ListBasics

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Making a list

foo = ["orange", "purple", "blue", "green", "red"]


Turning a string (text) into a list

Strings have a split function.

text = "Evil media studies deliberately courts the accusation of anachronism so as to both counter and to enhance the often tacit deception and trickery within the precincts of both theory and practice."
words = text.split()
print words

outputs:

['Evil', 'media', 'studies', 'deliberately', 'courts', 'the', 'accusation', 'of', 'anachronism', 'so', 'as', 'to', 'both', 'counter', 'and', 'to', 'enhance', 'the', 'often', 'tacit', 'deception', 'and', 'trickery', 'within', 'the', 'precincts', 'of', 'both', 'theory', 'and', 'practice.']


Turning a list into a string (text)

The quickest way is with the string function join. With join, you start with the text that you want to use to join up the different list elements:

"*".join(words)

output:

'all*work*and*no*play*makes*jack*a*dull*boy'


Here an example using the fact that Python can treat a string variable like a list of letters:

name = "Rachmaninov"
print "".join(reversed(name))
voninamhcaR


Reversing a list

Use the reverse function of a list

Every python list has a function called reverse that changes the list and reverses the order of things.

words = "all work and no play makes jack a dull boy".split()
words.reverse()
print words

outputs:

['boy', 'dull', 'a', 'jack', 'makes', 'play', 'no', 'and', 'work', 'all']


Use the reversed function.

The reversed function allows you to iterate over a list in reverse order, leaving the original list unchanged.

for word in reversed(words):
    print word,

# check that the original list is unchanged
print words

output:

boy dull a jack makes play no and work all
['all', 'work', 'and', 'no', 'play', 'makes', 'jack', 'a', 'dull', 'boy']

Use a negative "step" value with list slicing.

A very pythonic way to do this is with a negative "step" value in a list slice, this also leaves the original list unchanged:

words[::-1]

outputs

['boy', 'dull', 'a', 'jack', 'makes', 'play', 'no', 'and', 'work', 'all'


Doing something with every item in a list

Use a for loop:

Filtering a list

Use a for loop + an if statement.

Using list comprehension.

Putting two lists together

Use the "+" operator

The plus sign (+) will add two lists together. Neither list is changed.

words = "all work and no play makes jack a dull boy".split()
nums = range(10)
print words + nums

output:

['all', 'work', 'and', 'no', 'play', 'makes', 'jack', 'a', 'dull', 'boy', 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]


Using the list extend function

The extend function changes a list by adding a second list onto itself.

words.extend(nums)
print words

output:

['all', 'work', 'and', 'no', 'play', 'makes', 'jack', 'a', 'dull', 'boy', 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]


Making a (backup) copy of a list

This can be done with slicing as well (very Pythonic ;):

words = "all work and no play makes jack a dull boy".split()
backup = words[:]
words.reverse()
print words
print backup

output:

['boy', 'dull', 'a', 'jack', 'makes', 'play', 'no', 'and', 'work', 'all']
['all', 'work', 'and', 'no', 'play', 'makes', 'jack', 'a', 'dull', 'boy']