[Python] Why do we need frozenset ?

(1) 用frozenset 才可以用集合當作Key

Example:

itemsets=[{'B','C'},{'D'}] 

count[{'B','C'}]+=1  # Error : "unhashable type: 'set'"

in this case, we can use frozenset to make 'set' as a key

count[frozenset({'B','C'})]=1   

(2) 用frozenset 才可以做集合包含集合

  S=set()

e={'A','B','C'}

S.add (e)  ==> 想要表示 S={{'A'},{'B'},{'C'}} --> TypeError: unhashable type: 'set'    

S.add (frozenset(e))  -->OK   S={frozenset({'A', 'B', 'C'})}