In [11]: yy, xx = np.where(a%3 == 0)

In [12]: yy
Out[12]: array([0, 0, 1, 1, 2, 3, 3])

In [13]: xx
Out[13]: array([0, 3, 1, 4, 2, 0, 3])

In [33]: np.where( (a>2) & (a<12) )
Out[33]: (array([0, 0, 1, 1, 1, 1, 1, 2, 2]), array([3, 4, 0, 1, 2, 3, 4, 0, 1]))