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The old % formatting allowed arbitrary mappings:
 >>> class Default(dict):
 ... def __missing__(self, key):
 ... return key
 ...
 >>> s = '%(name)s was born in %(country)s'
 >>> s % Default(name='Guido')
 'Guido was born in country'
But the new str.format() demands mappings be first converted to a
regular dict using **kwargs, so something like the above is not possible.
 >>> s = '{name} was born in {country}'
 >>> s.format(**Default(name='Guido'))
 Traceback (most recent call last):
 File "<pyshell#27>", line 1, in <module>
 s.format(**Default(name='Guido'))
 KeyError: 'country'
There is a work-around using string.vformat() but it is obscure and awkward:
 >>> import string
 >>> string.Formatter().vformat(s, (), Default(name='Guido'))
 'Guido was born in country'
Instead, it would be better to offer an alternate method:
 >>> s.format_from_mapping(Default(name='Guido'))
 'Guido was born in country'