Python List Tutorial

Section Overview

• Lists
• Slices
• Exception Handling
• Looping through a list
• Sorting and Ranges
• Ranges

Section 1. Lists

• Lists are one of the most powerful data types in Python.
• It is a data type that holds an ordered collection of items.
• The items can be of various data types. You can even have lists of lists.

Lists are created using comma separated values betweeen square brackets. The format is:

list_name = [item_1, item_2, ..., item_N]
list_name = []

Using our understanding of Python types, we might think we could store each data point in its own variable. For instance, this is how we might store the first row’s data points:

first_name = 'Erdi'
last_name = 'Mollahüseyin'
email = 'erdimollahuseyin@gmail.com'
age = 29

Above, we stored:

• The first name “Erdi” as a string
• The last name “Mollahüseyin” as a string
• The email “erdimollahuseyin@gmail.com” as a string
• The age 29 as a int

We can store data more efficiently using lists. This is how we can create a list of data points for the first row:

user_row_1 = ['Erdi', 'Mollahüseyin', 'erdimollahuseyin@gmail.com', 29]
print(user_row_1)
['Erdi', 'Mollahüseyin', 'erdimollahuseyin@gmail.com', 29]
type(user_row_1)
list

After we created the list, we stored it in the computer’s memory by assigning it to a variable named users.

To create a list of data points, we only need to:

• Separate the data points with a comma.
• Surround the sequence of data points with brackets.

Now let’s create 3 lists:

user_row_1 = ['Erdi', 'Mollahüseyin', 'erdimollahuseyin@gmail.com', 29]
user_row_2 = ['Cengiz', 'Under', 'cengizunder@me.com', 23]
user_row_3 = ['Burak', 'Yilmaz', 'burakyilmaz@me.com', 32]

• A list like [7, 8, 9] has identical data types (only integers)
• While the list [‘Erdi’, ‘Mollahüseyin’, ‘erdimollahuseyin@gmail.com’, 29] has mixed data types:
• Three strings (‘Erdi’, ‘Mollahüseyin’, ‘erdimollahuseyin@gmail.com’)
• One integer (29)

The [‘Erdi’, ‘Mollahüseyin’, ‘erdimollahuseyin@gmail.com’, 29] list has four data points. To find the length of a list, we can use the len() command:

user_row_1 = ['Erdi', 'Mollahüseyin', 'erdimollahuseyin@gmail.com', 29]
print(len(user_row_1))
4
list_1 = [7, 8, 9] 
print(len(list_1))
3
list_2 = []
print(len(list_2))
0

In another example is drinks list that items in a list can be accessed by index. List indices are zero based. The format is:

list_name[index]
drinks = ['water', 'tea', 'orange juice', 'beer']
print(drinks)
['water', 'tea', 'orange juice', 'beer']
type(drinks)
list
print(drinks[0])
water
print(drinks[1])
tea
print(drinks[2])
orange juice
print(drinks[3])
beer
drinks[0] = 'cider'
print(drinks[0])
cider

Access items from the end of the list by using negative indices. The last item in a list is:

list_name[-1]
drinks = ['water', 'tea', 'orange juice', 'beer']
print(drinks[-1])
beer
print(drinks[-2])
orange juice
print(drinks[-3])
tea
print(drinks[-3])
water

Add items to a list by using the append() or extend() list methods.

drinks = ['water', 'tea', 'orange juice', 'beer']
drinks.append('wine')
print(drinks[-1])
wine
drinks = ['water', 'tea', 'orange juice', 'beer']
drinks.extend(['cider', 'wine'])
print(drinks)
['water', 'tea', 'orange juice', 'beer', 'cider', 'wine']
drinks = ['water', 'tea', 'orange juice', 'beer']
more_drinks = ['cider', 'wine']
drinks.extend(more_drinks)
print(drinks)
['water', 'tea', 'orange juice', 'beer', 'cider', 'wine']
drinks = ['water', 'tea', 'orange juice', 'beer']
drinks.insert(0, 'wine')
print(drinks)
['wine', 'water', 'tea', 'orange juice', 'beer']
drinks.insert(2, 'cider')
print(drinks)
['wine', 'water', 'cider', 'tea', 'orange juice', 'beer']

