correct and efficient way to flatten array in numpy in python?

I have:

a = array([[1,2,3],[4,5,6]])

and I'd like to flatten it, joining the two inner lists into one flat array entry. I can do:

array(list(flatten(a)))

but that seems inefficient due to the list cast (I want to end up with an array and not a generator.)

Also, how can this be generalized to an array like this:

b = array([[[1,2,3],[4,5,6]], [[10,11,12],[13,14,15]]])

where the result should be:

b = array([[1,2,3,4,5,6],
           [10,11,12,13,14,15]])

are there builtin/efficient numpy/scipy operators for this? thanks.