Pascals triangle

tri.py

import collections

class Pascals_Triangle(collections.Sequence):
    """ Build a triangle as indexes are requested """
    tri = [(1,)]
    def __len__(s): return len(s.tri)
    def __repr__(s):
        rows = [repr(a) for a in s.tri]
        size = max(len(a) for a in rows)
        return "\n".join(a.center(size) for a in rows)
    def next_row(s, last_row):
        yield 1
        for i in range(len(last_row) -1):
            yield last_row[i] + last_row[i + 1]
        yield 1
    def __getitem__(s, n):
        while n >= len(s.tri):
            type(s).tri.append(tuple(s.next_row(s.tri[-1])))
        return s.tri[n]