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.

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.
  "

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

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