'mushroomsin requested_toppings true 'pepperoniin requested_toppings false at and vthe keyword in tells python to check for the existence of 'mushroomsand 'pepperoniin the list requested_toppings this technique is quite powerful because you can create list of essential valuesand then easily check whether the value you're testing matches one of the values in the list checking whether value is not in list other timesit' important to know if value does not appear in list you can use the keyword not in this situation for exampleconsider list of users who are banned from commenting in forum you can check whether user has been banned before allowing that person to submit commentbanned_ users py banned_users ['andrew''carolina''david'user 'marieu if user not in banned_usersprint(user title("you can post response if you wish "the line at reads quite clearly if the value of user is not in the list banned_userspython returns true and executes the indented line the user 'marieis not in the list banned_usersso she sees message inviting her to post responsemarieyou can post response if you wish boolean expressions as you learn more about programmingyou'll hear the term boolean expression at some point boolean expression is just another name for conditional test boolean value is either true or falsejust like the value of conditional expression after it has been evaluated boolean values are often used to keep track of certain conditionssuch as whether game is running or whether user can edit certain content on websitegame_active true can_edit false boolean values provide an efficient way to track the state of program or particular condition that is important in your program if statements