Question about functions

Question

Can someone help in describing below functions how the output will be 18?!
All results count as (3+6+9) = 18 ? OR!??

def f(x=[]):
x+=[3]
return sum(x)

print(f()+f()+f())

The output is: 18

Answer 1

First call x becomes [3]. Second call x becomes [3, 3]. Third call x becomes [3, 3, 3]. Therefore, f returns 3, 6, and 9, which added together is your printed 18.

Answer 2

Thanks John

Answer 3

Right