bad error message when combining _ grouping specifier with invalid type

[benjaminp]

BPO	34868
Nosy	@benjaminp, @serhiy-storchaka, @miss-islington
PRs	bpo-34868: Improve error message with '_' is combined with an invalid type specifier. #9666 [3.7] closes bpo-34868: Improve error message with '_' is combined with an invalid type specifier. (GH-9666) #9668 [3.6] closes bpo-34868: Improve error message with '_' is combined with an invalid type specifier. (GH-9666) #9669

^{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 2018-10-02.04:54:44.824>
created_at = <Date 2018-10-02.04:10:59.018>
labels = ['interpreter-core', '3.7', '3.8']
title = 'bad error message when combining _ grouping specifier with invalid type'
updated_at = <Date 2018-10-02.05:18:18.421>
user = 'https://github.com/benjaminp'

bugs.python.org fields:

activity = <Date 2018-10-02.05:18:18.421>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-02.04:54:44.824>
closer = 'benjamin.peterson'
components = ['Interpreter Core']
creation = <Date 2018-10-02.04:10:59.018>
creator = 'benjamin.peterson'
dependencies = []
files = []
hgrepos = []
issue_num = 34868
keywords = ['patch']
message_count = 5.0
messages = ['326848', '326851', '326856', '326857', '326858']
nosy_count = 3.0
nosy_names = ['benjamin.peterson', 'serhiy.storchaka', 'miss-islington']
pr_nums = ['9666', '9668', '9669']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34868'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[benjaminp]

>>> format(34, '_n')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Cannot specify ',' with 'n'.

Why is it talking about ","?

[serhiy-storchaka]

See bpo-31780.

[benjaminp]

New changeset cbda8fc by Benjamin Peterson in branch 'master':
closes bpo-34868: Improve error message with '_' is combined with an invalid type specifier. (GH-9666)
cbda8fc

[miss-islington]

New changeset cd4dd93 by Miss Islington (bot) in branch '3.7':
closes bpo-34868: Improve error message with '_' is combined with an invalid type specifier. (GH-9666)
cd4dd93

[miss-islington]

New changeset 7455bf4 by Miss Islington (bot) in branch '3.6':
closes bpo-34868: Improve error message with '_' is combined with an invalid type specifier. (GH-9666)
7455bf4