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Sets

In Python, a set is very similar to a list, but with a few key differences.

  • A set is unordered, meaning the elements are not stored in a specific order. If order is important, you should use a list.
    • When printing a set, the elements sometimes appear in sorted order, but this is not guaranteed.
  • A set can only contain unique elements. If you try to add a duplicate element to a set, it will be ignored.
my_set = {1, 2, 3}

print(my_set)  # Output: {1, 2, 3}

my_set = {3, 2, 1}

print(my_set)  # Output: {1, 2, 3}

1. Creating Sets​

A set can be created using curly braces {} with elements separated by commas. We can declare an empty set with set().

my_set = set()
my_set.add(1)
my_set.add(2)
my_set.add(3)

# equivalent to 
my_set = {1, 2, 3}

Note: my_set = {} creates an empty dictionary in Python, not a set.

2. Set Operations​

1. add()​

We can add elements to a set using the add() method.

my_set = {1, 2, 3}

my_set.add(4)

print(my_set) # Output: {1, 2, 3, 4}

2. remove()​

We can remove elements from a set using the remove() method. If the element is not present in the set, a KeyError will be raised.

my_set = {1, 2, 3}

my_set.remove(2)

print(my_set)  # Output: {1, 3}

my_set.remove(4)  # Raises KeyError

3. len()​

You can call the len() function on a set to get the number of elements in the set.

my_set = {1, 2, 3}
print(len(my_set)) # Output: 3

4. Loop over Sets​

Just like with lists, we can loop over elements within a set using for loops. The difference is that we can't access elements by index because sets are unordered. The order that we loop over a set is not guaranteed.

my_set = {1, 2, 3}

for element in my_set:
    print(element)

5. Convert List into Set​

We can also convert a list into a set by passing the list into the set() function. We can then convert the set back into a list by passing it into the list() function.

This is an easy way to remove duplicates from a list.

my_list = [1, 2, 3, 4, 5, 1, 2, 5]

my_set = set(my_list)

print(my_set)  # Output: {1, 2, 3, 4, 5}

my_list_no_duplicates = list(my_set)

6. in keyword​

Just like with lists, we can also use the in keyword to check if an element is present in a set.

my_set = {"Cat", "Dog", "Mouse"}

contains_cat = "Cat" in my_set   # True
contains_lion = "Lion" in my_set # False
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