A list stores a series of items in a particular order. Lists allow you to store sets of information in one place, whether you have just a few items or millions of items. Lists are one of Python’s most powerful features readily accessible to new programmers, and they tie together many important concepts in programming.
Defining a list
Use square brackets to define a list, and use commas to separate individual items in the list. Use plural names for lists, to make your code easier to read.
Making a list
users = ['val', 'bob', 'mia', 'ron', 'ned']Accessing Elements
Individual elements in a list are accessed according to their position, called the index. The index of the first element is 0, the index of the second element is 1, and so forth. Negative indices refer to items at the end of the list. To get a particular element, write the name of the list and then the index of the element in square brackets.
Getting the first element
first_user = users[0]
Getting the second element
second_user = users[1]
Getting the last element
newest_user = users[-1]
Modifying individual items
Once you’ve defined a list, you can change individual elements in the list. You do this by referring to the index of the item you want to modify.
Changing an element
users[0] = 'valerie'
users[-2] = 'ronald'
Adding element
You can add elements to the end of a list, or you can insert
them wherever you like in a list.
Adding an element to the end of the list
users.append('amy')
Starting with an empty list
users = []
users.append('val')
users.append('bob')
users.append('mia')
Inserting elements at a particular position
users.insert(0, 'joe')
users.insert(3, 'bea')
Removing Element
You can remove elements by their position in a list, or by the value of the item. If you remove an item by its value, Python removes only the first item that has that value.
Deleting an element by its position
del users[-1]
Removing an item by its value
users.remove('mia')
Popping Elements
If you want to work with an element that you’re removing from the list, you can “pop” the element. If you think of the list as a stack of items, pop() takes an item off the top of the stack. By default pop() returns the last element in the list, but you can also pop elements from any position in the list.
Pop the last item from a list
most_recent_user = users.pop()
print(most_recent_user)
Pop the first item in a list
first_user = users.pop(0)
print(first_user)
Get length of a list
The len() function returns the number of items in a list.
Find the length of a list
num_users = len(users)
print("We have " + str(num_users) + " users.")
Sorting a list
The sort() method changes the order of a list permanently. The sorted() function returns a copy of the list, leaving the original list unchanged. You can sort the items in a list in alphabetical order, or reverse alphabetical order. You can also reverse the original order of the list. Keep in mind that lowercase and uppercase letters may affect the sort order.
Sorting a list permanently
users.sort()
Sorting a list permanently in reverse alphabetical order
users.sort(reverse=True)
Sorting a list temporarily
print(sorted(users))
print(sorted(users, reverse=True))
Reversing the order of a list
users.reverse()
Looping through the list
Lists can contain millions of items, so Python provides an efficient way to loop through all the items in a list. When you set up a loop, Python pulls each item from the list one at a time and stores it in a temporary variable, which you provide a name for. This name should be the singular version of the list name.
The indented block of code makes up the body of the loop, where you can work with each individual item. Any lines that are not indented run after the loop is completed.
Printing all items in a list
for user in users:
print(user)
Printing a message for each item, and a separate message afterward
for user in users:
print("Welcome, " + user + "!")
print("Welcome, we're glad to see you all!")
Looping the range of number
You can use the range() function to work with a set of numbers efficiently. The range() function starts at 0 by default and stops one number below the number passed to it. You can use the list() function to efficiently generate a large list of numbers.
Printing the numbers 0 to 1000
for number in range(1001):
print(number)
Printing the numbers 1 to 1000
for number in range(1, 1001):
print(number)
Making a list of numbers from 1 to a million
numbers = list(range(1, 1000001))
Simple Statistic
There are a number of simple statistics you can run on a list containing numerical data.
Finding the minimum value in a list
ages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77]
youngest = min(ages)
Finding the maximum value
ages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77]
oldest = max(ages)
Finding the sum of all values
ages = [93, 99, 66, 17, 85, 1, 35, 82, 2, 77]
total_years = sum(ages)
Slicing a list
You can work with any set of elements from a list. A portion of a list is called a slice. To slice a list start with the index of the first item you want, then add a colon and the index after the last item you want. Leave off the first index to start at the beginning of the list, and leave off the last index to slice through the end of the list.
Getting the first three items
finishers = ['kai', 'abe', 'ada', 'gus', 'zoe']
first_three = finishers[:3]
Getting the middle three items
middle_three = finishers[1:4]
Getting the last three items
last_three = finishers[-3:]
Copying a list in python
To copy a list make a slice that starts at the first item and ends at the last item. If you try to copy a list without using this approach, whatever you do to the copied list will affect the original list as well.
Making a copy of a list
finishers = ['kai', 'abe', 'ada', 'gus', 'zoe']
copy_of_finishers = finishers[:]
Python list comprehensions
You can use a loop to generate a list based on a range of numbers or on another list. This is a common operation, so Python offers a more efficient way to do it. List comprehensions may look complicated at first; if so, use the for loop approach until you’re ready to start using comprehensions.
To write a comprehension, define an expression for the values you want to store in the list. Then write a for loop to generate input values needed to make the list.
Using a loop to generate a list of square numbers
squares = []
for x in range(1, 11):
square = x**2
squares.append(square)
Using a comprehension to generate a list of square numbers
squares = [x**2 for x in range(1, 11)]
Using a loop to convert a list of names to upper case
names = ['kai', 'abe', 'ada', 'gus', 'zoe']
upper_names = []
for name in names:
upper_names.append(name.upper())Using a comprehension to convert a list of names to upper case
names = ['kai', 'abe', 'ada', 'gus', 'zoe']
upper_names = [name.upper() for name in names]
Python Tuples
A tuple is like a list, except you can’t change the values in a tuple once it’s defined. Tuples are good for storing information that shouldn’t be changed throughout the life of a program. Tuples are designated by parentheses instead of square brackets. (You can overwrite an entire tuple, but you can’t change the individual elements in a tuple.)
Defining a tuple
dimensions = (800, 600)
Looping through a tuple
for dimension in dimensions:
print(dimension)
Overwriting a tuple
dimensions = (800, 600)
print(dimensions)
dimensions = (1200, 900)