4. List Methods

The dot operator can also be used to access built-in methods of list objects. append is a list method which adds the argument passed to it to the end of the list. Continuing with this example, we show several other list methods. Many of them are easy to understand.

 
1
mylist = []
2
mylist.append(5)
3
mylist.append(27)
4
mylist.append(3)
5
mylist.append(12)
6
print(mylist)
7
​
8
mylist.insert(1, 12)
9
print(mylist)
10
print(mylist.count(12))
11
​
12
print(mylist.index(3))
13
print(mylist.count(5))
14
​
15
mylist.reverse()
16
print(mylist)
17
​
18
mylist.sort()
19
print(mylist)
20
​
21
mylist.remove(5)
22
print(mylist)
23
​
24
lastitem = mylist.pop()
25
print(lastitem)
26
print(mylist)
27
​
Output


There are two ways to use the pop method. The first, with no parameter, will remove and return the last item of the list. If you provide a parameter for the position, pop will remove and return the item at that position. Either way the list is changed.


The following table provides a summary of the list methods shown above. In the column labeled “result”, the word mutator means that the list is changed by calling the method on it but nothing is returned (actually None is returned). A hybrid method is one that not only changes the list but also returns a value as its result. Finally, if the result is simply a return, then the list is unchanged by the method.


Be sure to experiment with these methods to gain a better understanding of what they do.











Method
Parameters
Result
Description





append
item
mutator
Adds a new item to the end of a list



insert
position, item
mutator
Inserts a new item at the position given



pop
none
hybrid
Removes and returns the last item



pop
position
hybrid
Removes and returns the item at position



sort
none
mutator
Modifies a list to be sorted



reverse
none
mutator
Modifies a list to be in reverse order



index
item
return idx
Returns the position of first occurrence of item



count
item
return ct
Returns the number of occurrences of item



remove
item
mutator
Removes the first occurrence of item






Details for these and other methods can be found in the Python Documentation.


It is important to remember that methods like append, sort, and reverse all return None.  This means that re-assigning mylist to the result of sorting mylist will result in losing the entire list. Calls like these should never appear as part of an assignment statement (see line 8 below).


10
 
1
mylist = []
2
mylist.append(5)
3
mylist.append(27)
4
mylist.append(3)
5
mylist.append(12)
6
print(mylist)
7
​
8
mylist = mylist.sort()   # error
9
print(mylist)
10
​
Output


Check your understanding




What is printed by the following statements?


alist = [4, 2, 8, 6, 5]
alist.append(True)
alist.append(False)
print(alist)













What is printed by the following statements?


alist = [4, 2, 8, 6, 5]
alist.insert(2, True)
alist.insert(0, False)
print(alist)













What is printed by the following statements?


alist = [4, 2, 8, 6, 5]
temp = alist.pop(2)
temp = alist.pop()
print(alist)













What is printed by the following statements?


alist = [4, 2, 8, 6, 5]
alist = alist.pop(0)
print(alist)