为什么在使用exec时会得到NoneType？

Question

我正在尝试比较两个随机整数，但是它不起作用，当将{}转换为mode [0]时出现TypeError

if exec("doggies[mode[0][0]][0]") >= exec("doggies[mode[1][0]][0]"):

值：

mode = ["player", "computer"] #THESE MUST BE STRINGS
doggies = { n : [randint(1,5), randint(1,100), randint(1,10), randint(1,10)]  for n in dogs} 

dogs = []
file = open("dogs.txt")
for n in file:
    n = n.strip()
    dogs.append(n)
file.close()

player = ["marley", "barley"]
computer = ["ley the ma", "im bad at names"]
#these two lists above are supposed to have random names inside, but for now i'll just put them as fixed values so it's easier to understand

dogs.txt：

marley the barley
barley the marley
ley the ma
im bad at names

运行if exec("doggies[{}[0]][0]".format(mode[0])) >= exec("doggies[{}[0]][0]".format(mode[1])):会得到：

  

TypeError：“ NoneType”和“ NoneType”的实例之间不支持“> =”

如果您仍然感到困惑，请按此分类。

if exec("doggies[{}[0]][0]".format(mode[0])) >= exec("doggies[{}[0]][0]".format(mode[1])):

应该立即执行

if doggies[player[0]][0] >= doggies[computer[0]][0]:

已简化

if doggies[marley][0] >= doggies[ley the ma][0]:

已简化

if 1 >= 2

1和2应该是介于1和5之间的随机数，但现在它们是固定数。