add vs update in set operations in python

Question

What is the difference between add and update operations in python if i just want to add a single value to the set.

a = set()
a.update([1]) #works
a.add(1) #works
a.update([1,2])#works
a.add([1,2])#fails 

Can someone explain why is this so.

Answer 1

add method directly adds elements to the set while the update method converts first argument into set then it adds the list is hashable therefore we cannot add a hashable list to unhashable set.

Answer 2

add only accepts a hashable type. A list is not hashable.