Python Lists

Lists are are one of the 4 data types in Python used to store collections of data.

['John', 'Peter', 'Debora', 'Charles']

Getting individual values with indexes

>>> furniture = ['table', 'chair', 'rack', 'shelf']

>>> furniture[0]
# 'table'

>>> furniture[1]
# 'chair'

>>> furniture[2]
# 'rack'

>>> furniture[3]
# 'shelf'

Negative indexes

>>> furniture = ['table', 'chair', 'rack', 'shelf']

>>> furniture[-1]
# 'shelf'

>>> furniture[-3]
# 'chair'

>>> f'The {furniture[-1]} is bigger than the {furniture[-3]}'
# 'The shelf is bigger than the chair'

Getting sublists with Slices

>>> furniture = ['table', 'chair', 'rack', 'shelf']

>>> furniture[0:4]
# ['table', 'chair', 'rack', 'shelf']

>>> furniture[1:3]
# ['chair', 'rack']

>>> furniture[0:-1]
# ['table', 'chair', 'rack']

>>> furniture[:2]
# ['table', 'chair']

>>> furniture[1:]
# ['chair', 'rack', 'shelf']

>>> furniture[:]
# ['table', 'chair', 'rack', 'shelf']

Slicing the complete list will perform a copy:

>>> spam2 = spam[:]
# ['cat', 'bat', 'rat', 'elephant']

>>> spam.append('dog')
>>> spam
# ['cat', 'bat', 'rat', 'elephant', 'dog']

>>> spam2
# ['cat', 'bat', 'rat', 'elephant']

Getting a list length with len()

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> len(furniture)
# 4

Changing values with indexes

>>> furniture = ['table', 'chair', 'rack', 'shelf']

>>> furniture[0] = 'desk'
>>> furniture
# ['desk', 'chair', 'rack', 'shelf']

>>> furniture[2] = furniture[1]
>>> furniture
# ['desk', 'chair', 'chair', 'shelf']

>>> furniture[-1] = bed
>>> furniture
# ['desk', 'chair', 'chair', 'bed']

Concatenation and Replication

>>> [1, 2, 3] + ['A', 'B', 'C']
# [1, 2, 3, 'A', 'B', 'C']

>>> ['X', 'Y', 'Z'] * 3
# ['X', 'Y', 'Z', 'X', 'Y', 'Z', 'X', 'Y', 'Z']

>>> my_list = [1, 2, 3]
>>> my_list = my_list + ['A', 'B', 'C']
>>> my_list
# [1, 2, 3, 'A', 'B', 'C']

Using for loops with Lists

>>> furniture = ['table', 'chair', 'rack', 'shelf']

>>> for item in furniture:
...     print(item)
# table
# chair
# rack
# shelf

Getting the index in a loop with enumerate()

>>> furniture = ['table', 'chair', 'rack', 'shelf']

>>> for index, item in enumerate(furniture):
...     print(f'index: {index} - item: {item}')
# index: 0 - item: table
# index: 1 - item: chair
# index: 2 - item: rack
# index: 3 - item: shelf

Looping Through Multiple Lists with zip()

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> price = [100, 50, 80, 40]

>>> for item, amount in zip(furniture, price):
...     print(f'The {item} costs ${amount}')
# The table costs $100
# The chair costs $50
# The rack costs $80
# The shelf costs $40

The in and not in operators

>>> 'rack' in ['table', 'chair', 'rack', 'shelf']
# True

>>> 'bed' in ['table', 'chair', 'rack', 'shelf']
# False

>>> 'bed' not in furniture
# True

>>> 'rack' not in furniture
# False

The Multiple Assignment Trick

The multiple assignment trick is a shortcut that lets you assign multiple variables with the values in a list in one line of code. So instead of doing this:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> table = furniture[0]
>>> chair = furniture[1]
>>> rack = furniture[2]
>>> shelf = furniture[3]

You could type this line of code:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> table, chair, rack, shelf = furniture

>>> table
# 'table'

>>> chair
# 'chair'

>>> rack
# 'rack'

>>> shelf
# 'shelf'

The multiple assignment trick can also be used to swap the values in two variables:

>>> a, b = 'table', 'chair'
>>> a, b = b, a
>>> print(a)
# chair

>>> print(b)
# table

Finding a value with the index Method

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.index('chair')
# 1

Adding Values

append()

append adds an element to the end of a list:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.append('bed')
>>> furniture
# ['table', 'chair', 'rack', 'shelf', 'bed']

insert()

insert adds an element to a list at a given position:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.insert(1, 'bed')
>>> furniture
# ['table', 'bed', 'chair', 'rack', 'shelf']

Removing Values

del()

del removes an item using the index:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> del furniture[2]
>>> furniture
# ['table', 'chair', 'shelf']

>>> del furniture[2]
>>> furniture
# ['table', 'chair']

remove()

remove removes an item with using actual value of it:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.remove('chair')
>>> furniture
# ['table', 'rack', 'shelf']

Removing repeated items

If the value appears multiple times in the list, only the first instance of the value will be removed.

pop()

By default, pop will remove and return the last item of the list. You can also pass the index of the element as an optional parameter:

>>> animals = ['cat', 'bat', 'rat', 'elephant']

>>> animals.pop()
'elephant'

>>> animals
['cat', 'bat', 'rat']

>>> animals.pop(0)
'cat'

>>> animals
['bat', 'rat']

Sorting values with sort()

>>> numbers = [2, 5, 3.14, 1, -7]
>>> numbers.sort()
>>> numbers
# [-7, 1, 2, 3.14, 5]

furniture = ['table', 'chair', 'rack', 'shelf']
furniture.sort()
furniture
# ['chair', 'rack', 'shelf', 'table']

You can also pass True for the reverse keyword argument to have sort() sort the values in reverse order:

>>> furniture.sort(reverse=True)
>>> furniture
# ['table', 'shelf', 'rack', 'chair']

If you need to sort the values in regular alphabetical order, pass str.lower for the key keyword argument in the sort() method call:

>>> letters = ['a', 'z', 'A', 'Z']
>>> letters.sort(key=str.lower)
>>> letters
# ['a', 'A', 'z', 'Z']

You can use the built-in function sorted to return a new list:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> sorted(furniture)
# ['chair', 'rack', 'shelf', 'table']

The Tuple data type

Tuples vs Lists

The key difference between tuples and is that, while tuples are immutable objects, lists are mutable. This means that tuples cannot be changed while the lists can be modified. Tuples are more memory efficient than the lists.

>>> furniture = ('table', 'chair', 'rack', 'shelf')

>>> furniture[0]
# 'table'

>>> furniture[1:3]
# ('chair', 'rack')

>>> len(furniture)
# 4

The main way that tuples are different from lists is that tuples, like strings, are immutable.

Converting types with list() and tuple()

>>> tuple(['cat', 'dog', 5])
# ('cat', 'dog', 5)

>>> list(('cat', 'dog', 5))
# ['cat', 'dog', 5]

>>> list('hello')
# ['h', 'e', 'l', 'l', 'o']