Python exec命令不会更改变量

Question

代码

a = 2
print("a = {:d}".format(a))
my_dict = {'test_function': lambda x: exec("a = {:d}".format(x))}
my_dict['test_function'](3)
print("a = {:d}".format(a))

返回

a = 2
a = 2

代替

a = 2
a = 3

我在做什么错了？