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Python 2.7: Lists & Dictionaries

Hands-On | 04 Dec 2016


Lists are the same as arrays in other programming languages. List can be defined using assigment:

fruits = ["banana", "apple", "strawberry"]

and can be accessed by indices, similar to any other programming language:

# output: strawberry
print fruits[2]

# substitution of existing element
# will replace apple with kiwi in fruits list
fruits[1] = "kiwi"

defining an empty list:

vegetables = []

appending items to the existing list:


obtaining the length of the list, i.e. number of elements in the list:

print len(vegetables)

List slicing

You can extract certain chunks of the list, using ": " syntax. You have indicate from and to index, to represent the index range.

# output: cucumber, eggplant
print vegetables[0:2]

# output: apple, strawberry
print fruits[1:3]

animals = "catdogfrog"
cat = animals[:3]  # slicing elements until the third
dog = animals[3:6] # slicing based on range 
frog = animals[6:] # slicing elements starting from 7th

Searching through the list

You can search through the list using .index(item).

fruits = ['apple', 'kiwi', 'banana', 'peach']

# output: 1 - i.e. index of element kiwi
kiwi_index = fruits.index('kiwi')
print "Kiwi found at position %s" % kiwi_index

# insert watermelon in the kiwi position, pushing everything down
fruits.insert(kiwi_index, 'watermelon')

# output: apple, watermelon, kiwi, banana, peach
print fruits

Traversing through the list

You can traverse through the list using the for loop.

# output: 2,4,6,8,10
my_list = [1,2,3,4,5]
for number in my_list:
    print 2 * number

my_list = [5,6,3,2,1]
for i in range(0, len(my_list)):
    print my_list[i]

Remove specific item from the list:

furniture = ['sofa', 'chair', 'table', 'bed']

# output: sofa, table, bed
print furniture

Other list functions

# sorting array in ascending order 
# output: 1,2,3,5,8
my_list = [2,5,1,3,8] 
print my_list

# using .pop(index) - returns element from the list and removes it 
furniture = ['sofa', 'chair', 'table', 'bed']
item = furniture.pop(2)
# output: table
print item

# output: sofa, chair, bed
print furniture
# output: list of numbers from 2 to 8, excluding 8
# output: 2,3,4,5,6,7
list = range(2, 8)
print list

Joining lists

Lists can be joined using summation operator

n = [1,2,3]
m = [7,4,5]

def join_lists(x, y):
    return x + y

# output: 1,2,3,7,4,5
print join_lists(n, m)

Also list can be multiplied by a certain number and that it will join with itself several times

m = ["A"]

# output: ["A","A","A","A","A","A","A","A","A","A"]
print m * 10

List elements can be "imploded" to a string with a specified separator using .join(list) function.

my_list = ["I", "believe", "I", "can", "fly"]
joint_string = " ".join(my_list)

# output: "I believe I can fly"
print joint_string

Casting list elements to other data types

my_list_int = [1,2,3]

# will return the same array of int, but casted to str
my_list_str = map(str, my_list_int)

Check if number is a member of the list:

import random
my_list = []

guess_number = int(raw_input("Guess a number between 0 and 10: "))

if guess_number not in range(0,10):
    print "The number is even not between 0 and 10"
    if guess_number in my_list:
        print "Congratulations, you found the number!"
        print "Sorry, you missed it!"
    print " ".join(map(str, my_list))


Dictionaries are similar to associative arrays. They consist of key-value pairs.

score = {"James": 12, "Samantha": 43, "Andre": 81}

# output: 12
print score["James"]

In contrast to lists which are enclosed by [] brackes, dictionaries are enclosed by curly braces {}.

Elements can be appened to dictionaries using dictionary[key] assignment

score["Kate"] = 34

# output: 4
print len(score)

Removing items from the dictionary:

score["Rene"] = 29

# output: 5 
print len(score)

del score["Rene"]
# output: 4
print len(score)

Traversing through the dictionary

# output: Price of apple is 32.5
# output: Price of potato is 20
prices = {"potato": 20, "apple": 32.5}
for key in prices:
    print "Price of %s is %s" % (key, prices[key])