get_type_hints fails to resolve forward references in nested function

[cjw296]

BPO	41872
Nosy	@gvanrossum, @cjw296, @ilevkivskyi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-09-27.19:59:08.263>
created_at = <Date 2020-09-27.13:58:03.622>
labels = ['type-bug', '3.8', '3.9', '3.10', 'invalid', '3.7', 'library']
title = 'get_type_hints fails to resolve forward references in nested function'
updated_at = <Date 2020-09-27.22:00:19.694>
user = 'https://github.com/cjw296'

bugs.python.org fields:

activity = <Date 2020-09-27.22:00:19.694>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-27.19:59:08.263>
closer = 'gvanrossum'
components = ['Library (Lib)']
creation = <Date 2020-09-27.13:58:03.622>
creator = 'cjw296'
dependencies = []
files = []
hgrepos = []
issue_num = 41872
keywords = ['3.6regression']
message_count = 4.0
messages = ['377569', '377578', '377579', '377581']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'cjw296', 'levkivskyi']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41872'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[cjw296]

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

[gvanrossum]

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())