ast_unparser.c doesn't handle := expressions

[ericvsmith]

BPO	36826
Nosy	@ericvsmith, @ambv, @emilyemorehouse, @pablogsal, @isidentical
PRs	bpo-36826: Handle := in ast_unparse.c #13337

^{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 2019-05-18.22:10:57.378>
created_at = <Date 2019-05-07.08:39:53.109>
labels = ['interpreter-core', 'type-bug', '3.8', 'release-blocker']
title = "ast_unparser.c doesn't handle := expressions"
updated_at = <Date 2019-05-18.22:10:57.377>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2019-05-18.22:10:57.377>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2019-05-18.22:10:57.378>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2019-05-07.08:39:53.109>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 36826
keywords = ['patch']
message_count = 3.0
messages = ['341699', '342560', '342820']
nosy_count = 5.0
nosy_names = ['eric.smith', 'lukasz.langa', 'emilyemorehouse', 'pablogsal', 'BTaskaya']
pr_nums = ['13337']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36826'
versions = ['Python 3.8']

[ericvsmith]

If either of these lines are added to test_annotations() in Lib/test/test_future.py, a SystemError will be raised:

    eq("(x:=10)")
    eq("f'{(x:=10):=10}'")

Traceback (most recent call last):
  File "/Users/eric/local/python/cpython/Lib/test/test_future.py", line 258, in test_annotations
    eq("(x:=10)")
  File "/Users/eric/local/python/cpython/Lib/test/test_future.py", line 134, in assertAnnotationEqual
    actual = self.getActual(annotation)
  File "/Users/eric/local/python/cpython/Lib/test/test_future.py", line 121, in getActual
    exec(self.template.format(ann=annotation), {}, scope)
SystemError: unknown expression kind

[isidentical]

I have patch, i'm going to submit it.

[pablogsal]

Thanks for the patch Batuhan!