List operations beginner

List operations beginner
0

Could somebody explain why the output of the code is such ?
a = [[1, 2, 3], [4, 5], [6], [7, 8, 9]]
s = sum(a,[])
print(s)
-------------------------output-----------------------
[1, 2, 3, 4, 5, 6, 7, 8, 9]

It is because doing [1,2,3] + [4,5] you are concatenating the two lists and getting [1,2,3,4,5]
sum() adds the elements inside a list, but the elements inside your list are other lists