# Python Programming – List Methods

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## Python Programming – List Methods

List methods

Below are the methods of list-objects.

list . append ( x )
Add an item to the end of the list. It is same as list [ len ( list ) : len ( list ) ] = [ x ] .

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . append ( 45 )
>>> a
[ 66 . 25 , 333 , 333 , 1 , 1234 . 5 , 45 ]
>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a [ len ( a ) : len ( a ) ]=[ 45 ]
>>> a
[ 66 . 25 , 333 , 333 , 1 , 1234 . 5 , 45 ]

list . extend ( L )
Extend a list by appending all the items of a given list. It is same as list [ len ( list ) : len ( list ) ] =L.

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> b= [ 7 . 3 , 6 . 8 ]
>>> a . extend ( b )
>>> a
[ 66 . 25 , 333 , 333 , 1 , 1234 . 5 , 7 . 3 , 6 . 8 ]
>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> b= [ 7 . 3 , 6 . 8 ]
>>> a [ len ( a ) : len ( a ) ]=b
>>> a
[ 66 . 25 , 333 , 333 , 1 , 1234 . 5 , 7 . 3 , 6 . 8 ]

list.insert ( i , x )
Insert an item at a given position in the list. The first argument i is the index before which an item x need to be inserted. It is same as list [i : i] = [x].

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . insert ( 2 , 5 . 7 )
>>> a
[ 66 . 25, 333 , 5 . 7 , 333 , 1 , 1234 . 5 ]
>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a [ 2 : 2 ] = [ 5 . 7 ]
>>> a
[ 66 . 25 , 333 , 5 . 7 , 333 , 1 , 1234 . 5 ]

list.index ( x [ , i [ , j ] ] )
Return the index in the list of the first occurrence of item x. In other words, it returns the smallest index k such that list [k]==x and i<=k<j. A ValueError exception is raised in absence of item x.

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . index ( 333 )
1

list.remove ( x )
Remove the first item from the list whose value is x. An error (ValueError exception) occur in absence of item x. It is same as del list [list. index (x) ].

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . remove ( 333 )
>>> a
[ 66 . 25 , 333 , 1 , 1234 . 5 ]
>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> del a [ a . index ( 333 ) ]
>>> a
[ 66 . 25 , 333 , 1 , 1234 . 5 ]

list . pop ( [ i ] )
Remove the item at the given position i in the list, and return it. If no index is specified (defaults to -1), pop () removes and returns the last item in the list.

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . pop ( 3 )
1
>>> a
[ 66 . 25 , 333 , 333 , 1234 . 5 ]
>>> a . pop ( )
1234 . 5
>>> a
[ 66 . 25 , 333 , 333 ]

list.count ( x )
Return the number of times item x appears in the list.

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . count ( 333 )
2

list.reverse ( )
Reverse the element’s position in the list; no new list is returned. It is same as list=list [: : -1 ].

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . reverse ( )
>>> a
[ 1234 . 5 , 1 , 333 , 333 , 66 . 25 ]
>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a=a [ : : -1 ]
[ 1234 . 5 , 1 , 333 , 333 , 66 . 25 ]

list.sort ( [ cmp [ , key [ , reverse ] ] ] )
Sort the items of the list; no new list is returned. The optional arguments have same meaning as given in sorted () built-in function.

>>> a= [ 66 . 25 , 333 , 333 , 1 , 1234 . 5 ]
>>> a . sort ( )
>>> a
[ 1 , 66 . 25 , 333 , 333 , 1234 . 5 ]
>>> a= [ 66 . 25 , 333 , ' abc ' , 333 , 1 , ' ab ' , 1234 . 5 ]
>>> a . sort ( )
>>> a
[ 1 , 66 . 25 , 333 , 333 , 1234 . 5 , ' ab ' , ' abc ' ]
>>> a= [ 66 . 25 , 333 , ' abc ' , 333 , 1 , ' ab ' , 1234 . 5 ]
>>> a . sort ( reverse=True )
>>> a
[ ' abc ' , ' ab ' , 1234 . 5 , 333 , 333 , 66 . 25 , 1 ]

Using list as Stack

The list methods make it very easy to use a list as a stack, where the last element added is the first element retrieved (“last-in, first-out” approach). To add an item to the top of the stack, use append (). To retrieve an item from the top of the stack, use pop ( ) without an explicit index. For example:

>>> stack= [ 3 , 4 , 5 ]
>>> stack . append ( 6 )
>>> stack . append ( 7 )
>>> stack
[ 3 , 4 , 5 , 6 , 7 ]
>>> stack . pop ( )
7
>>> stack [ 3 , 4 , 5 , 6 ]
>>> stack . pop ( )
6
>>> stack . pop ( )
5
>>> stack
[ 3 , 4 ]

Using list as queue

It is also possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, list is not efficient for this purpose. While appending and popping of elements from the end of list are fast, doing inserting and popping from the beginning of a list is slow (because all of the other elements have to be shifted by one).

To implement a queue, use collections. deque which was designed to have fast appends and pops from both ends. For example:

>>> from collections import deque
>>> queue=deque ( [ " Eric " , " John " , " Michael " ] )
>>> queue . append ( " Terry " )
>>> queue . append ( " Graham " )
>>> queue
deque ( [ ' Eric ' , ' John ' , ' Michael ' , ' Terry ' , ' Graham ' ] )
>>> queue . popleft ( )
' Eric '
>>> queue . popleft ( )
' John '
>>> queue
deque ( [ ' Michael ' , ' Terry ' , ' Graham ' ] )