string.Template documentation incorrectly references "identifiers"

[tifv]

BPO	24351
Nosy	@warsaw, @taleinat, @tifv

^{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/warsaw'
closed_at = <Date 2015-06-09.18:27:19.090>
created_at = <Date 2015-06-01.19:05:07.872>
labels = ['docs']
title = 'string.Template documentation incorrectly references "identifiers"'
updated_at = <Date 2015-06-15.06:57:51.018>
user = 'https://github.com/tifv'

bugs.python.org fields:

activity = <Date 2015-06-15.06:57:51.018>
actor = 'Arfrever'
assignee = 'barry'
closed = True
closed_date = <Date 2015-06-09.18:27:19.090>
closer = 'barry'
components = ['Documentation']
creation = <Date 2015-06-01.19:05:07.872>
creator = 'july'
dependencies = []
files = []
hgrepos = []
issue_num = 24351
keywords = []
message_count = 4.0
messages = ['244607', '244871', '245071', '245080']
nosy_count = 5.0
nosy_names = ['barry', 'taleinat', 'july', 'docs@python', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue24351'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6']

[tifv]

Documentation of Template says:

$identifier names a substitution placeholder matching a mapping key of "identifier". By default, "identifier" must spell a Python identifier. The first non-identifier character after the $ character terminates this placeholder specification.

While it was true when these lines were written, Python 3 has greatly extended identifier character set, and it no longer matches the set of characters used by Template:

>>> Template('$foo123щъ').substitute(foo=1, foo123=2, foo123щъ=3)
'2щъ'

I propose clarifying documentation.

Actually extending character set is backward-incompatible, although it won't be if restricted to "${identifier}" syntax.

[warsaw]

Rather than change the code, which strives to keep the "Simple" in PEP-292's title, I'd rather just update the documentation to define what "identifier" means here. E.g.

"

• $identifier names a substitution placeholder matching a mapping key of
  "identifier". By default, "identifier" is restricted to any
  case-insensitive ASCII alphanumeric string (including underscores) that
  starts with an underscore or ASCII letter. The first non-identifier
  character after the $ character terminates this placeholder
  specification.
  "

[taleinat]

+1 for keeping the current behavior and fixing the docs.

[python-dev]

New changeset 20bf84cd38bc by Barry Warsaw in branch '3.4':

• Issue bpo-24351: Clarify what is meant by "identifier" in the context of
  https://hg.python.org/cpython/rev/20bf84cd38bc

New changeset 0fa48f581fe3 by Barry Warsaw in branch '3.5':

• Issue bpo-24351: Clarify what is meant by "identifier" in the context of
  https://hg.python.org/cpython/rev/0fa48f581fe3

New changeset 1cdedc9a9e9f by Barry Warsaw in branch 'default':

• Issue bpo-24351: Clarify what is meant by "identifier" in the context of
  https://hg.python.org/cpython/rev/1cdedc9a9e9f