[Solved] Python 3.4, add numbers up to an integer [duplicate]

About your code:

if y == 1: 
    return y 
elif y <= 1 and y <= 999999999: 
    print(sum(y))

First, why are you using return in if part but using print() in elif part?

Then, simply do something like this:

def function_name(number):
    return sum(range(1, number+1))

Demo:

For example, the number is 10:

print(function_name(10))

Output:

55

range() will give a generator like 1, 2, 3...8, 9, 10 here. And sum() can do something like 1 + 2 + 3...8 + 9 + 10 that you want.