为何我写成这样是错的：

a = eval(input())
b = eval(input())
if b = 0:
    print('除零错误')
else:
    print(round(a/b,2))


而这样是对的;

a = eval(input())
b = eval(input())
if b != 0:
    print(round(a/b,2))
else:
    print("除零错误")