Until now all the variables we have used have contained a single piece of information, for example,
a = 4 makes a variable
a containing a single number,
4. It’s very common in programming (and in fact real life) to want to refer to collections of this. For example a shopping list contains a list of items you want to buy, or a car park contains a set of cars.
In Python a list is an ordered collection of values that can be accessed by their index. Create a script list.py and write this:
= ["cat", "dog", 261] my_list print(my_list)
Run this script with
python list.py and look at the output.
This will create a Python
list with three elements and assign it to the variable
my_list. The square brackets
] in this case mean “create a list” and the elements of the list are then separated by commas. As with previous variable types, you can print lists by passing their name to the
Since lists are containers for data and each item in the list has a fixed position, you can request a single item by giving its index. To get an item from a list you give the name of the list followed by square brackets with the index of the item you want between them. Add the following lines to the end of
list.py and run it again:
print(my_list) print(my_list) print(my_list)
Did this do what you expect? Notice that the index
0 returns the first element and the index
1 returns the second. This is because Python (along with most other programming languages) count their indices from zero.
If you want to find out how many elements a list has, you can use the
print("my_list contains %s items" % (len(my_list)))
You can iterate over the elements in a list using a
for loop. Each time around the loop, the loop variable will be set to point at another element of the list. If you pass the list to
enumerate() first then the loop is provided with a pair of values: the index of the element and the element itself.
= [3, 5, "green", 5.3, "house", 100, 1] my_list for elem in my_list: print(elem) for i, elem in enumerate(my_list): print("Element %s is %s" % (i, elem))
As well as accessing individual elements in a list looping over all the elements, it is possible to grab subsets of a list by using slicing:
= [3, 5, "green", 5.3, "house", 100, 1] my_list print(my_list[-1]) # Get the last element of the list print(my_list[2:5]) # Get elements from index 2 to (but not including) index 5 print(my_list[3:]) # Get elements from index 3 until the end of the list print(my_list[:4]) # Get elements from the beginning to (but not including) index 4 print(my_list[::2]) # Get every other element from the list print(my_list[::-1]) # Get all the elements in reverse order
Inside the square brackets you can have up to three arguments, separated by
:. The first is the starting point for the slice, the second is the stopping point for the slice and the third is the ‘step’.