Printing all possible permutations of valid parenthesis pairs

PrintParenthesis.py

class PrintParenth:
    def __init__(self, n):
        self.n = n
        self.s = [None for i in range(2*n)]

    def P(self, l, r):
        if l==self.n and r==self.n:
            yield ''.join(self.s)
        if l < self.n:
            self.s[l+r] = '('
            for s in self.P(l+1, r):yield s
        if l > r:
            self.s[l+r] = ')'
            for s in self.P(l, r+1): yield s

    def solve(self):
        return self.P(0,0)

c = PrintParenth(4)
for s in c.solve():
    print s