set.issubset() method in Python

The issubset() method in sets checks whether a set is a subset of  another set given as argument.

A set is a subset of another set if  all of its elements are present in the second set. For example both sets, {1, 3, 5} and {0, 2, 4} are subsets of  the set {0, 1, 2, 3, 4, 5} since all of their elements are also present in this set. 

Logically, every set is a subset of itself.

set1.issubset(set2)

The method returns True, if set1 is a subset of set2, else False. 

integers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

evens = {0, 2, 4, 6, 8}
odds = {1, 3, 5, 7, 9}

#check for subsets
print(integers.issubset(integers)) #it is a subset of itself
print(evens.issubset(integers)) #True
print(odds.issubset(integers)) #True

In the above case, the issubset() method returns True because  all the sets are sub sets of integers, including the integers set itself.

integers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

evens = {0, 2, 4, 6, 8}
odds = {1, 3, 5, 7, 9}

#check for subsets
print(integers.issubset(evens)) #False
print(integers.issubset(odds)) #False

Using the <= operator instead

The <= is the operator version of the issubset() function. Similarly, it returns True if the first set is a subset of the second set, otherwise False.

set1 <= set2

Returns True if set1 is a subset of set2, else False.

integers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

evens = {0, 2, 4, 6, 8}
odds = {1, 3, 5, 7, 9}

#check for subsets
print(integers <= integers) #it is a subset of itself
print(evens <= integers) #True
print(odds <=integers)#True
print(evens <= odds) #False
print(odds <= evens) #False

Check for strict Subset

The < operator is used to check whether a set is a strict subset of another set.

A subset is a strict subset when it is a subset of another set, but it does not contain all the elements of the larger set. In other words, the subset is smaller in size than the other set.

A set is not a strict subset of itself.

integers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

evens = {0, 2, 4, 6, 8}
odds = {1, 3, 5, 7, 9}

print(integers < integers) #it is a NOT its own subset
print(evens < integers) #True
print(odds < integers) #True