limited exec: can't assign new value to variable?

Question

Having this code:

x = 10
exec('x += 5', {}, {'x': x})
print(x)  # prints 10.

# This works of course
exec('x += 5')
print(x)  # prints 15.

Why it does ignore my x += 5 expression? Is there something else I remove, by limiting globals/locals on exec, so it does not allow to change variable?

P.S. Though it does work if I modify dictionary. Is it related with immutable types maybe?

Answer 1

Here's what is going on:

>>> x = 5
>>> loc = dict(x=x)
>>> exec('x += 5', {}, loc)
>>> print(loc)
{'x': 10}
>>> x
5