Fix punctuation in isinstance() error message

[wyz23x2]

BPO	44640
Nosy	@ambv, @pablogsal, @miss-islington, @wyz23x2, @jdevries3133
PRs	bpo-44640: Improve punctuation in isinstance() error message #27144 [3.10] bpo-44640: Improve punctuation consistency in isinstance/issubclass error messages (GH-27144) #28436

^{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 2021-09-19.19:31:14.561>
created_at = <Date 2021-07-14.17:40:20.209>
labels = ['interpreter-core', 'type-bug', '3.9', '3.10', '3.11']
title = 'Fix punctuation in isinstance() error message'
updated_at = <Date 2021-10-04.19:18:42.710>
user = 'https://github.com/wyz23x2'

bugs.python.org fields:

activity = <Date 2021-10-04.19:18:42.710>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-09-19.19:31:14.561>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2021-07-14.17:40:20.209>
creator = 'wyz23x2'
dependencies = []
files = []
hgrepos = []
issue_num = 44640
keywords = ['patch']
message_count = 6.0
messages = ['397499', '397501', '397502', '402110', '402163', '403160']
nosy_count = 5.0
nosy_names = ['lukasz.langa', 'pablogsal', 'miss-islington', 'wyz23x2', 'jack__d']
pr_nums = ['27144', '28436']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44640'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[wyz23x2]

>>> isinstance(2, 1)
Traceback (most recent call last):
  ...
TypeError: isinstance() arg 2 must be a type, a tuple of types or a union
>>> issubclass(int, 1)
Traceback (most recent call last):
  ..
TypeError: issubclass() arg 2 must be a class, a tuple of classes, or a union.

1. It should be "an union", not "a union".
2. The punctuation marks aren't the same -- there's a comma before "or" in issubclass, but not isinstance(). And issubclass()'s ends with a period, which isn't the same with other builtins' messages.

[jdevries3133]

The current string is correct. The word "union" is actually an exception to the "a/an" rule. Here is more detail:

https://english.stackexchange.com/questions/266309/why-is-union-an-exception-to-the-a-an-rule

[wyz23x2]

Changed to only 2).

[ambv]

New changeset f481338 by wyz23x2 in branch 'main':
bpo-44640: Improve punctuation consistency in isinstance/issubclass error messages (GH-27144)
f481338

[pablogsal]

New changeset 9c23a1e by Miss Islington (bot) in branch '3.10':
bpo-44640: Improve punctuation consistency in isinstance/issubclass error messages (GH-27144) (GH-28436)
9c23a1e

[pablogsal]

New changeset 39c4fe5 by Pablo Galindo (Miss Islington (bot)) in branch '3.10':
bpo-44640: Improve punctuation consistency in isinstance/issubclass error messages (GH-27144) (GH-28436)
39c4fe5