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LESSON 230 OF 673 · SETS

Sets in Python: Values Without Repeats

Lesson Read the full explanation

TL ; DR:

A set holds values without repeats. Adding a value that is already in it changes nothing.


colors = set()             # an empty set
colors.add("red")
colors.add("blue")
colors.add("red")          # already in it, no change

print(len(colors))         # 2
print("blue" in colors)    # True

pets = set(["cat", "dog", "cat"])
print(len(pets))           # 2

Full lesson:

A list keeps every value you put in it, repeats too. Sometimes you only need to know which values there are. A set is made for that.

A set holds each value once. Adding a value that is already in it changes nothing.

  • set() makes an empty set.
  • s.add(x) puts x in the set s.
  • x in s is True when x is in the set.
  • len(s) gives how many values the set holds.
  • set(lst) builds a set from the list lst. Its repeats go in once.

Checking x in s is fast, even for a big set. On a list, in looks at the values one by one.

A set has no order and no indexes. s[0] is an error. A printed set may show its values in any order.

Watch out for {}. It makes an empty dictionary, not an empty set. Write set() instead.

Your task

The camp's sign-up list, names, has some names twice. Build a set named kids from it.

Then Zoe signs up, and Ava signs up again. Add both names to kids.

Last, if "Leo" is in kids, print Leo is coming.

The code already prints the number of kids twice.

Expected output:

3 kids
4 kids
Leo is coming
STEP 1 OF 3
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    PYTHON · SETS

    Sets in Python, explained

    What a set is

    A list keeps every value you put in it, repeats included. Sometimes you only care which values are there, not how often. A set holds each value once. Adding a value that is already in the set changes nothing.

    The basic operations

    • set() makes an empty set.
    • s.add(x) puts x in the set s.
    • x in s is True when x is in the set.
    • len(s) gives how many values the set holds.
    • set(lst) builds a set from a list. Its repeats go in once.
    colors = set()
colors.add("red")
colors.add("blue")
colors.add("red")

print(len(colors))
print("blue" in colors)
print("pink" in colors)

pets = set(["cat", "dog", "cat"])
print(len(pets))
    2
True
False
2

    Checking x in s is fast, even for a big set: Python does not look at the values one by one. On a list, in does exactly that, so the work grows with the length of the list.

    Traps

    • No order, no indexes. A set has no first element. kids[0] stops with TypeError: 'set' object is not subscriptable. A printed set may show its values in any order.
    • {} is not an empty set. It makes an empty dictionary: type({}) is dict. Write set() for an empty set.
    • Equal sets ignore order. set([1, 2, 3]) == set([3, 2, 1]) is True.

    What can go in a set

    A set's values must be hashable, which is why the common cases work: numbers, strings and tuples can be set members, while lists cannot. s.add([1]) stops with TypeError: unhashable type: 'list'. Counting different values is the classic use: len(set("banana")) is 3, because the string has only the letters b, a and n, and sorted(set(["b", "a", "b"])) gives ['a', 'b'] when you do need an order.

    When to choose a set over a list

    Reach for a set when the questions are "is this value present?" and "how many different values are there?". Reach for a list when position or repeats matter: the third name on the sign-up sheet, or how many times a name was written down. A common pattern is to read data into a list, then build a set from it to answer the first kind of question quickly.

    The exercise, step by step

    A camp's sign-up list names has some names twice. The task is to build a set kids from it, add Zoe and then Ava, and print Leo is coming if Leo is in the set. The starter code already prints the number of kids twice.

    1. Build the set with kids = set(names). Ava and Leo are in the list twice but go in the set once, so it holds Ava, Leo and Mia: 3 names.
    2. Call kids.add("Zoe") and kids.add("Ava"). Zoe is new, so the set grows to 4. Ava is already there, so that call changes nothing.
    3. At the end, test "Leo" in kids and print the message when it is true.
    names = ["Ava", "Leo", "Ava", "Mia", "Leo"]

kids = set(names)
print(len(kids), "kids")

kids.add("Zoe")
kids.add("Ava")
print(len(kids), "kids")

if "Leo" in kids:
    print("Leo is coming")
    3 kids
4 kids
Leo is coming

    Key takeaways

    • A set stores each value once; add on an existing value does nothing.
    • Use in on a set for a fast membership check.
    • Sets have no order and no indexes.
    • Make an empty set with set(), never with {}.
    • set(list) is the quickest way to drop repeats and count the different values.
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