from yt.mods import *

#### define some helper quantities
def _ColdGasMass(data, Tcold=1e4):
    """
    This function returns the total mass of all cold gas, with T < Tcold.
    """

    gas_mass = data['CellMassMsun']
    gas_temperature = data['Temperature']
    ind = (gas_temperature < Tcold)
    coldgas_mass = gas_mass[ind].sum()
    return coldgas_mass

def _combColdGasMass(data, coldgas_mass):
    return coldgas_mass.sum()

add_quantity("ColdGasMass", function=_ColdGasMass, combine_function=_combColdGasMass, n_ret=1)


pf = load('DD0030/galaxy0030')
region = pf.h.region((0.5,0.5,0.5),(0.0,0.0,0.0),(1.0,1.0,1.0))
ColdGasMass = region.quantities['ColdGasMass']()
print ColdGasMass