- VARIABLES AND SIMPLE DATA TYPES
- INTRODUCING LISTS
3.1. Accessing Elements in a List
3.2. Using Individual Values from a List
3.3. Adding Elements to a List
3.3.1. Appending Elements to the End of a List
3.3.2. Inserting Elements into a List
3.4. Removing Elements from a List
3.4.1. Removing an Item Using the del Statement
3.4.2. Removing an Item Using the pop() Method
3.4.3. Removing an Item by Value
3.5. Organizing a List
3.5.1. Sorting a List Permanently with the sort() Method
3.5.2. Sorting a List Temporarily with the sorted() Function
3.5.3. Reverse the original order of a list
3.5.4. Finding the Length of a List - WORKING WITH LISTS
4.1. Looping Through an Entire List
4.2. Using the range() Function
4.2.1. Using range() to Make a List of Numbers
4.3. Simple Statistics with a List of Numbers
4.4. List Comprehensions
4.5. Working with Part of a List
4.5.1. Slicing a List
4.5.2. Looping Through a Slice
4.5.3. Copying a List
4.6. Tuples
4.6.1. Defining a Tuple
4.6.2. Writing Over a Tuple - IF STATEMENTS
5.1. Conditional Tests
5.1.1. Checking for Equality
5.1.2. Checking for Inequality
5.1.3. Numerical Comparisons
5.1.4. Using and to Check Multiple Conditions
5.1.5. Using or to Check Multiple Conditions
5.2. Checking Whether a Value Is in a List
5.3. Checking Whether a Value Is Not in a List
5.4. Boolean Expressions
5.5. Simple if Statements
5.6. The if-elif-else Chain - DICTIONARIES
6.1. A Simple Dictionary
6.2. Adding New Key-Value Pairs
6.3. Modifying Values in a Dictionary
6.4. Removing Key-Value Pairs
6.5. Using get() to Access Values
Variable names can contain only letters, numbers, and underscores. They can start with a letter or an underscore, but not with a number. For instance, you can call a variable message_1 but not 1_message.
Spaces are not allowed in variable names, but underscores can be used to separate words in variable names. For example, greeting_message works but greeting message will cause errors.
The Python variables you’re using at this point should be lowercase.
A list is a collection of items in a particular order.
Because a list usually contains more than one element, it’s a good idea to make the name of your list plural, such as letters, digits, or names.
Square brackets ([]) indicate a list, and individual elements in the list are separated by commas.
bicycles = ['trek', 'cannondale', 'redline', 'specialized']
print(bicycles) # ['trek', 'cannondale', 'redline', 'specialized']you can access any element in a list by telling Python the position of the item desired.
bicycles = ['trek', 'cannondale', 'redline', 'specialized']
print(bicycles[0]) # trekYou can use individual values from a list just as you would any other variable. For example, you can use f-strings to create a message based on a value from a list.
bicycles = ['trek', 'cannondale', 'redline', 'specialized']
message = f"My first bicycle was a {bicycles[0].title()}."
print(message) # My first bicycle was a Trek.The simplest way to add a new element to a list is to append the item to the list. The new element is added to the end of the list.
motorcycles = ['honda', 'yamaha', 'suzuki']
print(motorcycles) # ['honda', 'yamaha', 'suzuki']
motorcycles.append('ducati')
print(motorcycles) # ['honda', 'yamaha', 'suzuki', 'ducati']You can add a new element at any position in your list by using the insert() method.
motorcycles = ['honda', 'yamaha', 'suzuki']
motorcycles.insert(0, 'ducati')
print(motorcycles) # ['ducati', 'honda', 'yamaha', 'suzuki']If you know the position of the item you want to remove from a list, you can use the del statement.
motorcycles = ['honda', 'yamaha', 'suzuki']
print(motorcycles) # ['honda', 'yamaha', 'suzuki']
del motorcycles[0]
print(motorcycles) # ['yamaha', 'suzuki']The pop() method removes the an item in a list, but it lets you work with that item after removing it. If you use only pop() method removes the last item. If you pass a argument like pop(0) method removes the element.
