2，4，2算出來小於0了，程式走了你的else，然後else這裡又沒有給x1，x2賦值，怎麼返回？

def quadratic(a,b,c):

d=b**2-4*a*c

if d>0:

x1=(-b+d**0.5/2/a)

x2=(-b-d**0.5)/2/a

print("The quadratic is :{0}x^2+{1}x+{2}=0".format(a,b,c))

return x1,x2

else:

print "There is no real root"

return None,None

x1,x2=quadratic(2,4,2)

#There is no real root

print x1,x2

#None None

x1,x2=quadratic(2,16,2)

#The quadratic is :2x^2+16x+2=0

print x1,x2

#-12.1270166538 -7.87298334621

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