I am getting a NameError that is not defined and not sure why it happen

[rick0922]

This is my Code:

def test():
    exec("a=100")
    print(a)
    
test()

The Traceback i get is this:

Error:
Traceback (most recent call last):
  File "C:\Users\lab135\Desktop\test.py", line 5, in <module>
    test()
  File "C:\Users\lab135\Desktop\test.py", line 3, in test
    print(a)
NameError: name 'a' is not defined

It does not work.
However,

exec("a=100")
print(a)

It works, why?
In my situation, I need to put it in the function.