'Yields the Frobenius matrix norm (cached).
n = the square root of the sum of the absolute squares of the values.
The matrix norm is used to normalize (0.0-1.0) cosine similarity between documents.'

@property
def l2_norm(self):

    if (self._norm is None):
        self._norm = (sum(((w * w) for w in self.values())) ** 0.5)
    return self._norm
