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

    First method:
       Le(Sc =  Schmidt number,
          Pr = Prandtl number)

    Second method:
        Le(D = mass diffusivity,
           alpha = thermal diffusivity)
    """

    if ('Sc' in kwargs) and ('Pr' in kwargs):
        lewis_number = kwargs['Sc'] / kwargs['Pr']
    elif ('alpha' in kwargs) and ('D' in kwargs):
        lewis_number = kwargs['alpha'] / kwargs['D']
    else:
        raise KeyError('Incorrect variable assignment')

    return lewis_number