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*Draft* patch fixing the issue: render_field() raises an error if the argument is an unicode argument, string_format() catchs this error and converts self to unicode and call unicode.format(*args, **kw).
Pseudo-code:
 try:
 # self.format() raises an error if any argument is 
 # an unicode string)
 return self.format(*args,**kw)
 except UnicodeError:
 unicode = self.decode(default_encoding)
 return unicode.format(*args, **kw)
The patch changes the result type of '{}'.format(u'ascii'): it was str and it becomes unicode. The new behaviour is consistent with "%s" % u"ascii" => u"ascii" (unicode).
I'm not sure that catching *any* unicode error is a good idea. I think that it would be better to use a new exception type dedicated to this issue, but it looks complex to define a new exception. I will may do it for the next patch version ;-)