence element access"""
IndexError
if subscript
stored at specified
subscriptlies out of bounds
return self.__sequence[ subscript ]
 2002 Prentice Hall.
All rights reserved.
Python 2.2b2 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> from NewRange import MyRange
>>> myRange = MyRange( 0, 10, 1 )
>>>
>>> print myRange[ 0 ]
0
>>> print myRange[ 8 ]
8
>>>
>>> for value in myRange:
...
print value,
...
0 1 2 3 4 5 6 7 8 9
Outline
Fig. O.1
__getitem__
method to emulate
iteration
 2002 Prentice Hall.
All rights reserved.
Python 2.2b2 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> from NewRange import MyRange
>>> myRange = MyRange( 0, 10, 1 )
>>>
>>> currentIndex = 0
>>>
>>> while 1:
...
try:
...
value = myRange[ currentIndex ]
...
except IndexError:
...
break
...
else:
...
print value,
...
currentIndex += 1
...
0 1 2 3 4 5 6 7 8 9
Outline
Fig. O.2 __getitem__
and for loops
 2002 Prentice Hall.
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# Fig. O.3: NewRangeIterator.py
# Iterator class that defines a sequence.
class RangeIterator:
"""Simple class to simulate a range"""
Outline
NewRangeIterator
.py
def __init__( self, start, stop, step ):
"""RangeIterator constructor; takes start, stop and step"""
self.__sequence = range( start, stop, step )
self.__nextValue = 0 # subscript of next value to produce
def __iter__( self ):
"""Returns iterator for object of class RangeIterator"""
return self
def next( self ):
"""Iterator method to produce next value in sequence"""
try:
value = self.__sequence[ self.__nextValue ]
except IndexError:
raise StopIteration
else:
self.__nextValue += 1
return value
def main():
Create object range1 that contains values 0-9
# create object of class RangeIterator, use for loop to iterate
range1 = RangeIterator( 0, 10, 1 )
print "Iterate over the values in range1 using a for loop:"
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for value in range1:
print value,
Outline
Uses for loop to iterate over object’s elements
print
NewRangeIterator
Retrieve
iterator forcall
object
by calling
method iter
.py
class
RangeIterator,
next
to iterate
# create object of
range2 = RangeIterator( 0, 10, 1 )
range2Iterator = iter( range2 ) # retrieve iterator for range2
print "\nCall method next for range2Iterator:"
while 1:
next for an object’s iterator progresses through values in an object’s sequence
next raises StopIteration when there are no more values in the sequence
try:
value = range2Iterator.next()
except StopIteration:
break
else:
print value,
print
# create one object of class RangeIterator two iterators
# for that object
Objects can have more than one iterator
range3 = RangeIterator( 0, 10, 1 )
range3Iterator1 = iter( range3 )
range3Iterator2 = iter( range3 )
print "\nCall next for two iterators of the same object:"
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for i in range( Using
10 ): more than one iterator on the same object can
print "Loop iteration %d: range3Iterator1.next() = %d" % \
( i, range3Iterator1.next() )
print "Loop iteration %d: range3Iterator2.next() = %d" % \
( i, range3Iterator2.next() )
print
lead to subtleOutline
logic errors
NewRangeIterator
.py
if __name__ == "__main__":
main()
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Iterate over the values in range1 using a for loop:
0 1 2 3 4 5 6 7 8 9
Outline
Call method next for range2Iterator:
0 1 2 3 4 5 6 7 8 9
Call next for two iterators of the same object:
Loop iteration 0: range3Iterator1.next() = 0
Loop iteration 0: range3Iterator2.next() = 1
Loop iteration 1: range3Iterator1.next() = 2
Loop iteration 1: range3Iterator2.next() = 3
Loop iteration 2: range3Iterator1.next() = 4
Loop iteration 2: range3Iterator2.next() = 5
Loop iteration 3: range3Iterator1.next() = 6
Loop iteration 3: range3Iterator2.next() = 7
Loop iteration 4: range3Iterator1.next() = 8
Loop iteration 4: range3Iterator2.next() = 9
Traceback (most recent call last):
File "newrange3.py", line 74, in ?
