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.