Pascal triangle

pascal_triangle.py

from __future__ import print_function

def sum_one_to_n(n):
    return (n * (n + 1)) / 2

def get_index(row, col):
    return sum_one_to_n(row) + col

def get_value(row, col, l):
    if col < 0:
         return 0
    return l[get_index(row, col)]

n = 5

how_many_elements = sum_one_to_n(n)
l = [None] * how_many_elements

l[0] = 1
row = 1
col = 0

for _ in range(1, how_many_elements):
    l[get_index(row, col)] = get_value(row-1, col-1, l) + get_value(row-1, col, l)

    col += 1
    if col > row:
        row = col
        col = 0

# printing works prettily for n < 6

dots = n - 1
index = 0
for i in range(0, n):
    for _ in range(0, dots):
        print('.', end='')
    for k in range(0, i+1):
        print(l[index], end='')
        index += 1
        if k >= i:
            break
        print('_', end='')
        
    for _ in range(0, dots):
        print('.', end='')
    dots -= 1
    print('')