motorcycles = ['honda', 'yamaha', 'suzuki']
print(motorcycles) # ['honda', 'yamaha', 'suzuki']
popped_motorcycle = motorcycles.pop()
print(motorcycles) # ['honda', 'yamaha']
print(popped_motorcycle) # suzukimotorcycles = ['honda', 'yamaha', 'suzuki']
first_owned = motorcycles.pop(0)
print(f"The first motorcycle I owned was a {first_owned.title()}.") # The first motorcycle I owned was a Honda.Sometimes you won’t know the position of the value you want to remove from a list. If you only know the value of the item you want to remove, you can use the remove() method.
motorcycles = ['honda', 'yamaha', 'suzuki', 'ducati']
print(motorcycles) # ['honda', 'yamaha', 'suzuki', 'ducati']
motorcycles.remove('ducati')
print(motorcycles) # ['honda', 'yamaha', 'suzuki']*The remove() method deletes only the first occurrence of the value you specify. If there’s a possibility the value appears more than once in the list, you’ll need to use a loop to make sure all occurrences of the value are removed.
cars = ['bmw', 'audi', 'toyota', 'subaru']
cars.sort()
print(cars) # ['audi', 'bmw', 'subaru', 'toyota']You can also sort this list in reverse-alphabetical order by passing the argument reverse=True to the sort() method.
cars = ['bmw', 'audi', 'toyota', 'subaru']
cars.sort(reverse=True)
print(cars) # ['toyota', 'subaru', 'bmw', 'audi']To maintain the original order of a list but present it in a sorted order, you can use the sorted() function.
cars = ['bmw', 'audi', 'toyota', 'subaru']
print("Here is the original list:") # Here is the original list:
print(cars) # ['bmw', 'audi', 'toyota', 'subaru']
print("\nHere is the sorted list:") # Here is the sorted list:
print(sorted(cars)) # ['audi', 'bmw', 'subaru', 'toyota']
print("\nHere is the original list again:") # Here is the original list again:
print(cars) # ['bmw', 'audi', 'toyota', 'subaru']The sorted() function can also accept a reverse=True argument if you want to display a list in reverse-alphabetical order.
To reverse the original order of a list, you can use the reverse() method.
cars = ['bmw', 'audi', 'toyota', 'subaru']
print(cars) # ['bmw', 'audi', 'toyota', 'subaru']
cars.reverse()
print(cars) # ['subaru', 'toyota', 'audi', 'bmw']You can quickly find the length of a list by using the len() function.
>>> cars = ['bmw', 'audi', 'toyota', 'subaru']
>>> len(cars)
4*Python counts the items in a list starting with one, so you shouldn’t run into any off-by-one errors when determining the length of a list.
Keep in mind that whenever you want to access the last item in a list, you should use the index -1. This will always work, even if your list has changed size since the last time you accessed it:
motorcycles = ['honda', 'yamaha', 'suzuki']
print(motorcycles[-1]) # suzukiThe only time this approach will cause an error is when you request the last item from an empty list.
When you want to do the same action with every item in a list, you can use Python’s for loop.
magicians = ['alice', 'david', 'carolina']
for magician in magicians:
print(f"{magician.title()}, that was a great trick!")
# Alice, that was a great trick!
# David, that was a great trick!
# Carolina, that was a great trick!When writing your own for loops that you can choose any name you want for the temporary variable that will be associated with each value in the list. However, it’s helpful to choose a meaningful name that represents a single item from the list.
for cat in cats:
for dog in dogs:
for item in list_of_items:Python’s range() function makes it easy to generate a series of numbers.
for value in range(1, 5):
print(value)
# 1
# 2
# 3
# 4If you want to make a list of numbers, you can convert the results of range() directly into a list using the list() function.
numbers = list(range(1, 6))
print(numbers) # [1, 2, 3, 4, 5]We can also use the range() function to tell Python to skip numbers in a given range. If you pass a third argument to range(), Python uses that value as a step size when generating numbers.
even_numbers = list(range(2, 11, 2))
print(even_numbers) # [2, 4, 6, 8, 10]A few Python functions are helpful when working with lists of numbers. For example, you can easily find the minimum, maximum, and sum of a list of numbers:
>>> digits = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
>>> min(digits)
0
>>> max(digits)
9
>>> sum(digits)
45A list comprehension allows you to generate lists in just one line of code. A list comprehension combines the for loop and the creation of new elements into one line, and automatically appends each new element.
squares = [value**2 for value in range(1, 11)]
print(squares) # [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]To make a slice, you specify the index of the first and last elements you want to work with.
