here is a code show what happend:
>>> def func():
	exec('ans=1')
	print(dir())
	return ans

>>> func()
['ans']
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    func()
  File "<pyshell#4>", line 4, in func
    return ans
NameError: name 'ans' is not defined

i tried this code in version 2.7,it is ok