Numpy, adding a row to a matrix

[sapsi]

Hello,
I have a numpy array (2 rows 3 colums)

import numpy
a=numpy.array(  [ [1,2,3] , [3,3,1] ])

I wish to add a row, this is how i do it

s=a.shape
numpy.resize(a,s[0]+1,s[1])
a[s[0]]=new row vector.

Q: Is this a costly operation? What happens if i have to it several
(and unknown) number of times?
is there a simpler way to add a row?

Thank you in advance
Saptarshi