Python List Functions: in, sort, min, max, sum

Check If an Item Is in a List with in

The in operator asks: "is this value somewhere in the list?" The answer is True or False. not in asks the opposite question.

Think of the guest list at a party door. The guard checks whether your name is on it before letting you in.

1234guests = ["Sunny", "Rocky", "Lucky"]
print("Rocky" in guests)
print("sunny" in guests)
print("sunny" not in guests)

The rules

• value in list is True when at least one item equals the value.
• value not in list is True when no item does.
• The match must be exact: "sunny" is not "Sunny", and the text "5" is not the number 5.

in with if

in gives True or False, so it makes a perfect if condition (chapter 8). Here a café checks an order typed by the user:

123456menu = ["tea", "coffee", "juice"]
order = input()
if order in menu:
    print(f"One {order} coming up!")
else:
    print(f"Sorry, no {order} today.")

Try typing lassi in the Input box.

Good: check before you remove

In topic 4, remove() stopped with a ValueError when the value was missing. With in, you check first:

12345cart = ["milk", "eggs"]
if "bread" in cart:
    cart.remove("bread")
else:
    print("No bread in the cart")

Avoid: calling remove() on a value that might not be there. One missing item would stop the whole program.

value in list is True when the list has that item. Check with in before you remove().

Sort a List: sort() vs sorted()

Python can put a list in order for you. sort() sorts the list itself. sorted() leaves the list alone and gives you a new, sorted list.

Think of the books on your shelf. sort() is rearranging the books on the shelf. sorted() is writing the titles in order on a new piece of paper, while the shelf stays as it was.

12345marks = [72, 95, 60, 88]
marks.sort()
print(marks)
marks.sort(reverse=True)
print(marks)

1234marks = [72, 95, 60, 88]
ranked = sorted(marks)
print(ranked)
print(marks)

The rules

• Numbers sort from small to big.
• Strings sort in alphabet order, and capital letters come first.

12fruits = ["mango", "Banana", "apple"]
print(sorted(fruits))

To Python, every capital letter comes before every small letter. You'll learn to handle that in the Strings chapter.

Avoid: storing what sort() gives back

123marks = [72, 95, 60]
marks = marks.sort()
print(marks)

Same trap as append(): sort() changes the list and gives back None. Use marks.sort() on its own line, or marks = sorted(marks).

Avoid: mixing numbers and text

12items = [3, "two", 1]
items.sort()

Python can't say whether "two" is smaller than 3, so it stops. Sort lists that hold one type.

list.sort() sorts the list itself. sorted(list) gives a new sorted list and leaves the old one alone.

min(), max() and sum() of a List

Give a list of numbers to min(), max() or sum(), and you get the smallest item, the largest item, or all the items added up.

Think of a teacher with a sheet of marks. In one glance they want the top mark, the lowest mark and the total. These three functions do that for you.

1234marks = [72, 95, 60, 88]
print(min(marks))
print(max(marks))
print(sum(marks))

The average

The average is the total divided by the number of items. You already know both parts: sum() and len(). Round it with :.2f in an f-string from chapter 5:

123marks = [72, 95, 60, 88, 71]
average = sum(marks) / len(marks)
print(f"Average: {average:.2f}")

The rules

• sum() works only with numbers.
• min() and max() also work with strings. They use alphabet order, like sorted().
• sum([]) is 0, but min([]) and max([]) give a ValueError: an empty list has no smallest item.

123names = ["Rocky", "Sunny", "Lucky"]
print(min(names))
print(max(names))

Avoid: numbers stored as text

12marks = ["72", "95"]
print(sum(marks))

The quotes make the marks strings, and Python can't add text to a number. This happens when you store input() without int().

Good: convert as you store: marks = [int(input()), int(input())].

min() and max() find the smallest and largest item. sum() adds them all up.

count() and index(): How Many and Where

Two more methods search a list for you. count() tells you how many times a value appears. index() tells you the index where it first appears.

Think of counting votes after a class election: how many votes did "yes" get, and who voted first?

1234votes = ["yes", "no", "yes", "yes", "no"]
print(votes.count("yes"))
print(votes.count("maybe"))
print(votes.index("no"))

The rules

• list.count(value) gives a whole number. A missing value gives 0, with no error.
• list.index(value) gives the index of the first match.
• A missing value in index() is a ValueError, like remove().

12votes = ["yes", "no"]
print(votes.index("maybe"))

Good: check with in first

123456friends = ["Sunny", "Rocky", "Lucky"]
name = input()
if name in friends:
    print(f"{name} is at index {friends.index(name)}")
else:
    print(f"{name} is not in the list")

All the list tools in one table

count() tells you how many times a value appears. index() tells you where it first appears.

Practice: List tools

Solve these to lock in what you learned in this topic. Each one runs in your browser.

1. 78.Python Program to Check If an Item Is on the MenuEasySolve
2. 79.Python Program to Make a Class Marks ReportHardSolve