An Introduction to Python List

Let’s learn about python list in detail

The list is a data structure in Python which contains a mutable, ordered sequence of elements.

Creating a list:

l1 = [1, 2, 3]
l2 = [‘apple’, ‘banana’, ‘orange’, ‘grapes’, ‘mango’]

List constructor: list()
 List constructor takes sequence types and converts to list.It is used to convert tuple to list.
t1=(1,2,3)
l1=list(t1)
print (l1)
Output:
[1, 2, 3]

Accessing elements from list:
 List indices start from 0.

l1[0] -> 1 (first element in list l1)
 l2[2] -> orange (third element in list l2)
 l1[-1] -> 3 (Negative index -1 means starting from the end of the list)
 l1[-2] -> 2

List Slicing:
 We can specify range of indexes. start and stop. stop index is not included.

l1=[0,1,2,3,4,5]
print (l1[1:4])

Output:

[1, 2, 3]

Updating List:
 Inserting elements into the list. insert method will insert elements into the list at a specified index.
l1=[1,2,3]

#It will insert element 4 at index 3.

l1.insert(3,4)
print (l1)

#It will update the element in the second index to ‘a’

l1[2]=’a’ 
print (l1)

Output:

[1, 2, 3, 4]
[1, 2, ‘a’, 4]

Updating Nested List:

l1=[1,2,3,[4,5]]
l1[3][0]=’a’
print (l1)
Output:
[1, 2, 3, [‘a’, 5]]

Adding items to the list:

1.append
 2.extend

append

It will append the argument as a single item to the end of the list. The length of the list will be increased by 1.

 a1=[1,2,3]
 a2=[4,5]
 a1.append(a2)
 print (a1)
 Output:
 [1, 2, 3, [4,5]]

 b1=[4,5,6]
 b1.append(7)
 print (b1)
 Output:
 [4, 5, 6, 7]

append method will modify the original list

extend

It will iterate through the argument and add each element to the list.

c1=[1,2,3]
c2=[4,5,6]
c1.extend(c2)
print (c1)
Output:
[1, 2, 3, 4, 5, 6]

Removing Items from the list

• del
• pop()
• remove()
• clear()clear()
   clear method will empty the list. It will return an empty list.

a1=[1,2,3]
a1.clear()
print (a1)
Output:
[]

del
 del keyword delete the list itself.

a1=[1,2,3]
del a1
print (a1)
Output:
NameError: name ‘a1’ is not defined.

remove()
 remove method will remove the element specified from the list.

a1=[1,2,3]
a1.remove(1)
print (a1)
Output:
[2,3]

If the element mentioned is not in the list, it will raise ValueError.

a1.remove(4)
print (a1)
Output:
ValueError: list.remove(x): x not in list

pop()
 pop method will remove the item in the specified index from the list. If the index not specified, it will remove the last item from the list.

a1=[1,2,3]
a1.pop(1)
print (a1)
Output:
[1,3]

a1.pop()
print (a1)
Output:
[1]

a1.pop(4)
print (a1)
Output:
IndexError: pop index out of range

Joining two lists:

a1=[1,2,3]
a2=[4,5,6]
a3=a1+a2
print (a3)
Output:
[1,2,3,4,5,6]

count:

count will return the number of occurrences of a particular element mentioned.

a1=[1,2,3,4,1,5,6,1]
print (a1.count(1))
Output:
3

Sort

sort method will sort the original list.

Sorting in ascending order using sort function:

a1=[2,4,6,8,0,7,5,3,1]
a1.sort()
print (a1)
Output: 
[0, 1, 2, 3, 4, 5, 6, 7, 8]

Sorting in descending order using the sort method.

a2=[2,4,6,8,0,7,5,3,1]
a2.sort(reverse=True)
print (a2)
Output:
[8, 7, 6, 5, 4, 3, 2, 1, 0]

reverse

The reverse method is used to reverse the list. It will reverse the original list.

l1=[1,3,5,2,4,6,8]
l1.reverse()
print (l1)

Output:
[8, 6, 4, 2, 5, 3, 1]

List functions:

sorted()

It will sort the list and return a new list. It won’t modify the original list.

