def METHOD_NAME(self):
    net = op.network.Demo()
    o2 = op.phase.StandardGas(network=net, species='o2', name='pure_O2')
    n2 = op.phase.StandardGas(network=net, species='n2', name='pure_N2')
    air = op.phase.GasMixture(network=net, components=[n2, o2])
    del o2['pore.viscosity']
    _ = air['pore.viscosity.pure_N2']
    with pytest.raises(KeyError):
        _ = air['pore.viscosity.pure_O2']