1.4 lists

This lesson describes lists in python.

Important

This lesson is still under development.

A list can be defined as a collection of objects or a container in which we hold different objects.

mylist = []

Above we an empty list. How do we know that it is list? We can always check the type of an object in python as below.

print(type(mylist))

<class 'list'>

We can also make a list which is not empty as below

imperialists = ['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola']
print(type(imperialists))

<class 'list'>

In above mentioned list, all of its (6) elements are strings. However, the elements/members a list need not to be of same type.

imperialists = ["Bush", {"years": 8}, 2000, (2000, 2008)]
print(type(imperialists))

<class 'list'>

In above list, the first member is string, the second member is dictionary, the third member is integer and the fourth member is a tuple. We will study about string, dictionary, integer and tuple in upcoming lessons.

There are two ways to convert a python object into list

• using []

• usnig list function

a = 1,2
print(type(a))
a_as_list_using_slice_op = [a]
print(type(a_as_list_using_slice_op))

<class 'tuple'>
<class 'list'>

a_as_list_using_list_fn = list(a)
print(type(a_as_list_using_list_fn))

<class 'list'>

However there is a major difference in these two. [] converts the whole object as it is into a list. On the other hand, list function is more like element wise operator. This can be verified by printing the converted lists created above.

print(a_as_list_using_slice_op)
print(a_as_list_using_list_fn)

[(1, 2)]
[1, 2]

print(len(a_as_list_using_slice_op))
print(len(a_as_list_using_list_fn))

1
2

Then length of list using slice operator [] is always 1, while the length of list created using list function depends upon the number of values in the object.

Once we have a list, we can perform different operations on it. Some of them are given below.

append

imperialists = ['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola']
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola']

imperialists.append('clinton')
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton']

append changes the original list and it itself returns None.

new_imperialists = imperialists.append('netanyahu')
print(new_imperialists)

None

print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton', 'netanyahu']

If we add a similar but new element in list, then the list will have 2 such elements as its member.

imperialists.append('netanyahu')
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton', 'netanyahu', 'netanyahu']

pop

last_element = imperialists.pop(-1)
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton', 'netanyahu']

print(last_element)

netanyahu

# uncomment following 1 line
# imperialists.pop(8)

imperialists.pop()

'netanyahu'

print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton']

# uncomment following 1 line
# imperialists.pop('Bush')

extend

If we want to add multiple elements to a list, using append will put a new list in the previous list

imperialists = ['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton']
uk_imperialists = ['churchil', 'Tony Blair', 'BBC']
imperialists.append(uk_imperialists)
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton', ['churchil', 'Tony Blair', 'BBC']]

imperialists = ['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton']
imperialists.extend(uk_imperialists)
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton', 'churchil', 'Tony Blair', 'BBC']

extend actually takes any sequence as input. It must not be a list. It can be a string or tuple.

new_one = "Murdoch"
imperialists.extend(new_one)
print(imperialists)

['Bush', 'Obama', 'Trump', 'Zuckerberg', 'Bezos', 'coca cola', 'clinton', 'churchil', 'Tony Blair', 'BBC', 'M', 'u', 'r', 'd', 'o', 'c', 'h']

imperialists = ['Bush', 'Obama', 'Trump', 'clinton']
capitalists = ('Zuckerberg', 'Bezos', 'coca cola')
imperialists.extend(capitalists)
print(imperialists)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola']

using + operator

We can also append lists using the + operator.

media_houses = ['bbc', 'cnn', 'reuters', 'springer']
print(imperialists + media_houses)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola', 'bbc', 'cnn', 'reuters', 'springer']

print(imperialists)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola']

imperialists = imperialists + media_houses
print(imperialists)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola', 'bbc', 'cnn', 'reuters', 'springer']

morons = ['sam haris', 'richard dawkins', 'baghdadi', 'bin ladan']
imperialists += morons
print(imperialists)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola', 'bbc', 'cnn', 'reuters', 'springer', 'sam haris', 'richard dawkins', 'baghdadi', 'bin ladan']

remove

imperialists.remove('springer')
print(imperialists)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola', 'bbc', 'cnn', 'reuters', 'sam haris', 'richard dawkins', 'baghdadi', 'bin ladan']

If we repeat the above operation, it will result in error because springer has already been removed from the list imperialists.

# uncomment following 2 line
# imperialists.remove('springer')
# print(imperialists)

insert

puts an the value before the index

imperialists.insert(-1, 'DW')
print(imperialists)

['Bush', 'Obama', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola', 'bbc', 'cnn', 'reuters', 'sam haris', 'richard dawkins', 'baghdadi', 'DW', 'bin ladan']

Finding position/index of a member of a list

imperialists.index('bbc')

7

# uncomment following 1 line
# imperialists.index('bbc', 8)

# uncomment following 1 line
# imperialists.index('bbc', 3, 6)

if an element is present in a list twice, index of its first position is returned.

last_value = imperialists[-1]
imperialists.insert(2, last_value)
print(imperialists)

['Bush', 'Obama', 'bin ladan', 'Trump', 'clinton', 'Zuckerberg', 'Bezos', 'coca cola', 'bbc', 'cnn', 'reuters', 'sam haris', 'richard dawkins', 'baghdadi', 'DW', 'bin ladan']

imperialists.index(last_value)

2

reverse

imperialists = ['bbc', 'cnn', 'reuters', 'springer', 'voa']
imperialists.reverse()

The function does not return anything itself but the original list is reversed.

print(imperialists)

['voa', 'springer', 'reuters', 'cnn', 'bbc']

sort

print(imperialists)

['voa', 'springer', 'reuters', 'cnn', 'bbc']

imperialists.sort()

print(imperialists)

['bbc', 'cnn', 'reuters', 'springer', 'voa']

imperialists.sort(reverse=True)
print(imperialists)

['voa', 'springer', 'reuters', 'cnn', 'bbc']

numbers = [4,3,6,2]
numbers.sort()
print(numbers)

[2, 3, 4, 6]

# uncomment following 2 line
# imperialists = ['bbc', 1, 'cnn', 3, 'voa', 2]
# imperialists.sort()

Notes

I have been using the word function for append, sort etc, however in Object Oriented Programming, it can be seen that they are actually called methods.

There are a lot more powerful list manipulations which can be done by combining conditional and looping statements. We will come back to them once looping and conditioning statements are covered.