Skip to main content

Sets

Updated Oct 28, 2019 ·

Overview​

A set stores unique elements (no duplicates).

  • Sets are unordered, so elements don't have a specific index.
  • We can add or remove values, but cannot change them once added.

Sets don’t have an index, so we can't access elements like a list using square brackets.

Creating Sets​

There are two ways to create a set:

  1. Using braces to create a set from scratch:

    attendees = {"Alice", "Bob", "Alice"}
    print(attendees)

    Output:

    {'Bob', 'Alice'}

    Even though "Alice" was added twice, the set only keeps one instance.

  2. Using set to create a set from a list.

    In the example below, cookies_list contains repeated cookies which will be removed when list is converted to a set:

    cookies_list = ["chocolate chip", "oatmeal", "chocolate chip"]

    cookie_set = set(cookies_list)
    print(cookie_set)

    Output:

    {'oatmeal', 'chocolate chip'}

Modifying Sets​

You can add new elements to a set using add(). It only adds the item if it isn’t already in the set.

cookie_set = {'oatmeal', 'chocolate chip'}

cookie_set.add("biscotti")
print(cookie_set)

Output:

{'oatmeal', 'chocolate chip', 'biscotti'}

Adding Multiple Items​

update() lets you add several elements at once from a list or another set.

cookie_set = {'oatmeal', 'chocolate chip', 'biscotti'}
more_cookies = ["snickerdoodle", "oatmeal"]

cookie_set.update(more_cookies)
print(cookie_set)

Output:

{'snickerdoodle', 'chocolate chip', 'oatmeal', 'biscotti'}

Removing Elements​

You can remove items using the following:

  • discard() removes a specific item safely
  • pop() removes and returns an arbitrary element.

Example:

cookie_set = {'snickerdoodle', 'chocolate chip', 'oatmeal', 'biscotti'}

cookie_set.discard("biscotti")
removed = cookie_set.pop()
print(cookie_set)
print(removed)

Output (may vary due to unordered nature):

{'chocolate chip', 'oatmeal'}
snickerdoodle

Set Operations: union and intersection​

Sets can be combined or compared with other sets:

  • union() returns all unique elements from both sets
  • intersection() returns only the elements present in both sets

Examples:

my_cookies = {"chocolate chip", "oatmeal"}
hugo_cookies = {"oatmeal", "snickerdoodle"}

all_cookies = my_cookies.union(hugo_cookies)
common_cookies = my_cookies.intersection(hugo_cookies)

print(all_cookies)
print(common_cookies)

Output:

{'snickerdoodle', 'chocolate chip', 'oatmeal'}
{'oatmeal'}

Set Operations: difference​

difference() finds items in one set that aren’t in another. The set you call it on is the reference.

my_cookies = {"chocolate chip", "oatmeal"}
hugo_cookies = {"oatmeal", "snickerdoodle"}

my_unique = my_cookies.difference(hugo_cookies)
hugo_unique = hugo_cookies.difference(my_cookies)

print(my_unique)
print(hugo_unique)

Output:

{'chocolate chip'}
{'snickerdoodle'}

Set Operations: sorted​

Although sets are unordered, we can sort them using sorted:

attendees = {"Alice", "Joe", "Bob", "Charlie", "Zoey", "Ted"}

sorted_attendees = sorted(attendees)
print(sorted_attendees)

Output:

['Alice', 'Bob', 'Charlie', 'Joe', 'Ted', 'Zoey']