Python lists

Module3
LISTS
Prof. Krishnananda L
Department of ECE
Govt SKSJTI
Bengaluru

Contents:
Introduction
Features
The list constructor ( ):
List slicing
Replace list items
Check if item exists in a list
Loop list
List comprehension
Built-in function
List methods
List operation
Unpacking list.

PYTHON DATA TYPES
Python supports 3 sequence data types:
1. LISTS
2. TUPLES
3. STRINGS
Other two important data structures in Python are:
1. SET
2. DICTIONARY

List is a collection which is ordered , mutable and indexed
Lists are created by square brackets and the elements in the list are separated
by commas.
Lists are used to store multiple items in a single entity.
Lists allow duplicates
LIST:
# exampleof lists
>>>games=['cricket','badminton','volleyball']
>>>print (games)
['cricket', 'badminton','volleyball']
>>>List=[1,2,,3,4,]
>>>print(List)
[1, 2, 3, 4]

List items are ordered means, items can be accessed using positional index. In general, new items
will be placed at the end of the list.
#Lists allow duplicate values:
>>>games=['cricket','badminton','volleyball','cricket','volleyball']
>>>print(games)
#output
['cricket','badminton', 'volleyball', 'cricket', 'volleyball']
>>> len(games)
5
Lists are mutablemeans we can change, add and remove items in a list after it has been created.
Since the list is indexed , lists can have items with same value.
Ordered:
Mutable:
Allows duplicate:

1. List can contain elements of different data types i.e., string , int, Boolean, tuple etc. Lists are
Mutable
Features:
>>>list=['cricket','abc',42,29, 0, True]
>>>print(list)
#output
['cricket','abc', 42,29, 0, True]
>>>list=['cricket','abc',42,29.0,True]
>>>print(type(list))
#output
<class 'list'>
>>> list =[23, 45, 'krishna', (34,56)]
>>> print (list)
[23, 45, 'krishna', (34, 56)]
2. Lists are defined as objects with the data type ‘list’.
>>> list[1]='hello‘ ## mutability
>>> print (list) ## same memory reference
[23, 'hello', 'krishna', (34, 56)]
>>> list[3]=(89,'welcome')#mutability
>>> print (list)
[23, 'hello', 'krishna', (89, 'welcome')]
>>>len(list)
>>> list =[23, 45, 'krishna', (34,56)]
>>> print (list[2])# accessing list element
krishna

The list constructor ( ) :
It is also possible to use the
list ( ) constructor when
creating a new list.
x=list(('cricket','abc',42,29.0,True))
"""note
the double round-brackets"""
print(x)
#output:
['cricket','abc', 42,29.0, True]
Simple Operations on Lists
>> list1=[23, 55,77]
>>> list2=[23, 55, 77]
>>> list1==list2
True
>>> list1 is list2
False
>>> list2=list1
>>> list2 is list1
True
>>> d1=['a','b','c']
>>> d2=['a','b','d']
>>> d1<d2
True
>>> min(d1)
'a'
>>> max(d2)
'd‘
>>> sum(list1)
>>> list1=[20, 'hi', 40]
>>> list2=[20, 40, 'hi']
>>> list1==list2
False
>>> list1>list2
Traceback (most recent call last):
File "<pyshell#16>", line 1, in
<module>
list1>list2
TypeError: '>' not supported
between instances of 'str' and 'int'
>>> ["abc"]>["bac"]
False
>>> [45, 67, 89] + ['hi', 'hello']
[45, 67, 89, 'hi', 'hello']

Assignment and References
>>>x = 3
• First, an integer 3 is created and stored in memory
• A name x is created
• A reference to the memory location storing 3 is then
assigned to the name x
• So: When we say that the value of x is 3, we mean
that x now refers to the integer 3
Ex with immutable data like integers:
>>> x=10 # Creates 10, name x refers to 10
>>> y=x # Creates name y, refers to 10.
>>> id(x)
1400633408
>>> id(y) # x and y refer to same location
1400633408
>>> print(y)
10
>>> y=20 ## creates reference for 20
>>> id(y) ### changes y
1400633568 ## y points to a new mem location
>>> print (y)
20
>>> print(x) # no effect on x. Refers to 10
10
Note: Binding a variable in Python means
setting a name to hold a reference to some
object.
• Assignment creates references, not copies
 x = y does not make a copy of the object y
references
 x = y makes x reference the object y
references