players = ['charles', 'martina', 'michael', 'florence', 'eli']
print(players[0:3]) # ['charles', 'martina', 'michael']If you omit the first index in a slice, Python automatically starts your slice at the beginning of the list:
players = ['charles', 'martina', 'michael', 'florence', 'eli']
print(players[:4]) # ['charles', 'martina', 'michael', 'florence']A similar syntax works if you want a slice that includes the end of a list.
players = ['charles', 'martina', 'michael', 'florence', 'eli']
print(players[2:]) # ['michael', 'florence', 'eli']You can use a slice in a for loop if you want to loop through a subset of the elements in a list.
players = ['charles', 'martina', 'michael', 'florence', 'eli']
print("Here are the first three players on my team:") # Here are the first three players on my team:
for player in players[:3]:
print(player.title())
# Charles
# Martina
# Michaelmy_foods = ['pizza', 'falafel', 'carrot cake']
friend_foods = my_foods[:]
my_foods.append('cannoli')
friend_foods.append('ice cream')
print("My favorite foods are:") # My favorite foods are:
print(my_foods) # ['pizza', 'falafel', 'carrot cake', 'cannoli']
print("\nMy friend's favorite foods are:") # My friend's favorite foods are:
print(friend_foods) # ['pizza', 'falafel', 'carrot cake', 'ice cream']Sometimes you’ll want to create a list of items that cannot change. Python refers to values that cannot change as immutable, and an immutable list is called a tuple.
A tuple looks just like a list, except you use parentheses instead of square brackets.
dimensions = (200, 50)
print(dimensions[0]) # 200
print(dimensions[1]) # 50You can loop over all the values in a tuple using a for loop, just as you did with a list.
Although you can’t modify a tuple, you can assign a new value to a variable that represents a tuple.
dimensions = (200, 50)
print("Original dimensions:") # Original dimensions:
for dimension in dimensions: # 200
print(dimension) # 50
dimensions = (400, 100)
print("\nModified dimensions:") # Modified dimensions:
for dimension in dimensions: # 400
print(dimension) # 100The following example shows how if tests let you respond to special situations correctly.
cars = ['audi', 'bmw', 'subaru', 'toyota']
for car in cars:
if car == 'bmw':
print(car.upper())
else:
print(car.title())
# Audi
# BMW
# Subaru
# Toyota>>> car = 'bmw'
>>> car == 'bmw'
Truerequested_topping = 'mushrooms'
if requested_topping != 'anchovies':
print("Hold the anchovies!")
# Hold the anchovies!>>> age = 19
>>> age < 21
True
>>> age <= 21
True
>>> age > 21
False
>>> age >= 21
False>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 and age_1 >= 21
False
>>> age_1 = 22
>>> age_0 >= 21 and age_1 >= 21
True>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 or age_1 >= 21
True
>>> age_0 = 18
>>> age_0 >= 21 or age_1 >= 21
FalseTo find out whether a particular value is already in a list, use the keyword in.
>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'mushrooms' in requested_toppings
True
>>> 'pepperoni' in requested_toppings
Falsebanned_users = ['andrew', 'carolina', 'david']
user = 'marie'
if user not in banned_users:
print(f"{user.title()}, you can post a response if you wish.")
# Marie, you can post a response if you wish.A Boolean expression is just another name for a conditional test.
age = 17
if age >= 18:
print("You are old enough to vote!")
print("Have you registered to vote yet?")
else:
print("Sorry, you are too young to vote.")
print("Please register to vote as soon as you turn 18!")age = 12
if age < 4:
price = 0
elif age < 18:
price = 25
else:
price = 40
print(f"Your admission cost is ${price}.")
# Your admission cost is $25.You can use as many elif blocks in your code as you like. Python does not require an else block at the end of an if-elif chain.
A dictionary in Python is a collection of key-value pairs.
