scope of globals, locals, use with exec()

In your first example, locals is the globals dict, because you're on a module level.

You can check that with print(id(globals()), id(locals()))

In your second example, locals is a different dict in the scope of the function.

So to make example 2 work, either use:

def f1(glob, loc):
    exec("b = 5", glob, loc)
def f2():
    f1(globals(), globals())
    print(b + 1)
f2()

or

def f1(glob, loc):
    exec("b = 5", glob, glob)
def f2(loc):
    f1(globals(), loc)
    print(b + 1)
f2(locals())

So this isn't really a matter of exec() but of in which scope you're calling locals().

And ofc it's also a matter of what the code in exec does, because an assignment changes the scope of the name to the current (local) scope, unless global/nonlocal is involved. That's why b=5 is executed in the local scope - except that in your first example, the "local" scope is the global scope.

Follow-up:

if you want to re-assign a value to b in f2 you will need the global keyword:

def f1(glob, loc):
    exec("b = 5", glob, loc)
def f2():
    f1(globals(), globals())
    global b
    b += 1
    print(b + 1) # 7
f2()

Note that this has nothing to do with exec and passing around locals() / globals() - you can reduce it to this example, which would also fail w/o the global keyword (it would throw an UnboundLocalError: local variable 'b' referenced before assignment error).

b = 5
def f2():
    global b
    b += 1
    print(b + 1) # 7
f2()