List Assignment
>>> d=[1,2,3,4] ## d references the list [1,2,3,4]
>>> id (d)
56132040
>>> b=d ## b now references what d references
>>> id(b)
56132040
>>> b[3]=10 ## changes the list which b references
>>> print (b)
[1, 2, 3, 10]
>>> print (d) ### change reflected in original also
[1, 2, 3, 10]
>>>a=[23, 45, 'hi']
>>> b=a # two names refer to same memory
>>> id(a)
24931464
>>> id(b)
24931464 ## only one list. Two names refer to it
>>> a.append('welcome') ## change list element
>>> id(a)
24931464 # refer to same object
>>> print (a) [23, 45, 'hi', 'welcome']
>>> print (b)
## Change in one name affects the other
[23, 45, 'hi', 'welcome']
Lists are “mutable.”
When we change the values of list elements,
we do it in place.
We don’t copy them into a new memory
address each time.
If we type y=x and then modify y, both x
and y are changed
Note: b=a[:] creates copy of list a. Now we have
two independent copies and two references

List Slicing:
x=list(('cricket','abc',42,29.0,True))
print(x[-1])
print(x[2])
print(x[0])
#Output:
True
42
cricket
Positive indexing 0 1 2 3 4
List Cricket abc 42 29.0 True
Negative indexing -5 -4 -3 -2 -1
Note: Slicingis helpful to get a subset of the
original list

 Range of positive indexes:
We can specify a range of indexes by specifying where to start and where to end
the range.
# RETURN THE THIRD ,FOURTH AND FIFTH TERM:
>>>x=['cricket','badminton','volleyball','football','hockey','cheese']
print(x[2:5])
#output:
['volleyball', 'football', 'hockey']
2 3 4 Index number
NOTE: 1) The search will start at Index 2 ( included) and end at index 5 (not included).
2)Remember that the first item has index 0.
3)The index ranges from 0 to (n-1).

By leaving out the start value, by default the range will start
at the first item.
x=['cricket', 'badminton', 'volleyball', 'football','hockey','shuttle']
print(x[:4])
#output:
[‘cricket’,’badminton’,’volleyball', 'football']
By leaving out the end value, by default the range will go
on to the end of the list.
print(x[2:])
#output:
['volleyball', 'football', 'hockey', 'shuttle']
Herehockeyhasa
index value 4 and
isexcluded
becausetherange
isfrom0:(n-1)
0:(4-1)
i.e.,0:3
Herevolleyball hasa
index value 2 andis
included becausethe
range isfrom 2: last
itemin thelist

If you want to search the item from the end of the list , negative indexes can be
used.
>>>x=['cricket', 'badminton', 'volleyball',
'football','hockey', 'shuttle']
print(x[-3:-1])
#output:
['football', 'hockey']
-6 -5 -4
-3 -2 -1
Here the range is from
-3(included) : -1(excluded).
Shuttle is excluded because of
the range of index(-1).
 Range of negative indexes:

To replace the value of a specific item ,refer to the index number.
 Replace/Modify items in a List
#CHANGE THE SECOND ITEM IN A LIST:
x[2]='PUBG‘
print(x)
#output:
['cricket', 'badminton', 'PUBG', 'football', 'hockey', 'shuttle']
Here volleyballis
replacedbyPUBG
• Replace a range of elements
To replace the value of items within a specific range, define a list with the new
values, and refer to the range of index numbers where you want to insert the new
values.

x[4:]='PUBG','FREEFIRE’
print(x)
#Output:
['cricket', 'badminton', 'volleyball', 'football', 'PUBG', 'FREEFIRE']
We can also replace this line
by
x[4:6]=[‘PUBG,’FREEFIRE’]
If you insert more items than you replace , the new items will be inserted where you
specified, and the remaining itemsmove accordingly.
>>>x=['cricket', 'badminton', 'volleyball', 'football','hockey','shuttle']
>>>len(x)
6
>>>x[2:3]='PUBG','FREEFIRE’
>>>print (x)
>>>len(x)
7
['cricket', 'badminton', 'PUBG', 'FREEFIRE', 'football', 'hockey', 'shuttle']