To get the value associated with a key, give the name of the dictionary and then place the key inside a set of square brackets.
alien_0 = {'color': 'green', 'points': 5}
print(alien_0['color']) # green
print(alien_0['points']) # 5Dictionaries are dynamic structures, and you can add new key-value pairs to a dictionary at any time. To add a new key-value pair, you would give the name of the dictionary followed by the new key in square brackets, along with the new value.
alien_0 = {'color': 'green', 'points': 5}
print(alien_0) # {'color': 'green', 'points': 5}
alien_0['x_position'] = 0
alien_0['y_position'] = 25
print(alien_0) # {'color': 'green', 'points': 5, 'x_position': 0, 'y_position': 25}To modify a value in a dictionary, give the name of the dictionary with the key in square brackets and then the new value you want associated with that key.
alien_0 = {'color': 'green'}
print(f"The alien is {alien_0['color']}.") # The alien is green.
alien_0['color'] = 'yellow'
print(f"The alien is now {alien_0['color']}.") # The alien is now yellow.When you no longer need a piece of information that’s stored in a dictionary, you can use the del statement to completely remove a key-value pair.
alien_0 = {'color': 'green', 'points': 5}
print(alien_0) # {'color': 'green', 'points': 5}
del alien_0['points']
print(alien_0) # {'color': 'green'}Using keys in square brackets to retrieve the value you’re interested in from a dictionary might cause one potential problem: if the key you ask for doesn’t exist, you’ll get an error.
The get() method requires a key as a first argument. As a second optional argument, you can pass the value to be returned if the key doesn’t exist:
alien_0 = {'color': 'green', 'speed': 'slow'}
point_value = alien_0.get('points', 'No point value assigned.')
print(point_value) # No point value assigned.If there’s a chance the key you’re asking for might not exist, consider using the get() method instead of the square bracket notation.
You can loop through all of a dictionary’s key-value pairs, through its keys, or through its values.
user_0 = {
'username': 'efermi',
'first': 'enrico',
'last': 'fermi',
}
for key, value in user_0.items():
print(f"\nKey: {key}")
print(f"Value: {value}")
# Key: username
# Value: efermi
# Key: first
# Value: enrico
# Key: last
# Value: fermiThe keys() method is useful when you don’t need to work with all of the values in a dictionary. Let’s loop through the favorite_languages dictionary and print the names of everyone who took the poll:
favorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'rust',
'phil': 'python',
}
for name in favorite_languages.keys():
print(name.title())
# Jen
# Sarah
# Edward
# Phil Looping through the keys is actually the default behavior when looping through a dictionary, so this code would have exactly the same output if you wrote: for name in favorite_languages:
You can choose to use the keys() method explicitly if it makes your code easier to read, or you can omit it if you wish.
favorite_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'rust',
'phil': 'python',
}
print("The following languages have been mentioned:") # The following languages have been mentioned:
for language in favorite_languages.values():
print(language.title())
# Python
# C
# Rust
# PythonTo see each language chosen without repetition, we can use a set(). A set is a collection in which each item must be unique:
favorite_languages = {
--snip--
}
print("The following languages have been mentioned:") #
for language in set(favorite_languages.values()):
print(language.title())
# Python
# C
# RustYou can build a set directly using braces and separating the elements with commas:
>>> languages = {'python', 'rust', 'python', 'c'}
>>> languages
{'rust', 'python', 'c'}It’s easy to mistake sets for dictionaries because they’re both wrapped in braces. When you see braces but no key-value pairs, you’re probably looking at a set. Unlike lists and dictionaries, sets do not retain items in any specific order.
alien_0 = {'color': 'green', 'points': 5}
alien_1 = {'color': 'yellow', 'points': 10}
alien_2 = {'color': 'red', 'points': 15}
aliens = [alien_0, alien_1, alien_2]
for alien in aliens:
print(alien)A more realistic example would involve more than three aliens with code that automatically generates each alien. In the following example, we use range() to create a fleet of 30 aliens:
# Make an empty list for storing aliens.
aliens = []
# Make 30 green aliens.
for alien_number in range(30):
new_alien = {'color': 'green', 'points': 5, 'speed': 'slow'}
aliens.append(new_alien)
# Show the first 5 aliens.
for alien in aliens[:5]:
print(alien)
print("...")
# Show how many aliens have been created.
print(f"Total number of aliens: {len(aliens)}")These aliens all have the same characteristics, but Python considers each one a separate object, which allows us to modify each alien individually.
