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I came across this piece of code in Lib/string.py:146:
 # We use this idiom instead of str() because the latter will
 # fail if val is a Unicode containing non-ASCII characters.
 return '%s' % (mapping[named],)
This seems vestigial. I think it'd be appropriate to "fix" the string.Template by replacing the printf-style formatting with newer str.format everywhere in the Template's source. The obvious advantage is that by tweaking some regexes we'll make possible formatting using the following syntax
 ${thing.attribute_or_key}
by dropping to the str.format
 return '{named}'.format(**mapping) # <-- new version
It'd also make sense to use the str.format_map to implement the Template.safe_substitute.
Borrowing some ideas from issue1198569, we can then expose an additional attribute called Template.bracepattern to allow programmers use the str.format-based substitution extensively:
	$name
	${name.thing}
	${name.thing: >16}
This change won't break any existing code.
But I'm not exactly sure string.Template should be in Python 3 at all. It's one of the least used features and PEP 292 states it was added as an alternative to %-based substitution. I think it'd be deprecated and removed from the standard library.
So what's the resolution? Should we fix it or deprecate it or both?