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.