Python knows a number of compound data types, used to group together other values.
The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets.
Lists might contain items of different types, but usually the items all have the same type.
Like strings, lists can be indexed and sliced.
>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]
>>> squares[0] # indexing returns the item
1
>>> squares[-1]
25
>>> squares[-3:] # slicing returns a new list
[9, 16, 25]
All slice operations return a new list containing the requested elements. This means that the following slice returns a new (shallow) copy of the list:
>>>
>>> squares[:]
[1, 4, 9, 16, 25]
Lists also support operations like concatenation:
>>>
>>> squares + [36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
Unlike strings, which are immutable, lists are a mutable type, i.e. it is possible to change their content:
>>>
>>> cubes = [1, 8, 27, 65, 125] # something's wrong here
>>> 4 ** 3 # the cube of 4 is 64, not 65!
64
>>> cubes[3] = 64 # replace the wrong value
>>> cubes
[1, 8, 27, 64, 125]
You can also add new items at the end of the list, by using the append() method (we will see more about methods later):
>>>
>>> cubes.append(216) # add the cube of 6
>>> cubes.append(7 ** 3) # and the cube of 7
>>> cubes
[1, 8, 27, 64, 125, 216, 343]
Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:
>>>
>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> letters
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> # replace some values
>>> letters[2:5] = ['C', 'D', 'E']
>>> letters
['a', 'b', 'C', 'D', 'E', 'f', 'g']
>>> # now remove them
>>> letters[2:5] = []
>>> letters
['a', 'b', 'f', 'g']
>>> # clear the list by replacing all the elements with an empty list
>>> letters[:] = []
>>> letters
[]
The built-in function len() also applies to lists:
>>>
>>> letters = ['a', 'b', 'c', 'd']
>>> len(letters)
4
It is possible to nest lists (create lists containing other lists), for example:
>>>
>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'