Lists and Dictionaries

This page is a cheatsheet for using lists and dictionaries in Python.

Caption text

Header text	Lists	Dictionaries	Dictionary in List	List in Dictionary in List
Create	x = [1, 2]	x = { 'a': 2, 'b': 3, }	x = [1, { 'a': 2, 'b': 3, } ]	x = [1, { 'a': 2, 'b': [3, 4], } ]
Retrieve	x[1]	x['b']	x[1]['b']	x[1]['b][1]
Example	Example	Example	Example	Example

Lists

Create a list

people = []                                # empty list
animals = ['daisy','pat','wabbit','bunny']

Access elements

print(animals[0])                        # first item
print(animals[-1])                       # last item
print(animals[-2])                       # penultimate item
print(f"{animals[0].title()} is a cow.") # in an f-string

Modify, add, insert elements

animals[3] = 'veau'      # replace item 3
animals.append('croc')   # add new item to end of list
animals.insert(1,'pato') # insert item at index 1

Remove elements

del animals[0]                 # delete item at index 0
popped_animal = animals.pop()  # remove last item
popped_animal = animals.pop(2) # remove item at index 2
animals.remove('veau')         # remove (first instance of) item by value

Organize Lists

animals.sort()             # sort values a-z
animals.sort(reverse=True) # sort values z-a
animals.reverse()          # reverse order (not z-a)
print(sorted(animals))     # temporarily sort
len(animals)               # returns number of items

Loop on a List

for animal in animals:                    # for loop requires a colon
    print(animal)                         # indent each line in the loop
    print(f"Hello {animal.title()}")
print(f"We have {len(animals)} animals!") # stop indenting after the loop

Create a list of numbers and list statistics

numbers = list(range(1,11)) # list function makes a list, range function creates 1-10
odd_numbers = list(range(1, 11, 2)) #third parameter is step
even_numbers = list(range(2, 11, 2))

square_numbers = []
for value in range(1, 11):
    square_numbers.append(value ** 2)
    print(square_numbers)

square_nos = [value ** 2 for value in range(1, 11)] # list comprehension
print(square_nos)
print(min(square_nos))
print(max(square_nos))
print(sum(square_nos))

Slice a list

print(square_nos[0:3]) # first three
print(square_nos[2:6]) # 3rd, 4th, 5th, 6th
print(square_nos[:5]) # first five
print(square_nos[5:]) # from 5th to end
print(square_nos[-3:]) # last three
print(square_nos[0:5:2]) # first five, step 2

for number in square_nos[::3]: # every 3rd number of entire list
    print(number)

Copy a list

new_squares = square_nos # new list is linked to old one
new_squares = square_nos[:] # creates a new, separate list

Tuples

Tuples are immutable lists. You cannot modify values in a tuple, but you can replace the entire tuple.

limits = (10, 20) # defines the tuple
limits = (10, 30) # redefines the tuple

If Statements

Conditional Tests

number = 1
number == 1 # returns True
number == 2 # returns False
number != 2 # returns True (because 1 is not equal to 2)
number < 2  # returns True
number > 2  # returns False

letter = A
number = 2 and letter = A # returns false as only one condition is true
number = 2 or letter = A # returns true as one condition is true

numbers =[1, 2, 3]
1 in numbers                  # returns true
1 in numbers and 4 in numbers # returns false
4 not in numbers              # returns true

If Statements

age = 18
if age >= 18:                               # use a colon like a for list
    print(f"At age {age}, you can drive.")
elif age = 17:                              # one or more optional
    print("Next year!")
else:                                       # optional
    print(f"{age} is too young!")

numbers = list(range(0,11))
print(numbers)

if numbers:                  # is there anything in numbers?
    for number in numbers:
        if number == 0:
            print(f"{number} is nothing at all!")
        elif number % 2:
            print(f"{number} is an even number.")
        else:
            print(f"{number} is an odd number.")
else:
    print("No numbers!")

Check a list against a list

library_books = ["Gravity's Rainbow", "Naked Lunch", 
    "Hopscotch", "Crash"]
orders = ["Gravity's Rainbow", "The Soft Machine", 
    "Hopscotch", "The Atrocity Exhibition"]

for order in orders:
    if order in library_books:
        print(f"{order} added to package.")
    else:
        print(f"Sorry, we don't have {order}.")

Dictionaries

Create a dictionary

book_0 = {}                              # create empty dictionary

book_1 = {
    "title": "Gravity's Rainbow", 
    "author": "Thomas Pynchon"}          # create dictionary with key-value pairs
book_1["pub_date"] = "1973"              # add key-value pair
book_1["pages"] = "750 odd" 
book_1["pages"] = "776"                  # if key exists, update value         
del book_1["pages"]                      # delete key-value pair
print(book_1)

print(f"{book_1['title']} was written by {book_1['author']} in {book_1['pub_date']}.")

pages = book_1.get('pages', 'Not Available')
print(pages)

Loop Through a Dictionary

author_0 = {
    'first_name': 'Thomas',
    'last_name': 'Pynchon',
    'birth_date': '1937',
    }
for key, value in author_0.items():      # items() gets both keys and values
    print(f"Key: {key}, Value: {value}")

for k in author_0.keys():                # keys() gets just the keys
    print(f"{k}")

for v in author_0.values():              # values() gets just the values
    print(f"{v}")

Nest Dictionaries in a List and Lists in a Dictionary

author_0 = {                                           # each author is a dictionary
    'first_name': 'Thomas',
    'last_name': 'Pynchon',
    'books': [                                         # the books key contains a list
        "V",
        "Gravity's Rainbow",
        "The Crying of Lot 49",
        ]
    }
author_1 = {
    'first_name': 'William',
    'last_name': 'Burroughs',
    'books': [
        "Naked Lunch",
        "The Soft Machine",
        "The Ticket That Exploded",
        ]
    }
author_2 = {
    'first_name': 'JG',
    'last_name': 'Ballard',
    'books': [
        "The Drowned World",
        "Crash",
        "Concrete Island",
        ]
    }
authors = [author_0, author_1, author_2]                 # authors is a list of the dictionaries

print(authors[1])                                        # print the 2nd dictionary in the list
print(authors[1]['last_name'])                           # print the 'last_name' of the 2nd dictionary in the list
print(authors[1]['books'][2])                            # print the third 'book' of the 2nd dictionary in the list

for author in authors:                                   # loop over the authors list
    author_books = ""
    for book in author['books']:                         # loop over the books list
        author_books += book + ", "
    author_books = author_books.removesuffix(', ')       # remove the trailing comma
    print(f"{author['last_name']} wrote {author_books}.")