Core Python Programming (2nd Edition)

6.11. Lists
Like strings, lists provide sequential storage through an index offset and access to single or consecutive
elements through slices. However, the comparisons usually end there. Strings consist only of characters
and are immutable (cannot change individual elements), while lists are flexible container objects that
hold an arbitrary number of Python objects. Creating lists is simple; adding to lists is easy, too, as we
see in the following examples.
The objects that you can place in a list can include standard types and objects as well as user-defined
ones. Lists can contain different types of objects and are more flexible than an array of C structs or
Python arrays (available through the external array module) because arrays are restricted to containing
objects of a single type. Lists can be populated, empty, sorted, and reversed. Lists can be grown and
shrunk. They can be taken apart and put together with other lists. Individual or multiple items can be
inserted, updated, or removed at will.
Tuples share many of the same characteristics of lists and although we have a separate section on
tuples, many of the examples and list functions are applicable to tuples as well. The key difference is
that tuples are immutable, i.e., read-only, so any operators or functions that allow updating lists, such
as using the slice operator on the left-hand side of an assignment, will not be valid for tuples.
How to Create and Assign Lists
Creating lists is as simple as assigning a value to a variable. You handcraft a list (empty or with
elements) and perform the assignment. Lists are delimited by surrounding square brackets ( [ ] ). You
can also use the factory function.
>>> aList = [123, 'abc', 4.56, ['inner', 'list'], 7-9j]
>>> anotherList = [None, 'something to see here']
>>> print aList
[123, 'abc', 4.56, ['inner', 'list'], (7-9j)]
>>> print anotherList
[None, 'something to see here']
>>> aListThatStartedEmpty = []
>>> print aListThatStartedEmpty
[]
>>> list('foo')
['f', 'o', 'o']
How to Access Values in Lists
Slicing works similar to strings; use the square bracket slice operator ([ ] ) along with the index or
indices.
>>> aList[0]
123
>>> aList[1:4]
['abc', 4.56, ['inner', 'list']]
>>> aList[:3]
[123, 'abc', 4.56]
>>> aList[3][1]
'list'