def chi_eff(mass1, mass2, spin1z, spin2z):
    """Returns the effective spin from mass1, mass2, spin1z, and spin2z."""
    return (spin1z * mass1 + spin2z * mass2) / (mass1 + mass2)
def chi_eff ( mass1 , mass2 , spin1z , spin2z ) : return ( spin1z * mass1 + spin2z * mass2 ) / ( mass1 + mass2 )
Returns the effective spin from mass1, mass2, spin1z, and spin2z.