def Prandtl(**kwargs):
    """
    Calculates Prandtl number based upon either of two input variable sets.

    First method:
        Pr(C_p = specific heat capacity,
           mu = dynamic viscosity,
           k = thermal conductivity)

    Second method:
       Pr(nu = kinematic viscosity,
          alpha = thermal diffusivity)
    """

    if ('C_p' in kwargs) and ('k' in kwargs) and ('mu' in kwargs):
        prandtl_number = (kwargs['C_p'] * kwargs['mu']) / kwargs['k']
    elif ('nu' in kwargs) and ('alpha' in kwargs):
        prandtl_number = kwargs['nu'] / kwargs['alpha']
    else:
        raise KeyError('Incorrect variable assignment')

    return prandtl_number