Python Sets

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A Python set is a built-in data type that holds an unordered collection of unique items.

• purpose
  • to perform operations like the mathematical set operations
• uses
  • when the existence of an object in a collection is more important than the order or how many times it occurs
  • allow for efficient membership tests
• created by:
  1. placing comma-separated sequence of items inside curly braces {}
  2. using the set() function to create a set from another iterable

# Using curly braces
set1 = {'item1', 'item2', 'item3'}
 
# Using the set() function
set2 = set(['item1', 'item2', 'item3'])

Determine Number of Items in a Set

The number of items in a Python set can be determined using the len() function.

set1 = {'item1', 'item2', 'item3'}
number_of_items = len(set1)

set() Constructor

The set constructor set() is a built-in function for creating a set.

• takes an iterable as an argument
• returns a set containing unique elements of the iterable
• if no argument, creates an empty set

# Using a list as an argument
set1 = set(['item1', 'item2', 'item3', 'item1'])
 
# Using a string as an argument
set2 = set('hello')

Accessing Items in a Set

• items cannot be accessed by index or key
  • they are unordered
• can loop through a set
• can use the in keyword to determine if a value is in a set
• once a set is created, items cannot be changed
  • but new items can be added

set1 = {'item1', 'item2', 'item3'}
for item in set1:
    print(item)

Modify Sets

Add Items to a Set

To add an item to a Python set, use the add() method.

set1 = {'item1', 'item2', 'item3'}
set1.add('item4')

To add multiple items to a set, use update() method.

set1 = {'item1', 'item2', 'item3'}
set1.update(['item4', 'item5'])

Adding Sets

Use the update() method to add items from another set into the current set.

• takes one or more sets as arguments and adds all their elements to the current set

set1 = {'item1', 'item2', 'item3'}
set2 = {'item4', 'item5'}
set1.update(set2)

Remove an Item From a Set

• To remove an item from a set:
  • remove() method
    • removes the specified item from the set, but if the item does not exist, it raises an error
  • discard() method
    • removes the specified item from the set, but it does not raise an error if the item does not exist

# Using remove() method
set1 = {'item1', 'item2', 'item3'}
set1.remove('item1')
 
# Using discard() method
set1 = {'item1', 'item2', 'item3'}
set1.discard('item1')

Emptying a Set

The clear() method is used to empty a set.

set1 = {'item1', 'item2', 'item3'}
set1.clear()

Deleting a Set

Delete a set using the del keyword.

• removes the entire set from memory
• attempting to use the set afterward will result in a NameError

# Creating a set
network_devices = {'Router1', 'Switch1', 'Firewall1'}
# Using del to delete the set
del network_devices