main()
File "newrange3.py", line 67, in main
print "Loop iteration %d: range3Iterator1.next() = %d" % \
File "newrange3.py", line 24, in next
raise StopIteration
StopIteration
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# Fig. O.4: GuessingGame.py
# Class and program to simulate computer guessing game.
import random
Outline
GuessingGame.py
class ComputerGuessingGame:
"""Class to guess a number randomly"""
def __init__( self, value, lowerBound = 0, upperBound = 10 ):
"""ComputerGuesser constructor; takes secret number, lower
and upper bounds"""
self.realValue =Keeps
valuevalue
within upper and lower bounds
# keep value within upper and lower bound
if value < lowerBound:
self.lower = value
else:
self.lower = lowerBound
if value > upperBound:
self.upper = value + 1
else:
self.upper = upperBound
def __iter__( self ):
"""Return iterator for object of class ComputerGuesser"""
Produces
return self
a series of random guesses
def next( self ):
"""Guesses a new value. If correct, raises StopIteration;
otherwise returns guess"""
guess = random.randrange( self.lower, self.upper )
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If correct, raises StopIteration
Outline
if guess == self.realValue:
raise StopIteration
else:
return guess
GuessingGame.py
def main():
# retrieve an integer from the user
while 1:
try:
secretNumber = int(
raw_input( "Enter number for computer to guess: " ) )
except ValueError:
print "Please enter an integer."
else:
break
print
computerGuesser = ComputerGuessingGame( secretNumber )
numberOfWrongGuesses
= iterator
0
Implicitly creates
for ComputerGuessingGame
object computerGuesser
# print the incorrect guesses
for wrongGuess in computerGuesser:
numberOfWrongGuesses += 1
print "Computer guessed: %d" % wrongGuess
print "\nGot secret number after %d wrong guesses." % \
numberOfWrongGuesses
if __name__ == "__main__":
main()
 2002 Prentice Hall.
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Enter number for computer to guess: 6
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
Outline
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1
1
3
9
9
1
8
Got secret number after 9 wrong guesses
Enter number for computer to guess: 100
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
Computer
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
guessed:
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92
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6
7
Got secret number after 13 wrong guesses.
 2002 Prentice Hall.
All rights reserved.
O.3 Generators
• Functions that return iterators
• Resumable functions that are especially well
suited for computing the values of a recursive
sequence
• Special kind of functions that can suspend
computation, “remember” their local variables and
later resume where the previous computation left
off
 2002 Prentice Hall. All rights reserved.
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# Fig. O.5: figO_05.py
# Inefficient, recursive fibonacci calls.
def fibonacci( n ):
Outline
figO_05.py
if n < 0:
raise ValueError, \
"Cannot computer fibonacci on negative number"
elif n == 0 or n == 1:
return n
else:
return fibonacci( n - 1 ) + fibonacci( n - 2 )
n = 0
value = 0
Compute smallest fibonacci( n ) > 1000
# compute smallest fibonacci( n ) > 1000
while value < 1000:
value = fibonacci( n )
n += 1
print "The smallest fibonacci number greater than 1000 is:", value
The smallest fibonacci number greater than 1000 is: 1597
 2002 Prentice Hall.
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Outline
# Fig. O.6: figO_06.py
# fibonacci generator.
Import generators
Define fibonacci generator
from __future__ import generators
figO_06.py
Refers
def fibonacci():
to next value in the sequence
Return to value after the value indicated by nextItem
nextItem = 0 # next value in the sequence
beyondItem = 1 # value *after* the next value in the sequence
Keyword yield suspends execution and returns intermediate value
while 1:
yield nextItem
# return fibonacci( n )
# function resumes here when program calls next on
# fibonacci's
Functioniterator
resumes here when program calls next on
fibonacci’s iterator
# compute the next fibonacci( n ) and fibonacci( n + 1 )
nextItem, beyondItem
= beyondItem, nextItem + beyondItem
Create iterator
fibIterator = fibonacci() # create iterator for Fibonacci sequence
result = 0 Find smallest Fibonacci number greater than 1000
# find smallest Fibonacci number greater than 1000
while result < 1000:
result = fibIterator.next()
print "The smallest fibonacci number greater than 1000 is:", result
The smallest fibonacci number greater than 1000 is: 1597
 2002 Prentice Hall.
All rights reserved.