# Store information about a pizza being ordered.
pizza = {
'crust': 'thick',
'toppings': ['mushrooms', 'extra cheese'],
}
print(f"\nYou ordered a {pizza['crust']}-crust pizza with the following toppings:")
for topping in pizza['toppings']:
print(f"\t{topping}")You should not nest lists and dictionaries too deeply
You can nest a dictionary inside another dictionary, but your code can get complicated quickly when you do. For example, if you have several users for a website, each with a unique username, you can use the usernames as the keys in a dictionary.
users = {
'aeinstein': {
'first': 'albert',
'last': 'einstein',
'location': 'princeton',
},
'mcurie': {
'first': 'marie',
'last': 'curie',
'location': 'paris',
},
}
for username, user_info in users.items():
print(f"\nUsername: {username}")
full_name = f"{user_info['first']} {user_info['last']}"
location = user_info['location']
print(f"\tFull name: {full_name.title()}")
print(f"\tLocation: {location.title()}")
# Username: aeinstein
# Full name: Albert Einstein
# Location: Princeton
# Username: mcurie
# Full name: Marie Curie
# Location: ParisNotice that the structure of each user’s dictionary is identical. Although not required by Python, this structure makes nested dictionaries easier to work with. If each user’s dictionary had different keys, the code inside the for loop would be more complicated.
The input() function pauses your program and waits for the user to enter some text.
message = input("Tell me something, and I will repeat it back to you: ") # Tell me something, and I will repeat it back to you: Hello everyone!
print(message) # Hello everyone!Sometimes you’ll want to write a prompt that’s longer than one line. For example, you might want to tell the user why you’re asking for certain input. You can assign your prompt to a variable and pass that variable to the input() function. This allows you to build your prompt over several lines, then write a clean input() statement.
prompt = "If you share your name, we can personalize the messages you see."
prompt += "\nWhat is your first name? "
name = input(prompt)
print(f"\nHello, {name}!")
# If you share your name, we can personalize the messages you see.
# What is your first name? Eric
# Hello, Eric!current_number = 1
while current_number <= 5:
print(current_number)
current_number += 1
# 1
# 2
# 3
# 4
# 5prompt = "\nTell me something, and I will repeat it back to you:"
prompt += "\nEnter 'quit' to end the program. "
message = ""
while message != 'quit':
message = input(prompt)
print(message)
if message != 'quit':
print(message)prompt = "\nTell me something, and I will repeat it back to you:"
prompt += "\nEnter 'quit' to end the program. "
active = True
while active:
message = input(prompt)
if message == 'quit':
active = False
else:
print(message)prompt = "\nPlease enter the name of a city you have visited:"
prompt += "\n(Enter 'quit' when you are finished.) "
while True:
city = input(prompt)
if city == 'quit':
break
else:
print(f"I'd love to go to {city.title()}!")current_number = 0
while current_number < 10:
current_number += 1
if current_number % 2 == 0:
continue
print(current_number)
# 1
# 3
# 5
# 7
# 9# Start with users that need to be verified,
# and an empty list to hold confirmed users.
unconfirmed_users = ['alice', 'brian', 'candace']
confirmed_users = []
# Verify each user until there are no more unconfirmed users.
# Move each verified user into the list of confirmed users.
while unconfirmed_users:
current_user = unconfirmed_users.pop()
print(f"Verifying user: {current_user.title()}")
confirmed_users.append(current_user)
# Display all confirmed users.
print("\nThe following users have been confirmed:")
for confirmed_user in confirmed_users:
print(confirmed_user.title())
# Verifying user: Candace
# Verifying user: Brian
# Verifying user: Alice
# The following users have been confirmed:
# Candace
# Brian
# Alicepets = ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
print(pets)
while 'cat' in pets:
pets.remove('cat')
print(pets)
# ['dog', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
# ['dog', 'dog', 'goldfish', 'rabbit']responses = {}
# Set a flag to indicate that polling is active.
polling_active = True
while polling_active:
# Prompt for the person's name and response.
name = input("\nWhat is your name? ")
response = input("Which mountain would you like to climb someday? ")
# Store the response in the dictionary.
responses[name] = response
# Find out if anyone else is going to take the poll.
repeat = input("Would you like to let another person respond? (yes/ no) ")
if repeat == 'no':
polling_active = False
# Polling is complete. Show the results.
print("\n--- Poll Results ---")
for name, response in responses.items():
print(f"{name} would like to climb {response}.")
# What is your name? Eric
# Which mountain would you like to climb someday? Denali
# Would you like to let another person respond? (yes/ no) yes
# What is your name? Lynn
# Which mountain would you like to climb someday? Devil's Thumb
# Would you like to let another person respond? (yes/ no) no
# --- Poll Results ---
# Eric would like to climb Denali.
# Lynn would like to climb Devil's Thumb.