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

This seems to a behaviour change. In 2.7, return ans compiles to LOAD_NAME so it can find ans. But in 3.x, it compiles to LOAD_GLOBAL which searches ans from global scope. I don't know when this change is introduced.

In Python 2, using the exec statement makes the compiler disable fast locals: 

    >>> def foo(): pass
    ... 
    >>> def bar(): exec ''
    ... 
    >>> foo.__code__.co_flags & inspect.CO_OPTIMIZED
    1
    >>> bar.__code__.co_flags & inspect.CO_OPTIMIZED
    0

This has never been the case in Python 3, in which exec() is a function instead of a statement. The exec function can be shadowed by a global named "exec", so the hack to disable fast locals was removed.