Python Sets: Comprehensive Theory

Sets are used to store multiple items in a single variable. A set is a collection which is unordered, unchangeable*, and unindexed.

1. Characteristics of Sets

• Unordered: Sets do not have a defined order. Items can appear in a different order every time you use them, and they cannot be referred to by index or key.
• Unchangeable: Set items are unchangeable, meaning we cannot change the items after the set has been created.
  • Note: Although you cannot change items, you can remove items and add new items.
• Unindexed: You cannot access items in a set by referring to an index.
• No Duplicates: Sets cannot have two items with the same value. If a duplicate value is added, it is ignored.

2. Creating a Set

Sets are written with curly brackets {}.

myset = {"apple", "banana", "cherry"}

The set() Constructor

You can also use the set() constructor to create a set.

thisset = set(("apple", "banana", "cherry")) # note the double round-brackets

3. Accessing Set Items

Since sets are unindexed, you cannot access items by index. However, you can loop through the set items using a for loop, or ask if a specified value is present by using the in keyword.

thisset = {"apple", "banana", "cherry"}

for x in thisset:
  print(x)

print("banana" in thisset) # Output: True

4. Adding & Removing Items

Adding Items

• To add one item to a set use the add() method.
• To add items from another set (or any iterable) into the current set, use the update() method.

thisset = {"apple", "banana"}
thisset.add("cherry")

# Adding multiple items
tropical = {"pineapple", "mango", "papaya"}
thisset.update(tropical)

Removing Items

• remove(): Removes the specified element. Raises an error if the item does not exist.
• discard(): Removes the specified element. Does NOT raise an error if the item does not exist.
• pop(): Removes a random item (since sets are unordered, you don't know which item will be removed).
• clear(): Empties the set.

5. Join Sets (Operations)

Python has several ways to join two or more sets:

| Operation | Method | Description | | :--- | :--- | :--- | | Union | union() or | | Returns a new set with all items from both sets. | | Intersection | intersection() or & | Returns a new set with only items present in both sets. | | Difference | difference() or - | Returns a new set with items in the first set but NOT in the second. | | Symmetric Difference | symmetric_difference() or ^ | Returns a set with items that are in either set, but NOT in both. |

6. Set Methods Reference Table

| Method | Description | | :--- | :--- | | add() | Adds an element to the set | | clear() | Removes all the elements from the set | | copy() | Returns a copy of the set | | difference() | Returns a set containing the difference between two or more sets | | discard() | Remove the specified item | | intersection() | Returns a set, that is the intersection of two other sets | | isdisjoint() | Returns whether two sets have a intersection or not | | issubset() | Returns whether another set contains this set or not | | issuperset() | Returns whether this set contains another set or not | | pop() | Removes an element from the set | | remove() | Removes the specified element | | union() | Return a set containing the union of sets | | update() | Update the set with the union of this set and others |