tri.py

def tri_gen():
    tri_num = 0
    n = 1
    while True:
        yield tri_num
        tri_num += n
        n += 1

tri_numbers = tri_gen()
next(tri_numbers)  # 0
next(tri_numbers)  # 1
next(tri_numbers)  # 3
next(tri_numbers)  # 6
next(tri_numbers)  # 10
next(tri_numbers)  # 15
next(tri_numbers)  # 21