List methods
§
append(value) – appends
a new element to the end of the list.
list=[10,20,30]
# Append values 40, 50, and 60 to the list
list.append(40)
list.append(50)
list.append(60)
print(list)
Output:
[10, 20, 30, 40, 50, 60]
§
append(List) – appends
a new List to the end of the list.
list1=[10,20,30]
# Append another list
list2 = [8, 9]
list1.append(list2)
print(list1)
Output:
[10, 20, 30, [8, 9]]
list1=[10,20,30]
# Append an element of
a different type, as list elements do not need to have the same type
list2 = [8, 9]
list1.append(list2)
list1.append('python')
print(list1)
Output:
[10, 20, 30, [8, 9], 'python']
Note :
§ the
append() method only appends one new element to the end of the list. If you
append a list to another list, the list that you append becomes a single
element at the end of the first list.
extend(enumerable)
– extends the list by appending elements from another enumerable
list1=[10,20,30]
# Extend list by appending all elements from b
list2 = [40, 50]
list1.extend(list2)
print(list1)
# Extend list with elements from a non-list enumerable:
list1.extend(range(3))
print(list1)
Output:
[10, 20, 30, 40, 50]
[10, 20, 30, 40, 50, 0, 1, 2]
§
my_list=[1,2,3,4,5,6]
print(len(my_list)) #
the output will be 6
insert() method in Python List
§ With
the help of insert method, you can place an element on a specific index
position.
§ This
method takes two parameters.
index - position where an
element needs to be inserted
element - this is the
element to be inserted in the list
Return Value from insert()
§ It
only inserts the element to the list It doesn't return anything; returns None.
list1=[10,20,30]
print(list1)#The
Output [10, 20, 30]
list1.insert(3,40)
print(list1)#The
Output [10, 20, 30, 40]
list1.insert(1,50)
print(list1)#The
Output [10, 50, 20, 30, 40]
list1.insert(6,60)
print(list1)#The Output [10, 50, 20, 30, 40, 60]
remove() method in Python List
§ With
the help of remove () method we can remove the first matching element (which is
passed as an argument) from the list.
The syntax of the remove() method is:
list.remove(element)
remove() Parameters
§ The
remove() method takes a single element as an argument and removes it from the
list.
§ If
the element doesn't exist, it throws ValueError: list.remove(x): x not in list
exception.
Return Value from remove()
It only remove the element from the list It doesn't return anything; returns None
#cities list
cities=['agra','delhi','jaipur','kanpur','lucknow']
print('Original
Cities list: ', cities)
# 'agra' is
removed
cities.remove('agra')
# Updated Cities List
print('Updated Cities list: ', cities)
Output:
Original Cities
list: ['agra', 'delhi', 'jaipur',
'kanpur', 'lucknow']
Updated Cities
list: ['delhi', 'jaipur', 'kanpur',
'lucknow']
pop() method in Python List
§ With
the help of pop () method we can remove the last item from the list and returns
the removed item.
The syntax of the pop() method is:
list.pop()
Return Value from pop() method
§ pop()
method with-out parameter returns the last item of the list
#cities list
cities=['agra','delhi','jaipur','kanpur','lucknow']
print('Original Cities list: ', cities)
# last item is removed
cities.pop()
# Updated Cities List
print('Updated Cities list: ', cities)
# last item is removed
cities.pop()
# Updated Cities List
print('Updated Cities list: ', cities)
Output:
Original Cities list:
['agra', 'delhi', 'jaipur', 'kanpur', 'lucknow']
Updated Cities list: ['agra',
'delhi', 'jaipur', 'kanpur']
Updated Cities list:
['agra', 'delhi', 'jaipur']
pop(index) method in Python List
§ With
the help of pop () method we can remove the item at the given index from the
list and returns the removed item.
The syntax of the pop() method is:
§ list.pop(index)
pop() Parameters
§ The
remove () method takes a single element as an argument.
§ this list.pop([i]) method to return the item which
are present at the given specified index
position from the list and then remove that item from the list
§ If
the element doesn't exist, it throws ValueError: list.remove(x): x not in list
exception.
