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.
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. The column labeled
result gives an explanation as to what the return value is as it relates to the new value of the list. The word
mutator means that the list is changed by the method 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 others can be found in the Python Documentation.
It is important to remember that methods like append, sort,
and reverse all return None. They change the list; they don’t produce a new list.
So, while we did reassignment to increment a number, as in x = x + 1, doing the
analogous thing with these operations will lose the entire list contents (see line 8 below).
Check your understanding
- [4,2,8,6,5,False,True]
- True was added first, then False was added last.
- [4,2,8,6,5,True,False]
- Yes, each item is added to the end of the list.
- [True,False,4,2,8,6,5]
- append adds at the end, not the beginning.
rec-5-1: What is printed by the following statements?
alist = [4,2,8,6,5]
alist.append(True)
alist.append(False)
print(alist)