Searching a List: in

Is a name on the guest list? Is there milk on the shopping list? The operator in answers questions like these. value in items is True if the list items contains the value, and False if it doesn't:

guests = ["Anna", "Tom", "Lucy"]
print("Tom" in guests)
print("Paul" in guests)

This prints:

True
False

A boolean is exactly what a condition needs, so in is most often used with if:

guests = ["Anna", "Tom", "Lucy"]
visitor = "Tom"
if visitor in guests:
    print("Come in!")
else:
    print("Sorry, you are not on the list.")

This prints:

Come in!

The opposite is not in: value not in items is True when the value isn't in the list. It's handy for adding something only if it isn't in the list yet:

shopping = ["milk", "bread"]
item = "eggs"
if item not in shopping:
    shopping.append(item)
print(shopping)

This prints:

['milk', 'bread', 'eggs']

With in, you can also avoid the ValueError from the previous exercise. If you write if "butter" in shopping: before shopping.remove("butter"), the value is removed only when it is in the list.

Like ==, in compares exactly: "tom" in guests is False, because "tom" with a lowercase t isn't the same text as "Tom".