strange behaviour of locals() after exec()

Question

I have a function that accepts a string with source code with some definitions, execs it - and then gets new definitions from locals().

def do_exec(code):
    exec(code)
    print(locals()['x'])

do_exec('x = 5')

this works as expected.

However, when I try to do this:

def do_exec_2(code):
    exec(code)
    x = locals()['x']
    print(x)


do_exec_2('x = 5')

I get an error:

KeyError: 'x'

why is that?