Iterators in Python 2 - Drexel University

Iterators in Python 2 Kurt Schmidt

Intro Comprehensions zip Creating

Iterators in Python 2

Kurt Schmidt

Dept. of Computer Science, Drexel University

September 21, 2021

Overview

Iterators in Python 2 Kurt Schmidt

Intro Comprehensions zip Creating

? An iterator iterates over an iterable object ? Examples of iterable objects:

? str list tuple dict file ? An iterator serves up items one at a time

? Objects can serve up an iterator (often itself) ? Invoked in the for loop

1 l = [1, 2, 3]

2 for i in l :

3

print i

str doesn't have an iter method; the iter() operator sees the getitem (idx) method, builds an iterator.

Built-in Iterators

Iterators in Python 2 Kurt Schmidt

Intro Comprehensions zip Creating

? list, tuple, dict, others, provide an iterator upon asking

? next(...) accesses values, moves the iterator along ? Raises the StopIteration exception when it's done

1 l = [1, 2, 3]

2 it = iter( l )

3 try:

4

while True :

5

print( next( it ))

6

except StopIteration , e :

7

print( "\nAnd , that's all\n" )

Avoiding the StopIteration

Iterators in Python 2 Kurt Schmidt

Intro Comprehensions zip Creating

? There is no hasNext(...) predicate

? Raises the StopIteration exception when it's done ? next( iter [, default] ) will take a default value to

return, rather than raising an exception ? Choose a generic object, used for its unique reference

1 l = [1, 2, 3]

2 END_ITER = object() # just a sentinel reference

3

4 v = next( l, END_ITER )

5 while v is not END_ITER :

6

print( v )

7

v = next( l, END_ITER )

8 print( "\nAnd , that's all\n" )

for-loop calls iter(...)

Iterators in Python 2

Kurt Schmidt

Intro

Comprehensions zip

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? for i in iterable expects iterable to be iterable ? Class iter( iterable ) ? Takes care of catching StopIteration

l = [1, 2, 3]

for i in l : print( i )

print( "\nAnd , that's all\n" )

1 2 3

And, that's all

List Comprehensions

Iterators in Python 2

Kurt Schmidt

Intro

Comprehensions zip

1 Creating

2 3 4 5 6

? Construct lists in a natural way

? Behaves as map and filter

L = [ x**2 for x in range( 1, 11 )] M = [ x for x in L if x%2==0 ] print L print M # print the even squares print [ "BUZZ" if i%7==0 else i

for i in range (15) ]

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100] [4, 16, 36, 64, 100] [1, 2, 3, 4, 5, 6, 'BUZZ', 8, 9, 10, 11,

12, 13, 'BUZZ', 15]

Nested List Comprehensions

Iterators in Python 2

Kurt Schmidt

Intro

Comprehensions

zip

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Creating

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? Let's make a multiplication table:

t = [ [ x*y for x in range(1, 6) ] for y in range(1, 6) ]

for r in t : print r

[1, 2, 3, 4, 5] [2, 4, 6, 8, 10] [3, 6, 9, 12, 15] [4, 8, 12, 16, 20] [5, 10, 15, 20, 25]

Zipped Comprehensions

Iterators in Python 2

Kurt Schmidt

Intro

Comprehensions zip

Creating

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? We can iterate over two or more lists simultaneously

first = [ 'Hedda', 'Bilbo', 'John' ] last = [ 'Gabler', 'Baggins', 'Watson' ] names = [ ' '.join( (f,l) )

for (f,l) in zip( first , last ) ] print names

['Hedda Gabler', 'Bilbo Baggins', 'John Watson']

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