In Built Function

To get information

len()

• Returns the number of elements present in the list.

Example

n=[10,20,30,40]
print(len(n))

Output

4

count()

• It returns the number of occurrences of specified item in the list.

Example

n=[1,2,2,2,2,3,3]
print(n.count(1)) 
print(n.count(2)) 
print(n.count(3)) 
print(n.count(4))

Output

1
4
2
0

index()

• Returns the index of first occurrence of the specified item.

Example

n=[1,2,2,2,2,3,3]
print(n.index(1))
print(n.index(2))
print(n.index(3))
print(n.index(4))

Output

0
1
5
ValueError: 4 is not in list

Note

• If the specified element not present in the list then we will get ValueError. Hence before index() method we have to check whether item present in the list or not by using in operator.

To manipulate elements

append()

• We can use append() function to add item at the end of the list.

Example

list=[]
list.append("A")
list.append("B")
list.append("C")
print(list)

Output

['A', 'B', 'C'] 

Question

Add all elements to list unto 100 which are divisible by 10.

list=[]
for i in range(101):
  if i%10==0:
    list.append(i) 
print(list)

Output

[0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100] 

insert()

• To insert item at specified index position.

Example

n=[1,2,3,4,5]
n.insert(1,888)
print(n)

Output

[1, 888, 2, 3, 4, 5]

• If the specified index is greater than max index then element will be inserted at last position. If the specified index is smaller than min index then element will be inserted at first position.

Example

n=[1,2,3,4,5]
n.insert(10,777)
n.insert(-10,999)
print(n)

Output

[999, 1, 2, 3, 4, 5, 777]

extend()

• To add all items of one list to another list.

Example

order1=["Chicken","Mutton","Fish"]
order2=["RC","KF","FO"]
order1.extend(order2)
print(order1)

Output

['Chicken', 'Mutton', 'Fish', 'RC', 'KF', 'FO']

Example

order=["Chicken","Mutton","Fish"]
order.extend("Mushroom")
print(order)

Output

['Chicken', 'Mutton', 'Fish', 'M', 'u', 's', 'h', 'r', 'o', 'o', 'm']

remove()

• We can use this function to remove specified item from the list. If the item present multiple times then only first occurrence will be removed.

Example

n=[10,20,10,30]
n.remove(10)
print(n)

Output

[20, 10, 30]

• If the specified item not present in list then we will get ValueError

n=[10,20,10,30]
n.remove(40)
print(n)

Output

ValueError: list.remove(x): x not in list

Note

• Before using remove() method first we have to check specified element present in the list or not by using in operator.

pop()

• It removes and returns the last element of the list.
• This is only function which manipulates list and returns some element.

Example

n=[10,20,30,40]
print(n.pop())
print(n.pop())
print(n)

Output

40
30
[10, 20]

• If the list is empty then pop() function raises IndexError

Example

n=[]
print(n.pop())

Output

IndexError: pop from empty list

Note

• In general we can use append() and pop() functions to implement stack data structure by using list, which follows LIFO(Last In First Out) order.
• In general we can use pop() function to remove last element of the list. But we can use to remove elements based on index.

Example

n=[10,20,30,40,50,60]
print(n.pop())
print(n.pop(1))
print(n.pop(10))

Output

60
20
IndexError: pop index out of range

Note

• List objects are dynamic i.e. based on our requirement we can increase and decrease the size.

clear()

• We can use clear() function to remove all elements of List.

Example

n=[10,20,30,40]
print(n)
n.clear()
print(n)

Output

[10, 20, 30, 40]
[]

To ordering elements

reverse()

• We can use to reverse() order of elements of list.

Example

n=[10,20,30,40]
n.reverse()
print(n)

Output

[40, 30, 20, 10] 

sort()

• In list by default insertion order is preserved. If want to sort the elements of list according to default natural sorting order then we should go for sort() method.
• Default natural sorting order is ascending order.

Example

n=[20,5,15,10,0]
n.sort()
print(n)

s=["Dog","Banana","Cat","Apple"]
s.sort()
print(s)

Output

[0,5,10,15,20]
['Apple','Banana','Cat','Dog']

Note

• To use sort() function, compulsory list should contain only homogeneous elements. otherwise we will get TypeError.

Example

n=[20,10,"A","B"]
n.sort()
print(n)

Output

TypeError: '<' not supported between instances of 'str' and 'int'

Remark

• We can sort according to reverse of default natural sorting order by using reverse=True argument.

Example

n=[40,10,30,20]
n.sort()
print(n)
n.sort(reverse=True)
print(n)
n.sort(reverse=False) 
print(n)

Output

[10,20,30,40]
[40,30,20,10]
[10,20,30,40]

Aliasing of List

• The process of giving another reference variable to the existing list is called aliasing.

Example

x=[10,20,30,40]
y=x
print(id(x))
print(id(y))

Output

1234545678
1234545678

• The problem in this approach is by using one reference variable if we are changing content, then those changes will be reflected to the other reference variable.

x=[10,20,30,40]
y=x
y[1]=777
print(x)

Output

[10,20,30,40]

Cloning of List

• To overcome this problem we should go for cloning.
• The process of creating exactly duplicate independent object is called cloning.
• We can implement cloning by using slice operator or by using copy() function.

Using slice operator

x=[10,20,30,40] 
y=x[:] 
y[1]=777 
print(x)
print(y)

Output

[10,20,30,40]
[10,777,30,40]

Using copy()

x=[10,20,30,40]
y=x.copy()
y[1]=777
print(x)
print(y)

Output

[10,20,30,40]
[10,777,30,40]

Question

Difference between = operator and copy() function

Answer

= operator meant for aliasing and copy() function meant for cloning.

Ungraded Questions

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