Fix Python – python how to pad numpy array with zeros

I want to know how I can pad a 2D numpy array with zeros using python 2.6.6 with numpy version 1.5.0. But these are my limitations. Therefore I cannot use np.pad. For example, I want to pad a with zeros such that its shape matches b. The reason why I want to do this is so I can do:
b-a

such that
>>> a
array([[ 1., 1., 1., 1., 1.],
[ 1…..