If you insert less items than you replace , the new items will be inserted where you specified,
and theremaining items move accordingly.
>>>x=['cricket', 'badminton', 'volleyball', 'football','hockey','shuttle']
>>>x[2:4]=['PUBG']
print(x)
#Output:
['cricket', 'badminton', 'PUBG', 'hockey', 'shuttle']
Note: If the range of index which must be replaced is not equal to the item to be
replaced, the extra mentioned range of item is removed from the list … i.e., in the
range x[2:4] we can replace 2 items but we replaced only 1 item PUBG, hence
football is removedin the list.
Here square brackets are
must and necessary. If not
used the single item PUBG is
BY DEFAULT taken as 4
individual item as ‘P’,’U’
,’B’,’G’.
>>> list=[] #emptylist
>>> len (list)
0
>>> type(list)
>>> list=[34 ] # list with single element
>>> len (list)
1
>>> type(list)
<class 'list'>

x[2:4]='PUBG'
print(x)
#Output:
['cricket', 'badminton', 'P','U', 'B', 'G', 'hockey', 'shuttle']
Here squarebrackets are
notusedhence the single
item PUBG is BY
DEFAULTtakenas 4
individualitemas‘P’,’U’
,’B’,’G’.
NOTE : The length of the list will change when the number of items
inserteddoes not match thenumber of itemsreplaced.
 Check if item exists in a list:
To determine if a specified item is present in a list use the in keyword.
if 'volleyball' in x:
print(“yes, ‘volleyball’ is in the list x”)
#output:
yes ‘volleyball’ is in the list x

List Comparison, use of is, in operator
>>> l1=[1,2,3,4]
>>> l2=[1,2,3,4]
>>> l3=[1,3,2,4]
>>> l1==l2
True
>>> l1==l3
False
>>> id(l1)
1788774040000
>>> id(l2)
1788777176704
>>> id(l3)
1788777181312
>>> l1 is l2
False
>>>> l1=['a','b','c','d']
>>> l2=['a','b','c','d']
>>> l1 is l2
False
>>> l1==l2
True
>>> if 'a' in l1:
print (" 'a' is present")
'a' is present
>>> if 'u' not in l1:
print (" 'u' is not present")
'u' is not present

Loop Through a List
You can loop through the list items by using
a for loop:
Loop list:
fruits=['apple','cherry','grapes']
for x in fruits:
print(x)
#Output:
apple
cherry
grapes
 Loop Through the Index Numbers
• You can also loop through the list
items by referring to their index
number.
• Use the range() and len() functions
to create a suitable iterable.
fruits= ["apple", "banana", "cherry"]
for i in range(len(fruits)):
print(fruits[i])
#Output:
apple
banana
cherry

 Using a While Loop:
• You can loop through the list items by using a while loop.
• Use the len() function to determine the length of the list, then start at 0 and
loop your way through the list items by referring to their indexes.
• Update the index by 1 after each iteration.
thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
print(thislist[i])
i = i + 1
#Output:
apple
banana
cherry
thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist]
#Output:
apple
banana
cherry
 Looping Using List Comprehension:
List Comprehension offers the shortest syntax
for looping through lists:

List Comprehension:
List comprehension offers a shorter syntax when you want to create a new list
based on the values of an existing list.
Example:
• Based on a list of fruits, you want a new list, containing only the fruits with the
letter "a" in the name.
['apple', 'banana', 'mango']
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [ ]
for x in fruits:
if "a" in x:
newlist.append(x)
print(newlist)
#Output:
• Without list comprehension you will have to write a for statement with a
conditional test inside:
This line specifies
that the newlist must
appendonlythe
fruits whichcontains
the letter‘a’ in them.

• With list comprehension one line of code is enough to create a new list:
newlist = [x for x in fruitsif "a" in x]
print(newlist)
#Output:
['apple', 'banana', 'mango']
• The Syntax:
newlist = [expression for item in iterable if condition == True]
• Condition:
The condition is like a filter that only accepts the items that evaluates to True.
newlist = [x for x in fruits if x != "apple"]
print(newlist)
#Output:
['banana', 'cherry', 'kiwi', 'mango']
The
condition if x!= "apple" will
return True for all elements
other than "apple", making the
new list contain all fruits
except "apple".
Note: The iterable can be any iterable object, like a list, tuple, set etc.
Note: The expression is the
current item in the iteration, but
it is also the outcome, which you
can manipulate before it ends up
like a list item in the new list

