diff --git a/native-datatypes.html b/native-datatypes.html index 5870256..971624f 100644 --- a/native-datatypes.html +++ b/native-datatypes.html @@ -334,6 +334,72 @@ ValueError: list.index(x): x not in list
  • The index() method finds the first occurrence of a value in the list. In this case, 'new' occurs twice in the list, in a_list[2] and a_list[4], but the index() method will return only the index of the first occurrence.
  • As you might not expect, if the value is not found in the list, Python raises an exception. This is notably different from most languages, which will return some invalid index (like -1). While this may seem annoying at first, I think you will come to appreciate it. It means your program will crash at the source of the problem instead of failing strangely and silently later. + +

    Removing Items From A List

    + + + +

    Lists can expand and contract automatically. You’ve seen the expansion part. There are several different ways to remove items from a list as well. + +

    +>>> a_list = ['a', 'b', 'new', 'mpilgrim', 'new']
+>>> a_list[1]
+'b'
+>>> del a_list[1]         ①
+>>> a_list
+['a', 'new', 'mpilgrim', 'new']
+>>> a_list[1]             ②
+'new'
    +
      +
    1. You can use the del statement to delete a specific item from a list. +
    2. Accessing index 1 after deleting index 1 does not result in an error. All items after the deleted item shift their positional index to “fill the gap” created by deleting the item. +
    + +

    Don’t know the positional index? Not a problem; you can remove items by value instead. + +

    +>>> a_list.remove('new')  ①
+>>> a_list
+['a', 'mpilgrim', 'new']
+>>> a_list.remove('new')  ②
+>>> a_list
+['a', 'mpilgrim']
+>>> a_list.remove('new')
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+ValueError: list.remove(x): x not in list
    +
      +
    1. You can also remove an item from a list with the remove() method. The remove() method takes a value and removes the first occurrence of that value from the list. Again, all items after the deleted item will have their positional indices bumped down to “fill the gap.” Lists never have gaps. +
    2. You can call the remove() method has often as you like, but it will raise an exception if you try to remove a value that isn’t in the list. +
    + +

    Removing Items From A List: Bonus Round

    + +

    Another interesting list method is pop(). The pop() method is yet another way to remove items from a list, but with a twist. + +

    +>>> a_list = ['a', 'b', 'new', 'mpilgrim']
+>>> a_list.pop()   ①
+'mpilgrim'
+>>> a_list
+['a', 'b', 'new']
+>>> a_list.pop(1)  ②
+'b'
+>>> a_list
+['a', 'new']
+>>> a_list.pop()
+'new'
+>>> a_list.pop()
+'a'
+>>> a_list.pop()   ③
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+IndexError: pop from empty list
    +
  • When called without arguments, the pop() list method removes the last item in the list and returns the value it removed. +
  • You can pop arbitrary items from a list. Just pass a positional index to the pop() method. It will remove that item, shift all the items after it to “fill the gap,” and return the value it removed. +
  • All around the mulberry bush… calling pop() on an empty list raises an exception. + +

    Lists In A Boolean Context

    You can also use a list in a boolean context, such as an if statement.