Python: I'm trying to use math contained in a variable

How can I make this:

a = '3,3,3,3'
b = a.replace(',','+')
print(b)

Have the output of 12?
I want the math inside the variable to be solved.

>Solution :

a = '3,3,3,3'
b = [int(el) for el in a.split(",")]
print(sum(b))

What is going on in the code:

1. separate input into a list of numbers.
2. cast strings in that list to ints
3. sum a list of ints.