Return Value from pop() method
§ pop()
method with parameter returns the item at specified index position of the list
#cities list
cities=['agra','delhi','jaipur','kanpur','lucknow']
print('Original Cities list: ', cities)
# remove and return the
2th item
deleted_city=cities.pop(1)
print(deleted_city,'is delete from cities list')
# Updated Cities List
print('Updated Cities list: ', cities)
# remove and return the
11th item
Output
Original Cities list:
['agra', 'delhi', 'jaipur', 'kanpur', 'lucknow']
delhi is delete from cities list
Updated
Cities list: ['agra', 'jaipur',
'kanpur', 'lucknow']
cities.pop(10)
# Updated Cities List
print('Updated Cities list: ', cities)
cities.pop(10)
IndexError: pop index out of range
§ index
() method returns the index position of the element in the List.
§ If the
same element is present more than once, the method returns the index of the
first occurrence of the element.
§ If the
element is not present then it will return Error: ValueError
#cities list
cities=['agra','delhi','jaipur','kanpur','lucknow']
pos=cities.index('delhi')
print('delhi
is present at index: ' ,pos)
Output
delhi
is present at index: 1
sort() method in Python List
§ With the help
list.sort() method we can sort the elements of the list in place.
The syntax of sort() method is:
list.sort(key=...,
reverse=...)
§ Alternatively, you
can also use Python's in-built function sorted() for the same purpose.
sorted(list, key=..., reverse=...)
§ The difference
between sort() and sorted() method is only that sort() method doesn’t returns
any value while, sorted() method returns an iterable list.
sort() Parameters
§ By default, sort
() method doesn't take any extra parameters. However, it has two optional
parameters that can be used to customize the operation
reverse –
§ If true, the
sorted list is reversed (or sorted in Descending order)
key –
§ Specifies a
function that is used to extract a comparison key from each list element. The
default value is None (compares the elements directly).
#cities list
cities=['Jaipur','Delhi','Agra','kanpur','Lucknow']
print('Un-Sorting-cities: ',cities)
# sort the cities list
cities.sort()
print('Sorted cities',cities)
Output
Un-Sorting-cities:
['Jaipur', 'Delhi', 'Agra', 'kanpur', 'Lucknow']
Sorted cities ['Agra', 'Delhi', 'Jaipur', 'Lucknow', 'kanpur']
Example 2: Sort a
cities list in Descending Order
§ Setting
reverse=True sorts the list in the descending order.
list.sort(reverse=True)
#cities list
cities=['Jaipur','Delhi','Agra','kanpur','Lucknow']
print('Un-Sorting-cities: ',cities)
# sort the cities list
cities.sort(reverse=True)
print('Sorted cities',cities)
Output
Un-Sorting-cities: ['Jaipur', 'Delhi', 'Agra', 'kanpur',
'Lucknow']
Sorted cities ['kanpur',
'Lucknow', 'Jaipur', 'Delhi', 'Agra']
sorting
a list by using your own function with key parameter?
Example 3: Sort a
cities list by passing key function
cities.sort(key=len)
#cities list
cities=['Jaipur','Delhi','Agra','kanpur','Lucknow']
print('Un-Sorting-cities: ',cities)
# sort the cities list
cities.sort(key=len)
print('Sorted cities',cities)
Output
Un-Sorting-cities: ['Jaipur', 'Delhi', 'Agra', 'kanpur',
'Lucknow']
Sorted cities ['Agra',
'Delhi', 'Jaipur', 'kanpur', 'Lucknow']
§ Here,
no_digits is the Python's user-define function to count the no of digits of an
element.
§ We
want to sort this list in such a way that the number that have less number of
digit come first.
# take element for sort
def no_digits(n):
count=0
while n>0:
count+=1
n=n//10
return count
# number list
nums_list=[111,11111,1,1111,111111]
print('Un-Sorting-cities: ',nums_list)
# sort list with key
nums_list.sort(key=no_digits)
# print list
print('Sorted-List: ',nums_list)
Output
Un-Sorting-List: [111, 11111, 1, 1111, 111111]
Sorted-List: [1, 111, 1111, 11111, 111111]
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