Some examples of List Comprehension
>>> newlist = [x for x in range(10)]
>>> print (newlist)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:
>>> newlist = [x for x in range(10) if x < 5]
>>> print (newlist)
[0, 1, 2, 3, 4]
>>> list1=['cricket','badminton', 'volleyball', 'football','hockey']
>>> newlist = [x.upper() for x in list1]
>>> print (newlist)
['CRICKET', 'BADMINTON', 'VOLLEYBALL', 'FOOTBALL', 'HOCKEY']
>>> newlist1 = ['welcome' for x in list1]
>>> print (newlist1)
['welcome', 'welcome', 'welcome', 'welcome', 'welcome']

Method Description
append() Adds an element at the end of the list
clear() Removes all the elements from the list
copy() Returns a copy of the list (creates a new list with same elements)
count() Returns the number of occurrences of the specified element
extend() Add the elements of a list (or any iterable), to the end of the current
list
index() Returns the index of the first occurrence of the specified element
insert() Adds an element at the specified position (without replacement)
pop() Removes the element at the specified position
remove() Removes the first occurrence of the specified element
reverse() Reverses the order of the list
sort() Sorts the list
Python has a set of built-in methods that you can use on lists.
List Methods:

To add an item to the end of the list, use the append() method:
x=['cricket', 'badminton', 'volleyball', 'football']
x.append('PUBG')
print(x)
#Output:
['cricket', 'badminton', 'volleyball', 'football', 'PUBG']
• To insert a list item at a specified index, use the insert() method.
The insert() method inserts an item at the specified index (without replacing)
 Append items:
 Insert items:
x.insert(2,'PUBG')
print(x)
['cricket', 'badminton', 'PUBG', 'volleyball', 'football']
>>> d1=[23,45,67,3.14]
>>> d1.append(['hi',55])
>>> print (d1)
[23, 45, 67, 3.14, ['hi', 55]]
>>> len(d1)
5

• To append elements from another list to the current list, use
the extend() method. The items will be added at the end of the current list
sport=['cricket', 'badminton', 'volleyball', 'football']
fruits=['apple','cherry']
sport.extend(fruits)
print(sport)
['cricket', 'badminton', 'volleyball', 'football', 'apple', 'cherry']
#Output:
 Extend list:
>>> list1=[1,2,3,4]
>>> list2=['a','b','c','d']
>>> list1.extend(list2)
>>> print(list1)
[1, 2, 3, 4, 'a', 'b', 'c', 'd']
>>> d1=[23,45,67,3.14]
>>>d1.extend( [‘hi’, 55])
>>> print(d1)
[23, 45, 67, 3.14, 'hi', 55]
>>> len(d1)
6
Note: 1) Extend operates on the
existing list taking another list as
argument. So, refers to same
memory
2) Append operates on the existing
list taking a singleton as argument

The extend() method can be used with any iterable object like tuples, sets,
dictionaries etc..
x=['cricket', 'badminton', 'volleyball', 'football'] # list
fruits=('apple','cherry') #tuple
x.extend(fruits)
print(x)
#Output:
['cricket', 'badminton', 'volleyball', 'football', 'apple', 'cherry']
• Remove Specified Item:
The remove() method removes the specified item.
 Add any iterable:
 Remove list items:

x.remove('badminton')
print(x)
#Output:
['cricket', 'volleyball', 'football']
• Remove w.r.t Index:
The pop() method removes the specified index.
x.pop(2)
print(x)
#Output:
['cricket', 'badminton', 'football']
Note: If you do not
specify the index,
the pop() method
removes the last item.
>>> list1=['cricket', 'badminton', 'volleyball', 'football', 'hockey']
>>> list1.pop()
'hockey'
>>> print(list1)
['cricket', 'badminton', 'volleyball', 'football']
Note: we can use
negative Indexing also
with pop() method
>>> list1=['cricket', 'badminton', 'volleyball', 'footba
>>> list1.pop(-2)
'hockey'
>>> print(list1)
['cricket', 'badminton', 'football']

• Deleting items:
• The del keyword also removes the specified index:
del x[1]
print(x)
#Output:
['cricket', 'volleyball', 'football']
• The del keyword can also delete the list completely if index is not specified.
• The clear() method empties the list.
• The list still remains, but it has no content.
• Clear the List:
x.clear()
print(x)
#Output:
[ ]
>>> list2=['a','b','c','d']
>>> del(list2[1])
>>> print (list2)
['a', 'c', 'd']
>>> list2=['a','b','c','d']
>>> del(list2)
>>> print (list2)
File "<pyshell#50>", line 1, in
<module>
print (list2)
NameError: name 'list2' is not
defined

