Difference between str.format and string.Formatter.format

This issue/bug has been raised here. It has been fixed in Python3.5.

It has not (yet) been patched for Python2.7. Until then, you can subclass string.Formatter to implement auto-arg-indexing:

import string    

class AutoArgFormatter(string.Formatter):
    """
    Backport of https://hg.python.org/cpython/rev/ad74229a6fba to Python2.7
    """
    def _vformat(self, format_string, args, kwargs, used_args, recursion_depth,
                 auto_arg_index=0):
        if recursion_depth < 0:
            raise ValueError('Max string recursion exceeded')
        result = []

        for literal_text, field_name, format_spec, conversion in \
                self.parse(format_string):

            # output the literal text
            if literal_text:
                result.append(literal_text)

            # this is some markup, find the object and do
            #  the formatting

            # handle arg indexing when empty field_names are given.
            if field_name == '':
                if auto_arg_index is False:
                    raise ValueError('cannot switch from manual field '
                                     'specification to automatic field '
                                     'numbering')
                field_name = str(auto_arg_index)
                auto_arg_index += 1
            elif field_name.isdigit():
                if auto_arg_index:
                    raise ValueError('cannot switch from manual field '
                                     'specification to automatic field '
                                     'numbering')
                # disable auto arg incrementing, if it gets
                # used later on, then an exception will be raised
                auto_arg_index = False

            # given the field_name, find the object it references
            #  and the argument it came from
            obj, arg_used = self.get_field(field_name, args, kwargs)
            used_args.add(arg_used)

            # do any conversion on the resulting object
            obj = self.convert_field(obj, conversion)

            # expand the format spec, if needed
            format_spec = self._vformat(format_spec, args, kwargs,
                                        used_args, recursion_depth-1,
                                        auto_arg_index=auto_arg_index)

            # format the object and append to the result
            result.append(self.format_field(obj, format_spec))

        return ''.join(result)

fmt = AutoArgFormatter()
print(fmt.format('{} {}', 'hello', 'world'))

yields

hello world