pascal's trimangle, no recursion, no combinations

pascal.py

def triangle(r):
    def row(n, previous=None):
        if n == 1:
            return n+1, [1]
        else:
            rown = []
            rown.append(1)
            for i in xrange(n):
                if i + 1 < n-1:
                    rown.append(previous[i] + previous[i+1])
            rown.append(1)
        return n+1, rown
    
    rowd = []
    all_rows = []
    n = 1
    
    while n <= r:
        n, rowd = row(n, rowd)
        all_rows.append(rowd)
    return all_rows