from    math        import sqrt

 def l2(p):

     return sqrt(sum((x**2 for x in p)))

 # Pairwise distances between the elements of `points` with respect to some `norm`

 class PairwiseDistances:

     def __init__(self, points, norm = l2):

         self.points = points

         self.norm = norm

     def __len__(self):

         return len(self.points)

     def __call__(self, p1, p2):

         return self.norm((x - y for (x,y) in zip(self.points[p1], self.points[p2])))

 # Caches all distances specified by `distances`

 class ExplicitDistances:

     def __init__(self, distances):

         self.len = len(distances)

         self.distances = []

         for i in xrange(self.len): 

             self.distances.append([])

             for j in xrange(self.len):

                 self.distances[-1].append(distances(i,j))

     def __len__(self):

         return self.len

     def __call__(self, p1, p2):

         return self.distances[p1][p2]

 # Generator of all points in a file `filename` with one point per line

 def points_file(filename):

     fd = open(filename)

     for line in fd.xreadlines():

         if line.startswith('#'): continue

         yield map(float, line.strip().split())

     fd.close()