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) #         


        

Answer 1

add adds an element, update "adds" another iterable set, list or tuple, for example:

In [2]: my_set = {1,2,3}

In [3]: my_set.add(5)

In [4]: my_set
Out[4]: set([1, 2, 3, 5])

In [5]: my_set.update({6,7})

In [6]: my_set
Out[6]: set([1, 2, 3, 5, 6, 7])

Answer 2

a.update(1) in your code won't work. add accepts an element and put it in the set if it is not already there but update takes an iterable and makes a unions of the set with that iterable. It's kind of like append and extend for the lists.

Answer 3

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.