• Used to create lists using for and if clauses
• Introduced in Python 2.0
• This feature was introduced via PEP202

• Traditional imperative programming method, use a for loop

Create a list of the first ten powers of two:

powersof2 = []
for x in range(1, 11):
    powersof2.append(2 ** x)

>>> print powersof2
[2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]

• Functional programming approach
• Apply function to each item returned by iterable
• Syntax: map(function, iterable)

Powers of two example with map and anonymous lambda function:

powersof2 = map(lambda x:2 ** x, range(1,11))

>>> print powersof2
[2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]

• Syntactic sugar to easily generate lists
• Syntax for simple comprehension [ expression for variable in iterable ]

Powers of two example with list comprehension:

powersof2 = [ 2 ** x for x in range(1, 11) ]
>>> print powersof2
[2, 4, 8, 16, 32, 64, 128, 256, 512, 1024]

• for loop method

sentence = "She sells seashells by the seashore"
wordlist = []
for word in sentence.split():
    if word.lower().startswith("s"):
        wordlist.append(word)

print wordlist
['She', 'sells', 'seashells', 'seashore']

• Map has a companion, filter()

wordlist = filter(lambda word:word.lower().startswith("s"), sentence.split())
>>> print wordlist
['She', 'sells', 'seashells', 'seashore']

• Add a an if clause to the end of a comprehension to filter results
• Syntax for simple comprehension [ expression for variable in iterable if condition ]

wordlist = [ word for word in sentence.split() if word.lower().startswith("s") ]
>>> print wordlist
['She', 'sells', 'seashells', 'seashore']

• List comprehensions can be nested

>>> [ letter * number for number in [1,2,3] for letter in ["a","b","c"]  ]
['a', 'b', 'c', 'aa', 'bb', 'cc', 'aaa', 'bbb', 'ccc']

• The order of the for clauses is significant
• The above is equivalent to:

thelist = []
for number in [1,2,3]:
    for letter in ["a","b","c"]:
        thelist.append(letter * number)

• Convert a list of Celsius temparatures to Fahrenheit

Example from Python Course

Celsius = [39.2, 36.5, 37.3, 37.8]
Fahrenheit = [ "%.2f" % ((float(9)/5)*x + 32) for x in Celsius ]
print Fahrenheit
['102.56', '97.70', '99.14', '100.04']

• List of all drive letters in Windows

driveletters = [ "%s:" % letter for letter in string.ascii_uppercase ]

driveletters[:len(driveletters)/2]
['A:', 'B:', 'C:', 'D:', 'E:', 'F:', 'G:', 'H:', 'I:', 'J:', 'K:', 'L:', 'M:']
driveletters[len(driveletters)/2:]
['N:', 'O:', 'P:', 'Q:', 'R:', 'S:', 'T:', 'U:', 'V:', 'W:', 'X:', 'Y:', 'Z:']

• Unique IP addresses from an apache web server log file:

from pprint import pprint

uniqips = set( [ line.split()[0] for line in open("access.log") ] )

pprint(list(uniqips)[:5])
['180.76.5.65',
 '74.125.19.39',
 '220.181.51.109',
 '123.125.71.75',
 '178.255.215.65']

• For further reading

Generator expressions

(introduced in Python 2.4)

http://www.python.org/dev/peps/pep-0289/

Dict comprehensions

(introduced in Python 2.7)

http://www.python.org/dev/peps/pep-0274/

Set comprehensions

(introduced in Python 2.7)

http://docs.python.org/release/3.1.5/tutorial/datastructures.html#sets

• Presenter:	Shawn K. O'Shea

• Presented to:	GNHLUG's PySIG

• Presented on:	July 26, 2012

• Latest version:	You can find the latest version of this presentation on my website: http://eth0.net/