import yt

ds = yt.load('IsolatedGalaxy/galaxy0030/galaxy0030')

ds.index

index = ds.index

print index.grids

print index.grid_levels.T

grids_below_level_5 = (index.grid_levels < 5).T[0]

print index.grids[grids_below_level_5]

grid = index.grids[0]

grid_density =  grid['density']
print grid_density.shape
print grid_density[0,]