Python3：如何使用exec？

Question

如何在用户功能中使用exec？ 为什么功能未定义？

def func():
    exec('f = lambda x: x + 2')
    print(f(100))

if __name__ == "__main__":
    func()

Traceback (most recent call last):
  File "./playground/playground.py", line 10, in <module>
    func()
  File "./playground/playground.py", line 7, in func
    print(f(100))
NameError: name 'f' is not defined