Everything about lists¶

Lists are containers, containing items indexed by their location.

Location starts at 0 from left and -1 from right.

>>> animals
['dog', 'cat', 'pig']
>>> animals[0]
'dog'
>>> animals[1]
'cat'
>>> animals[2]
'pig'
>>> animals[3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> animals[-1]
'pig'
>>> animals[-2]
'cat'
>>> animals[-3]
'dog'
>>> animals[-4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

Valid list indices are between (0, len(L)-1) and (-1, -len(L)).

List operations¶

These operations do not return anything and alter the list that they are applied to (append, insert, remove, sort):

>>> animals
['dog', 'cat', 'pig']
>>> animals.append('cow')
>>> animals
['dog', 'cat', 'pig', 'cow']
>>> animals.insert(2,'bird')
>>> animals
['dog', 'cat', 'bird', 'pig', 'cow']
>>> animals.remove('cat')
>>> animals.remove('cat')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list
>>> animals
['dog', 'bird', 'pig', 'cow']
>>> animals.sort()
>>> animals
['bird', 'cow', 'dog', 'pig']

This operation returns a value from the list and modifies the list:

>>> animals
['dog', 'cat', 'pig', 'cow']
>>> animal = animals.pop()
>>> animals
['dog', 'cat', 'pig']
>>> animal
'cow'

These operations return a new list by copying the top level element in each list, hence called a shallow copy (+,*,list,sorted,slicing):

>>> birds = ['cardinal','bluebird','sparrow']
>>> animals = ['dog','cat','pig']
>>>
>>> birds + animals
['cardinal', 'bluebird', 'sparrow', 'dog', 'cat', 'pig']
>>> multibird = birds*2
>>> multibird
['cardinal', 'bluebird', 'sparrow', 'cardinal', 'bluebird', 'sparrow']
>>> birds_copy = list(birds)
>>> allanimals = birds + animals
>>> allanimals.append('hippo')
>>> someanimals = animals[1:3]
>>> someanimals
['cat', 'pig']
>>> allanimals
['cardinal', 'bluebird', 'sparrow', 'dog', 'cat', 'pig', 'hippo']
>>> birds
['cardinal', 'bluebird', 'sparrow']
>>> animals
['dog', 'cat', 'pig']
>>> ### see original lists are not changed

Slicing produces a new list, by copy and pasting values from the original list, L[s:e:i], from starting index s, to ending index e and incrementing by i.

>>> allanimals
['cardinal', 'bluebird', 'sparrow', 'dog', 'cat', 'pig', 'hippo']
>>> allanimals[1:5:2]
['bluebird', 'dog']
>>> allanimals[-2:-4]
[]
>>> allanimals[-2:-4:-1]
['pig', 'cat']
>>> allanimals[::-1]
['hippo', 'pig', 'cat', 'dog', 'sparrow', 'bluebird', 'cardinal']

You can apply the same principles to list of lists.

>>> names = [ ['Sun', 500], ['Moon', 200] ]
>>> names[0]
['Sun', 500]
>>> names[0][0]
'Sun'
>>> names[0][1]
500
>>> names[1]
['Moon', 200]
>>> names[1][-1]
200
>>> names[0][:1]
['Sun']
>>> names[0].append(200)
>>> names
[['Sun', 500, 200], ['Moon', 200]]

As this is a list of lists, then the 0th item is a list. You can apply the same list functions to this list as well.