To manipulate the contents of sets, the following methods are supported. This is not a complete list of set methods, but these are the most common ones.

add() and update()

Adding new items to a set you can do using the add() method, to add one new element that you provide as an argument. If you want to add multiple new elements at once, you can use the update() method, which you provide with a list of the new elements as argument. You can also use update() with a tuple as argument, and you can even use it with a string as argument. If you use it with a string, it will consider each letter of the string as a separate element to add.

Since sets can only contain unique elements, any duplicate element that you try to add will be ignored.

fruitset = { "apple", "banana", "cherry", "durian", "mango" }
print( fruitset )
fruitset.add( "apple" )
fruitset.add( "elderberry" )
print( fruitset )
fruitset.update( ["apple","apple","apple","strawberry",
    "strawberry","apple","mango"] )
print( fruitset )

remove(), discard(), and clear()

To remove elements from a set, you can use the remove() or discard() method. Both get the element to remove as argument. The difference between the two methods is that remove() will cause a runtime error if the element is not part of the set, while discard() will ignore such errors.

fruitset = { "apple", "banana", "cherry", "durian", "mango" }
print( fruitset )

fruitset.remove( "apple" )
print( fruitset )

clear() removes all elements of the set at once.

pop()

Calling the pop() method will remove an element from the set and return it. You cannot predict which element will be removed, as sets are unordered.

fruitset = { "apple", "banana", "cherry", "durian", "mango" }
while len( fruitset ) > 0:
    print( fruitset.pop() )

copy()

Just like lists, if you assign a variable that contains a set to another variable, you are creating an alias. Like with dictionaries (and probably because sets are implemented as dictionaries), you use the method copy() to create a copy of a set.

union()

The union of two sets is a set which contains elements of both of them. You can use the union() method for one set, with as argument a second set, to return the union of both sets involved. This does not change the sets themselves. Alternatively, you can use the special operator \(|\) (pipeline) to create the union of two sets. Note: you might suspect that you can also use the \(+\) operator to combine two sets, but \(+\) is not defined for sets, and neither is \(*\).

fruit1 = { "apple", "banana", "cherry" }
fruit2 = { "banana", "cherry", "durian" }
fruitunion = fruit1.union( fruit2 )
print( fruitunion )

fruitunion = fruit1 | fruit2
print( fruitunion )

intersection()

The intersection of two sets is a set which contains only the elements that they both have. You can use the intersection() method for one set, with as argument a second set, to return the intersection of the sets involved. This does not change the sets themselves. Alternatively, you can use the special operator & (ampersand) to create the intersection of two sets.

fruit1 = { "apple", "banana", "cherry" }
fruit2 = { "banana", "cherry", "durian" }
fruitintersection = fruit1.intersection( fruit2 )
print( fruitintersection )

fruitintersection = fruit1 & fruit2
print( fruitintersection )

difference()

The difference of two sets is a set which contains only the elements that the first set has that are not also in the second set. You can use the difference() method for one set, with as argument a second set, to return the difference whereby the elements of the argument set are removed from the first set. This does not change the sets themselves. Alternatively, you can use the special operator - (minus) to create the difference of two sets.

fruit1 = { "apple", "banana", "cherry" }
fruit2 = { "banana", "cherry", "durian" }
fruitdifference = fruit1.difference( fruit2 )
print( fruitdifference )

fruitdifference = fruit1 - fruit2
print( fruitdifference )

fruitdifference = fruit2 - fruit1
print( fruitdifference )

isdisjoint(), issubset(), and issuperset()

The methods isdisjoint(), issubset(), and issuperset() are all called as methods of one set, with a second set as argument. All return True or False. isdisjoint() returns True if the two sets share no elements. issubset() returns True if all the elements of the first set are also found in the argument set. issuperset() returns True if all the elements of the argument set are also found in the first set. Note that a set is both a subset and a superset of itself.

fruit1 = { "apple", "banana", "cherry" }
fruit2 = { "banana", "cherry" }

print( fruit1.isdisjoint( fruit2 ) )
print( fruit1.issubset( fruit2 ) )
print( fruit2.issubset( fruit1 ) )
print( fruit1.issubset( fruit1 ) )
print( fruit1.issuperset( fruit2 ) )
print( fruit2.issuperset( fruit1 ) )
print( fruit1.issuperset( fruit1 ) )

Practice

There is also a set method symmetric_difference() which returns a set that contains all the elements that are in the union of two sets, except those that are found in both sets. For example, if set 1 contains A, B, and C, and set 2 contains B, C, and D, the symmetric difference of sets 1 and 2 contains A and D. Can you implement the symmetric_difference() method by using only some of the methods found above?

In the chapter on iterations you were asked to write code that determines all the letters that two words have in common, whereby each letter should only be reported once. Using sets, you can do this very efficiently. Please write the appropriate code.