Python Collections - Sets in Python

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  • Sets

    Set Content

  • A set is a unique collection of objects in Python. You can denote a set with a curly bracket {}. Python will automatically remove duplicate items: 


    • 
# Create a set

set1 = {"pop", "rock", "soul", "hard rock", "rock", "R&B", "rock", "disco"}
set1

  • The process of mapping is illustrated in the figure:



  • You can also create a set from a list as follows: 


    • 
# Convert list to set

album_list = [ "Michael Jackson", "Thriller", 1982, "00:42:19", \
              "Pop, Rock, R&B", 46.0, 65, "30-Nov-82", None, 10.0]
album_set = set(album_list)             
album_set

  • Now let us create a set of genres: 


    • 
# Convert list to set

music_genres = set(["pop", "pop", "rock", "folk rock", "hard rock", "soul", \
                    "progressive rock", "soft rock", "R&B", "disco"])
music_genres

  • Set Operations

  • Let us go over set operations, as these can be used to change the set. Consider the set A: 


    • 
# Sample set

A = set(["Thriller", "Back in Black", "AC/DC"])
A

  • We can add an element to a set using the add() method: 


    • 
# Add element to set

A.add("NSYNC")
A

  • If we add the same element twice, nothing will happen as there can be no duplicates in a set: 


    • 

# Try to add duplicate element to the set

A.add("NSYNC")
A

  • We can remove an item from a set using the remove method: 


    • 
# Remove the element from set

A.remove("NSYNC")
A

  • We can verify if an element is in the set using the in command: 


    • 
# Verify if the element is in the set

"AC/DC" in A

    Sets Logic Operations

  • Remember that with sets you can check the difference between sets, as well as the symmetric difference, intersection, and union:


  • Consider the following two sets: 


    • 
# Sample Sets

album_set1 = set(["Thriller", 'AC/DC', 'Back in Black'])
album_set2 = set([ "AC/DC", "Back in Black", "The Dark Side of the Moon"])




    • 
# Print two sets

album_set1, album_set2

  • As both sets contain AC/DC and Back in Black we represent these common elements with the intersection of two circles.



  • You can find the intersect of two sets as follow using &: 


    • 
# Find the intersections

intersection = album_set1 & album_set2
intersection

  • You can find all the elements that are only contained in album_set1 using the difference method: 


    • 
# Find the difference in set1 but not set2

album_set1.difference(album_set2)

  • You only need to consider elements in album_set1; all the elements in album_set2, including the intersection, are not included.



  • The elements in album_set2 but not in album_set1 is given by: 


    • 
album_set2.difference(album_set1)


  • You can also find the intersection of album_list1 and album_list2, using the intersection method: 


    • 
# Use intersection method to find the intersection of album_list1 and album_list2

album_set1.intersection(album_set2)

  • This corresponds to the intersection of the two circles:



  • The union corresponds to all the elements in both sets, which is represented by coloring both circles:



  • The union is given by: 


    • 
# Find the union of two sets

album_set1.union(album_set2)

  • And you can check if a set is a superset or subset of another set, respectively, like this: 


    • 
# Check if superset

set(album_set1).issuperset(album_set2)   

# Check if subset

set(album_set2).issubset(album_set1)

  • Here is an example where issubset() and issuperset() return true: 


    • 
# Check if subset

set({"Back in Black", "AC/DC"}).issubset(album_set1) 

# Check if superset

album_set1.issuperset({"Back in Black", "AC/DC"})