str.format() doesn't handle unicode arguments:

Python 2.6.4 (r264:75706, Oct 27 2009, 15:18:04) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> '{0}'.format(u'\u3042')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\u3042' in
position 0: ordinal not in range(128)

Unicode arguments should be treated in the same way as the % operator
does it: by promoting the format string to unicode:

>>> '%s' % u'\u3042'
u'\u3042'

PyString_Format() uses a "goto unicode;" if a '%c' or '%s' argument is unicode. The unicode label converts the partial formatted result (byte string) to unicode, and use PyUnicode_Format() to finish to formatting.

I don't think that you can apply the same algorithm here (converts the partial result to unicode) because it requires to rewrite the format string: arguments can be used twice or more, and used in any order.

Example: "{0} {1}".format("bytes", u"unicode") => switch to unicode occurs at result="bytes ", format=" {1}", arguments=(u"unicode"). Converts "bytes " to unicode is easy, but the format have to be rewritten in " {0}" or something else.

Call trace of str.format(): do_string_format() -> build_string() -> output_markup() -> render_field(). The argument type is proceed in render_field().

*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 ;-)

My patch converts the format string to unicode using the default encoding. It's inconsistent with str%args: str%args converts str to unicode using the ASCII charset (if a least one argument is an unicode string), not the default encoding.

>>> "\xff%s" % u'\xe9'
...
UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 0: ordinal not in range(128)

I'm not sure I'm wild about doing the work twice, once as string and once as unicode if need be. But I'll consider it, especially since this is only a 2.7 issue.

There could be side effects of evaluating the replacement strings, but I'm not sure it's worth worrying about. Attribute (or index) access having side effects isn't something I think we need to cater to.