#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import math

def quadratic(a, b, c):
    if not isinstance(a, (int, float)):
        raise TypeError('bad operand type')
    if not isinstance(b, (int, float)):
        raise TypeError('bad operand type')
    if not isinstance(b, (int, float)):
        raise TypeError('bad operand type')

    delta = b * b - 4 * a * c
    if delta >= 0:
        e = math.sqrt(delta)
        m = (-b + e) / (2 * a)
        n = (-b - e) / (2 * a)
        print('The equation\'s roots are...\nx1 = %.1f , x2 = %.1f ...' % (m, n))
        return m,n
    else:
        print('There is no root...')
        pass

print('please enter the a, b, c of the equation: ax2 + bx + c = 0')
t1 = float(input('a = ',))
t2 = float(input('b = ',))
t3 = float(input('c = ',))

r = quadratic(t1, t2, t3)

最后一行，用 r = ，如果方程没有解，就运行正常， 如果写成 x1，x2 = ， 如果方程没有解，就NoneType object is not iterable ... 求解释...