Python supports many methods to change lists or get information from them. You do not need to import a module to use them. Since they are methods, you call them using the syntax <list>.<method>().

Important

Lists are mutable and these methods actually change the list! It is not as you are used to with string methods, where the methods create a new string, and return it, while the original string remains. Most list methods have an irrevocable effect on the list they work on. Usually they have no return value, and you do not need one either, as the purpose of the methods is to change the list.

append()

append() attaches an item at the end of a list. You call the method with the item you wish to add as argument.

fruitlist = ["apple", "banana", "cherry", "durian"]
print( fruitlist )
fruitlist.append( "orange" )
print( fruitlist )

An alternative for using the append() method is to add a list with one new element to the existing list with a \(+\), and assign the resulting list to the original list variable. However, the append() method is preferable as it is more readable. <list>.append(<element>) is equivalent to <list>[len(<list>):] = [<element>], or simply <list> += [<element>].

extend()

extend() makes a list longer by appending the elements of another list at the end. You call the method with the list of which you want to add the elements as argument.

fruitlist = ["apple", "banana", "cherry", "durian"]
print( fruitlist )
fruitlist.extend( ["raspberry", "strawberry", "blueberry"] )
print( fruitlist )

Just as with the append() method, you can extend an existing list with a new list by simply using the \(+\) operator, and assigning the result to the original list variable. And just as with the append() method, the extend() method is preferable. <list>.extend(<addlist>) is equivalent to <list>[len(<list>):] = <addlist>.

insert()

insert() allows you to insert an element at a specific position in a list. It is called with two arguments, the first being the index of the location where you wish to insert the new element, and the second the new element itself. To insert an element at the front of the list, you can use index 0.

fruitlist = ["apple", "banana", "cherry", "durian"]
print( fruitlist )
fruitlist.insert( 2, "orange" )
print( fruitlist )

<list>.insert(<i>,<element>) is equivalent to <list>[<i>:<i>] = [<element>].

remove()

remove() allows you to remove an element from a list. The element you wish to remove is given as argument. If the element occurs in the list multiple times, only the first occurrence will be removed. If you try to remove an element that is not on the list, a runtime error is generated.

fruitlist = ["apple", "banana", "cherry", "banana", "durian"]
print( fruitlist )
fruitlist.remove( "banana" )
print( fruitlist )

pop()

Like remove(), pop() removes an element from the list, but does so by index. It has one optional argument, which is the index of the element that you wish to remove. If you do not provide that argument, pop() removes the last element from the list. If the index is beyond the boundaries of the list, pop() generates a runtime error.

A major difference with remove() is that pop() actually has a return value, namely the element that gets removed. This allows you to quickly process all the elements of a list, while emptying the list at the same time.

fruitlist = ["apple", "banana", "cherry", "durian"]
print( fruitlist )
print( fruitlist.pop() )
print( fruitlist )
print( fruitlist.pop( 0 ) )
print( fruitlist )

del

del is neither a method nor a function, but since it is often metioned in one breath with remove() and pop(), I place it here. del is a keyword that allows you to delete a list element, or list slice, by index. It is similar to pop() in functionality, but does not have a return value. Also, pop() cannot be used on slices. To remove one element from a list, use del <list>[<index>]. To remove a slice, use del <list>[<index1>:<index2>].

fruitlist = ["apple", "banana", "cherry", "banana", "durian"]
del fruitlist[3]
print( fruitlist )

index()

index() returns the index of the first occurrence on the list of the element that is given to index() as argument. A runtime error is generated if the element is not found on the list.

fruitlist = ["apple", "banana", "cherry", "banana", "durian"]
print( fruitlist.index( "banana" ) )

count()

count() returns an integer that indicates how often the element that is passed to it as an argument occurs in the list.

fruitlist = ["apple", "banana", "cherry", "banana", "durian"]
print( fruitlist.count( "banana" ) )

sort()

sort() sorts the elements of the list, from low to high. If the elements of the list are strings, it does an alphabetical sort. If the elements are numbers, it does a numeric sort. If the elements are mixed, it generates a runtime error, unless certain arguments are given.

fruitlist = ["apple", "strawberry", "banana", "raspberry", "cherry", "banana", "durian", "blueberry"]
fruitlist.sort()
print( fruitlist )

numlist = [314, 315, 642, 246, 129, 999]
numlist.sort()
print( numlist )

To do a reverse sort (for instance, sorting numerical items from high to low, or sorting strings alphabetically from "z" to "a"), you can add an argument reverse=<boolean>.

fruitlist = ["apple", "strawberry", "banana", "raspberry", "cherry", "banana", "durian", "blueberry"]
fruitlist.sort( reverse=True )
print( fruitlist )

Another argument that you can give sort() is a key. You have to provide this argument as <list>.sort( key=<key> ), whereby <key> is a function that takes one argument (the element that is to be sorted) and returns a value that is used as key. A typical use for the key argument is if you want to sort a list of strings, but want to do the sorting case-insensitively. So as key you want to use the elements, but in lower case, i.e., you want to apply the function str.lower() to the element. You call the sort() method as in the following example:

fruitlist = ["apple", "Strawberry", "banana", "raspberry", "CHERRY", "banana", "durian", "blueberry"]
fruitlist.sort() 
print( fruitlist )
fruitlist.sort( key=str.lower ) # case-insensitive sort
print( fruitlist )

Note that for the key argument, you do not place parentheses after the function name. This is not a function call, it is an argument that tells Python which function to use to generate the key. You can write your own function to be used as key. For example, in the code below, numlist is sorted with the digits reversed:

def revertdigits( item ):
    return (item%10)*100 + (int(item/10)%10)*10 + int(item/100)

numlist = [314, 315, 642, 246, 129, 999]
numlist.sort( key=revertdigits )
print( numlist )

Here is another example, that sorts a list of strings by length, then alphabetical order:

def len_alphabetical( element ):
    return len( element ), element 

fruitlist = ["apple", "strawberry", "banana", "raspberry", "cherry", "banana", "durian", "blueberry"]
fruitlist.sort( key=len_alphabetical )
print( fruitlist )

Note that the len_alphabetical() function returns a tuple. When two tuples are compared, first the first elements of both tuples are compared, and if they are equal, the second elements are compared.

At this point I can give a typical example of the use of “anonymous functions,” which I introduced in the chapter explaining how functions work in Python. Using an anonymous function to specify the key for the sort() method keeps the code for the key next to where you call the sort(), instead of elsewhere in the program. This may improve readability.

fruitlist = ["apple", "strawberry", "banana", "raspberry", "cherry", "banana", "durian", "blueberry"]
fruitlist.sort( key=lambda x: (len(x),x) )
print( fruitlist )

reverse()

reverse() simply puts the elements of the list in reverse order.

fruitlist = ["apple","strawberry","banana","raspberry","durian"]
fruitlist.reverse()
print( fruitlist )

Practice

Write a program that asks the user to enter some data, for instance the names of their friends. When the user wants to stop providing inputs, he just presses Enter. The program then displays an alphabetically sorted list of the data items entered. Do not just print the list, but print each item separately, on a different line.

Sort a list of numbers using their absolute values; use the abs() function as key.

Count how often each letter occurs in a string (case-insensitively). You can ignore every character that is not a letter. Store the counts in a list of 26 items that all start at zero. Print the resulting counts. As index you can use ord(letter) - ord("a"), where letter is a lower case letter (the ord() function is explained in the section “Character encoding” of the chapter explaining strings ).