Sorting in ascending order using the sorted function:

a1=[2,4,6,8,0,7,5,3,1]
a2=sorted(a1)
print (f”Original List:{a1}”)
print (f”Sorted List:{a2}”)
Output:
Original List:[2, 4, 6, 8, 0, 7, 5, 3, 1]
Sorted List:[0, 1, 2, 3, 4, 5, 6, 7, 8]

Sorting in descending order using the sorted function:

a1=[2,4,6,8,0,7,5,3,1]
a2=sorted(a1,reverse=True)
print (f”Original List:{a1}”)
print (f”Sorted List:{a2}”)
Output:
Original List:[2, 4, 6, 8, 0, 7, 5, 3, 1]
Sorted List:[8, 7, 6, 5, 4, 3, 2, 1, 0]

Reversed()
The reversed function will reverse the elements in the list. It will return an iterator object. We can iterate through the iterator object or convert it to a list using a list().

a1=[1,3,5,2,4,6]
a2=reversed(a1) 
print (a2)
print (list(a2))

Output:
<list_reverseiterator object at 0x02817808>
[6, 4, 2, 5, 3, 1]

min, max, sum,len

min — returns the minimum number in the list
max- return the maximum number in the list
sum — return the sum of the numbers in the list
len — return the number of elements in the list

a1=[1,2,3,4,5,6,7,8,9,10]
print (min(a1)) 
print (max(a1))
print (sum(a1))
print (len(a1)) 
Output:
1
10
55
10

s1=[‘apple’,’banana’,’orange’,’grapes’]
print (min(s1))
print (max(s1))
print (len(s1))
print (sum(s1)
Output:
apple
orange
4
TypeError: unsupported operand type(s) for +: ‘int’ and ‘str’

Check if an item exists in the list.

s1=[‘apple’,’banana’,’orange’,’grapes’]
if ‘apple’ in s1:
 print (“YES”)
if ‘grapes’ not in s1:
 print (“Grapes is not in the list “)

Output:

YES

copy,deepcopy

 copy function will return a copy of the list. Modifications done in the original list will not be reflected in the copied list.
But modifications done in the nested list will be reflected in the copied list.
 deep copy function will return a deep copy of the list. Modifications done in the original list will not be reflected in the deep copied list.

import copy
l1=[1,2,3,[4,5]]
l2=l1.copy()
print (“Copied : {}”.format(l2))
l3=copy.deepcopy(l1)
print (“Deep Copied: {}”.format(l3))
l1[0]=’a’
print (“After updating elements in the list”)
print (f”Original List:{l1}”)
print (f”Copied List:{l2}”)
print (f”Deep Copied List:{l3}”)
l1[3][1]=’b’
print (“After updating elements in the nested list”)
print (f”Original List:{l1}”)
print (f”Copied List:{l2}”)
print (f”Deep Copied List:{l3}”)
Output:
Copied : [1, 2, 3, [4, 5]]
Deep Copied: [1, 2, 3, [4, 5]]
After updating elements in the list
Original List:[‘a’, 2, 3, [4, 5]]
Copied List:[1, 2, 3, [4, 5]]
Deep Copied List:[1, 2, 3, [4, 5]]
After updating elements in the nested list
Original List:[‘a’, 2, 3, [4, ‘b’]]
Copied List:[1, 2, 3, [4, ‘b’]]
Deep Copied List:[1, 2, 3, [4, 5]]

Assignment Operator:

Copy using assignment operator.

l1=[1,2,3]
l2=l1
print (l1)
print (l2)
l1[0]=’a’
l2[1]=’b’
print (“After modifying both list l1 and l2”)
print(l1)
print(l2)
Output:
[1, 2, 3]
[1, 2, 3]
After modifying both list l1 and l2
[‘a’, ‘b’, 3]
[‘a’, ‘b’, 3]

In the above example, both l1 and l2 will be referring to the same object in the memory. So changes made in one list will be reflected in the other list too.

---

Conclusion:

• List methods which will update the original list
  append,extend,remove,clear,pop,insert,sort,reverse
• List function which will return a new list and won’t modify the original list.
   sorted() , reversed(),copy(),deepcopy()

---

My other blogs on Python data structures

An Introduction to Python Set

String Methods in Python

An Introduction to Python Tuple

An Introduction to Python Dictionary