count and index methods. Methods are either mutating or non-mutating. Mutating methods are ones that change the object after the method has been used. Non-mutating methods do not change the object after the method has been used.
count and index methods are both non-mutating. Count returns the number of occurrences of the argument given but does not change the original string or list. Similarly, index returns the leftmost occurrence of the argument but does not change the original string or list. Below we’ll talk about list methods in general. Keep an eye out for methods that are mutating!
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.
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.
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.
| 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 |
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).
alist = [4,2,8,6,5]
alist.append(True)
alist.append(False)
print(alist)