 Copy Lists:
There are many ways to make a copy, one way is
to use the built-in List method copy().
fruits = ["apple", "banana", "cherry"]
newlist = fruits.copy()
print(newlist)
#Output:
['apple', 'banana', 'cherry']
Another way to make a copy is to use
the built-in method list().
fruits = ["apple", "banana", "cherry"]
newlist = list(fruits)
print(newlist)
#Output:
['apple', 'banana', 'cherry']
A new list can be created with ‘+’ operator
similar to string concatenation.
>>> a=['hi', 'welcome']
>>> b=['to', 'python class']
>>> c=a+b # new list, new memory reference
>>> print ("new list after concatenation is:")
new list after concatenation is:
>>> print (c)
['hi', 'welcome', 'to', 'python class']
 Creating new list using
existing lists (Concatenation)
Note: The * operator produces a new list that “repeats”
the original content
>>> print (a)
['hi', 'welcome']
>>> >>> print (a *3)
['hi', 'welcome', 'hi', 'welcome', 'hi', 'welcome']

 Sort Lists:
• Sort list alphabetically:
List objects have a sort() method that will sort the list alphanumerically, ascending, by default:
['banana', 'kiwi', 'mango', 'orange', 'pineapple']
fruits = ["orange", "mango", "kiwi", "pineapple", "banana"]
fruits.sort()
print(fruits)
#Output:
num = [100, 50, 65, 82, 23]
num.sort()
print(num)
#Output:
• Sort the list numerically:
[23, 50, 65, 82, 100]

• Sort Descending:
• To sort descending, use the keyword argument reverse = True
fruits = ["orange", "mango", "kiwi", "pineapple", "banana"]
fruits.sort(reverse = True)
print(fruits)
#Output:
['pineapple', 'orange', 'mango', 'kiwi', 'banana']
• The reverse() method reverses the current sorting order of the elements.
>>> list1=[23, 'krishna', 'a', 56.44, 'hello']
>>> list1.sort()
File "<pyshell#70>", line 1, in <module>
list1.sort()
TypeError: '<' not supported between instances of 'str' and 'int'

 List operations:
 Concatenation/joining lists
• There are several ways to
join, or concatenate, two or
more lists in Python.
• One of the easiest ways are
by using the + operator.
list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
#Output:
['a', 'b', 'c', 1, 2, 3]
list1 = ["a", "b" , "c"]
list2 = [1, 2,3]
for x in list2:
list1.append(x)
print(list1)
#Output:
['a', 'b', 'c', 1, 2, 3]
Another way to
join two lists are
by appending all
the items from
list2 into list1, one
by one:
 We can also use
the extend() method,
whose purpose is to
add elements from one
list to another list
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)
#Output:
['a', 'b', 'c', 1, 2, 3]

##list with append method
fruits = ["Apple", "Banana", "Mango"]
# using append() with user input
print(f'Current Fruits List {fruits}')
new = input("Please enter a fruit name:n")
fruits.append(new)
print(f'Updated Fruits List {fruits}')
Output:
Current Fruits List ['Apple', 'Banana', 'Mango']
Please enter a fruit name:
watermelon
Updated Fruits List ['Apple', 'Banana', 'Mango',
'watermelon']
## list with extend method
mylist = [ ] ### empty list
mylist.extend([1, "krishna"]) # extending list elements
print(mylist)
mylist.extend((34.88, True)) # extending tuple elements
print(mylist)
mylist.extend("hello") # extending string elements
print(mylist)
print ("number of elements in the list is:")
print (len(mylist))
Output:
[1, 'krishna']
[1, 'krishna', 34.88, True]
[1, 'krishna', 34.88, True, 'h', 'e', 'l', 'l', 'o']
numberof elements in the list is:
9

 Unpacking lists:
Unpack list and assign them into multiple variables.
numbers=[1,2,3,4,5,6,7,8,9]
first,second,*others,last=numbers
print(first,second)
print(others)
print(last)
#Output:
1 2
[3, 4, 5, 6, 7, 8]
9
NOTE : The number of variables in the left side of the operator must be equal to the number of the list.. If
there are many items in the list we can use *others to pack the rest of the items into the list called other.
Packs the rest of the items.
Unpacks the first and second
item of the list.
Unpacks the last item of the
list.

Python lists

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