Add number to set

Question

What am I doing wrong here?

a = set().add(1)
print a # Prints `None`

I'm trying to add the number 1 to the empty set.

Answer 1

The add method updates the set, but returns None.

a = set()
a.add(1)
print a

Answer 2

You should do this:

a = set()
a.add(1)
print a

Notice that you're assigning to a the result of adding 1, and the add operation, as defined in Python, returns None - and that's what is getting assigned to a in your code.

Alternatively, you can do this for initializing a set:

a = set([1, 2, 3])