Reproducer:

    def test_forward_type_references(self):
        def foo(a: 'Foo') -> 'Bar': pass

        class Foo: pass
        class Bar: pass

        get_type_hints(foo)


The above gives the following exception, rather than resolving the type:

/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py:1001: in get_type_hints
    value = _eval_type(value, globalns, localns)
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py:260: in _eval_type
    return t._evaluate(globalns, localns)
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/typing.py:464: in _evaluate
    eval(self.__forward_code__, globalns, localns),
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   NameError: name 'Foo' is not defined

This cannot be helped (unless we were to add an ugly sys._getframe() call to get_type_hints(), which I don't want to do). The solution is to pass `localns=locals()`, e.g.

get_type_hints(foo, None, locals())

or

get_type_hints(foo, localns=locals())

The tough one is that no-one wants an ugly sys._getframe() call, but by avoiding it in the standard library, we force each library that needs this to have the ugly sys._getframe() call rather than it being an unpleasant implementation detail of get_type_hints that users of the function don't have to know about.

That said, I've only hit this so far when writing a unit test, but will update this issue if I see real-world cases of this.

I've only seen it in test cases too -- that's one of the very few
situations where it makes sense to define a class inside a function.
(Creating a class is an expensive operation, so any function that expects
to be called more than once is better off moving the class out of the
function.)