Section 2. Slices

Access a portion of a list using a slice. The format is:

list_name(start, stop)
list[index1:index2]
list[:index2]
list[index1:]
drinks = ['wine', 'water', 'cider', 'tea', 'orange juice', 'beer']
some_drinks = drinks[1:4]
print('Some drinks: {}'.format(some_drinks))
Some drinks: ['water', 'cider', 'tea']
first_two = drinks[0:2]
print('First two drinks: {}'.format(first_two))
First two drinks: ['wine', 'water']
first_two_again = drinks[:2]
print('First two drinks: {}'.format(first_two_again))
First two drinks: ['wine', 'water']
drinks = ['wine', 'water', 'cider', 'tea', 'orange juice', 'beer']
last_two = drinks[4:6]
print('Last two drinks: {}'.format(last_two))
Last two drinks: ['orange juice', 'beer']
last_two_again = drinks[-2:]
print('Last two drinks: {}'.format(last_two_again))
Last two drinks: ['orange juice', 'beer']

# String Slices
part_of_a_water = 'water'[1:3]
print(part_of_a_water)
at

Section 3. Exception Handling

The list index() method accepts a value as a parameter and returns the index of the first value in the list or an exception if the value is not in the list.

# Finding an item in a list.
drinks = ['wine', 'water', 'cider', 'tea', 'orange juice', 'beer']
drinks_index = drinks.index('water')
print(drinks_index)
1
# Exception
drinks = ['water', 'tea', 'orange juice', 'beer']
wine_index = drinks.index('wine')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-42-d14f756f0077> in <module>
----> 1 wine_index = drinks.index('wine')
ValueError: 'wine' is not in list
drinks = ['water', 'tea', 'orange juice', 'beer']
try: 
    wine_index = drinks.index('wine') 
except: 
    wine_index = 'No wines found.' 
print(wine_index)
No wines found.

Unhandled exceptions cause Python programs to terminate. Handle exceptions using try/except blocks.

Section 4. Looping through a list

Section 4.1 FOR — LOOP

Loop through a list using a for loop. The format is:

for item_variable in list_name:
    # Code block
item_variable = list[0]
item_variable = list[1]
item_variable = list[N]
drinks = ['water', 'tea', 'orange juice', 'beer']
for drink in drinks:
    print(drink.upper())
WATER
TEA
ORANGE JUICE
BEER

Section 4.2 WHILE — LOOP

The code block in a while loop executes as long as the condition evaluates to true. The format is:

while condition
    # Code block
drinks = ['water', 'tea', 'orange juice', 'beer']
index = 0
while index < len(drinks):
    print(drinks[index])
    index += 1
water
tea
orange juice
beer

Section 5. Sorting and Ranges

To sort a list, use the sort() list method or hee built-in sorted() function.

drinks = ['water', 'tea', 'orange juice', 'beer']
print('Drinks list: {}'.format(drinks))
Drinks list: ['water', 'tea', 'orange juice', 'beer']
sorted_drinks = sorted(drinks)
print('Sorted animals list: {}'.format(sorted_drinks))
Sorted animals list: ['beer', 'orange juice', 'tea', 'water']
drinks.sort()
print('Drinks after sort method: {}'.format(drinks))
Drinks after sort method: ['beer', 'orange juice', 'tea', 'water']
drinks = ['water', 'tea', 'orange juice', 'beer']
more_drinks = ['wine']
all_drinks = drinks + more_drinks
print(all_drinks)
['water', 'tea', 'orange juice', 'beer', 'wine']
drinks = ['water', 'tea', 'orange juice', 'beer']
print(len(drinks))
4
drinks.append('wine')
print(len(drinks))
5

Section 6. Ranges

The built-in range() function generates a list of numbers. The format is:

for number in range(3):
    print(number)
0
1
2
for number in range(1, 3):
    print(number)
1
2
for number in range(1, 10, 2):
    print(number)
1
3
5
7
9
drinks = ['water', 'tea', 'orange juice', 'beer']
for number in range(0, len(drinks), 2):
    print(drinks[number])
water
orange juice

Section 7. Conclusion

In this article you’ve learned how to use a list with the Python programming.