Python for 循环不迭代

Question

迭代只运行一次，下面是我试图运行的代码，def heat_coeff(gas,Nu,d):    """Calculate heat transfer (at interior tube wall)"""    mass = gas *100    diff = mass * 2    for i in d:        U = 4.*diff*Nu / (i*i)        return Uheat_coeff(10,4, [1,2,3])