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# Fig O.7: figO_07.py
# fibonacci generator for nth Fibonacci sequence element.
Outline
Generator raises a StopIteration exception when it returns
from __future__ import generators
figO_07.py
def fibonacci( n ):
nextItem = 0 # next value in the sequence
beyondItem = 1 # value *after* the next value in the sequence
currentN = 0 # n for which the generator is producing a value
while currentN <= n:
yield nextItem # return fibonacci( n )
# compute the next fibonacci( n ) and fibonacci( n + 1 )
nextItem, beyondItem = beyondItem, nextItem + beyondItem
currentN += 1
while 1:
# retrieve number from user
try:
fibNumber = int( raw_input( "Enter a number: " ) )
except ValueError:
print "Please enter an integer."
else:
break
print
counter = 0
Python repeatedly
callsimplicitly
next to get
valuefirst
fortime
result
Iterator
created
fibonacci function is invoke
# print fibonacci( n ) for all n <= fibNumber
for result in fibonacci( fibNumber ):
print "fibonacci( %d ) = %d" % ( counter, result )
counter += 1
 2002 Prentice Hall.
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Enter a number: 20
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
fibonacci(
Outline
0 ) = 0
1 ) = 1
2 ) = 1
3 ) = 2
4 ) = 3
5 ) = 5
6 ) = 8
7 ) = 13
8 ) = 21
9 ) = 34
10 ) = 55
11 ) = 89
12 ) = 144
13 ) = 233
14 ) = 377
15 ) = 610
16 ) = 987
17 ) = 1597
18 ) = 2584
19 ) = 4181
20 ) = 6765
 2002 Prentice Hall.
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O.4 Nested Scopes
• Address how identifier lookup takes place in
nested functions
• Most useful for programs that employ a
functional-style of programming
• Nested scoping enables an inner function to access
variables that appear in the function’s outer scope
 2002 Prentice Hall. All rights reserved.
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Outline
# Fig. O.8: figO_08.py
# Nested functions.
def outer():Function
inner prints message to indicate that function is executing
figO_08.py
def inner(): Calls built-in function dir to print list of
print "\nFunction inner executing"
print "The objects in inner's scope are:", dir()
print "Function inner finishing"
print "Function outer executing"
print "The objects in outer's scope are:", dir()
inner()
print "\nFunction
outer
finishing"
Emphasize fact
that
inner can be accessed only
objects in the inner’s scope
in function outer’s local scope
print "The objects in the global scope are:"
print dir()
print "\nCalling function outer\n"
outer()
print "\nFunction outer finished"
 2002 Prentice Hall.
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The objects in the global scope are:
['__builtins__', '__doc__', '__name__', 'outer']
Outline
Calling function outer
Function outer executing
The objects in outer's scope are: ['inner']
Function inner executing
The objects in inner's scope are: []
Function inner finishing
Function outer finishing
Function outer finished
 2002 Prentice Hall.
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# Fig O.9: CenterLength.py
# Functional definition of string justification.
Function
contained
def centerLength(
length
):
Outline
within function centerLength
Inner function calls object’s center method CenterLength.py
def centerString( stringValue ):
Return centered string
return stringValue.center(
length )
return centerString
Python 2.2b2 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> import CenterLength
>>> centerString10 = CenterLength.centerLength( 10 )
>>>
>>> type( centerString10 )
<type 'function'>
>>>
>>> centerString10( "python" )
' python '
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Python 2.1.1 (#20, Jul 20 2001, 01:19:29) [MSC 32 bit (Intel)] on win32
Type "copyright", "credits" or "license" for more information.
>>>
>>> import CenterLength
CenterLength.py:4: SyntaxWarning: local name 'length' in 'centerLength'
shadows
use of 'length' as global in nested scope 'centerString'
def centerLength( length ):
>>>
>>> centerString10 = CenterLength.centerLength( 10 )
>>> centerString10( "python" )
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "CenterLength.py", line 7, in centerString
return stringValue.center( length )
NameError: global name 'length' is not defined
Outline
Fig. O.10 Nested
scopes and older
versions of
Python.
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