Python list

Python lists¶

Lists in python are collection of objects. They can include any python objects such as numerals, strings, other lists and dictionaries. The list is defined by square bracket as follows:

my_list = []

The above example shows an empty list. Following is an example of a list with numbers:

In [4]:

my_list = [1,2,4,65,43]
my_list

Out[4]:

[1, 2, 4, 65, 43]

Lists can also contain strings.

In [7]:

list_of_str = ['abc', 'goku', 'vada_pav', 'chicken_rice']

list_of_str

Out[7]:

['abc', 'goku', 'vada_pav', 'chicken_rice']

Single list can also contain different types of objects. Following is an example of a list containing number, string, another list and a dictionary.

In [9]:

list_mix = [3, 4, 'burger', ['Chicago', 23, 'Nile'], {'city_names': ['Delhi', 'London'], 'food_items': ['idli', 'mala-hotpot', 'rojak']}]
list_mix

Out[9]:

[3,
 4,
 'burger',
 ['Chicago', 23, 'Nile'],
 {'city_names': ['Delhi', 'London'],
  'food_items': ['idli', 'mala-hotpot', 'rojak']}]

When the command lines become too long we can break the line after a comma in the list.

In [11]:

list_mix = [3,
            4,
            'burger',
            ['Chicago', 23, 'Nile'],
            {'city_names': ['Delhi', 'London'],
             'food_items': ['idli', 'mala-hotpot', 'rojak']}]
list_mix

Out[11]:

[3,
 4,
 'burger',
 ['Chicago', 23, 'Nile'],
 {'city_names': ['Delhi', 'London'],
  'food_items': ['idli', 'mala-hotpot', 'rojak']}]

List indexing¶

List index starts with zero as all indexes in python. Following is the general format of indexing a list.

my_list[from_index:to_index:step]

The to_index is not inclusive. It means that the list item at to_index will not be included in the result of indexing. We will see this an example of a list of number from 0 to 20.

In [15]:

a = list(range(20))
a

Out[15]:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

In [18]:

a[2:18:3]

Out[18]:

[2, 5, 8, 11, 14, 17]

Here, we indexed the list from 2nd index to the 18th index with a step size of 3. The default stepsize is 1.

We will see different ways of indexing/slicing a list not.

From a defined index (e.g. 5) to end of the list.

In [21]:

a[5::]

Out[21]:

[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

From start of the list to a defined index (e.g. 15) including the item at that index.

In [23]:

a[:15+1]

Out[23]:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

In [24]:

a[:16]

Out[24]:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

The last element of a list can be indexed as -1, and the second last element is indexed as -2 and so on. Following are few example to index a list using negative numbers.

In [25]:

a[-1]

Out[25]:

19

In [26]:

a[-10:-1]

Out[26]:

[10, 11, 12, 13, 14, 15, 16, 17, 18]