def fluid_properties(fluid_str):  
   """
   Return the physical density and kinematic viscosity for the prescribed
   fluid.
   
   """
   fluid_lib = {'water':(1000., 1.0e-6), 
                'glycol':(965.3,0.06/965.3),
                'glycerin':(1260.0,1.49/1260.0)}
   if fluid_str in list(fluid_lib.keys()):
     return fluid_lib[fluid_str]
   else:
     print('valid fluids are:')
     for keys in fluid_lib:
       print(" '%s' " % keys)
     raise KeyError('invalid fluid specified')