Inaccurate error message for f-string

[seblin]

BPO	35494
Nosy	@ericvsmith, @serhiy-storchaka
PRs	bpo-35494: Improve syntax error messages for unbalanced parentheses in f-string. #11161
Dependencies	bpo-33306: Improving SyntaxError for unmatched parentheses

^{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 = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2019-01-12.07:47:17.498>
created_at = <Date 2018-12-14.13:34:17.302>
labels = ['interpreter-core', 'type-feature', '3.8']
title = 'Inaccurate error message for f-string'
updated_at = <Date 2019-01-12.07:47:17.497>
user = 'https://bugs.python.org/seblin'

bugs.python.org fields:

activity = <Date 2019-01-12.07:47:17.497>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2019-01-12.07:47:17.498>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2018-12-14.13:34:17.302>
creator = 'seblin'
dependencies = ['33306']
files = []
hgrepos = []
issue_num = 35494
keywords = ['patch']
message_count = 6.0
messages = ['331827', '331828', '331832', '331833', '331839', '333521']
nosy_count = 3.0
nosy_names = ['eric.smith', 'serhiy.storchaka', 'seblin']
pr_nums = ['11161']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35494'
versions = ['Python 3.8']

[seblin]

Python 3.7.1 (v3.7.1:260ec2c36a, Oct 20 2018, 14:05:16) [MSC v.1915 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> spam = 'spam'
>>> f'{spam[0}'
  File "<stdin>", line 1
SyntaxError: f-string: expecting '}'

The error message seems wrong because a "]" is missing rather than a "}".

[serhiy-storchaka]

I can take this issue if you do not mind.

[ericvsmith]

Go ahead, Serhiy. Thanks!

[seblin]

The same behavior applies to f'{spam[}' and f'{spam(}'. Also to f'{spam{}', but that might be expected.

This message is more clear:
>>> f'{spam('
  File "<stdin>", line 1
SyntaxError: f-string: mismatched '(', '{', or '['

Perhaps you want to apply that to the above error cases. Then you wouldn't have to catch any type of bracket. :-)

[serhiy-storchaka]

PR 11161 uses an approach similar to bpo-33306.

[serhiy-storchaka]

New changeset 58159ef by Serhiy Storchaka in branch 'master':
bpo-35494: Improve syntax error messages for unbalanced parentheses in f-string. (GH-11161)
58159ef