Cope with the absence of a file class in Python 3.

    _mapping_mixin = DictMixin

    _mutable_mapping_mixin = DictMixin

if sys.version >= '3':

    import io

    def _is_real_file(f):

        if not isinstance(f, io.IOBase):

            return False

        try:

            f.fileno()

            return True

        except (AttributeError, io.UnsupportedOperation):

            return False

else:

    def _is_real_file(f):

        return isinstance(f, file) and hasattr(f, 'fileno')

GPGV_DEFAULT_KEYRINGS = frozenset(['/usr/share/keyrings/debian-keyring.gpg'])

GPGV_EXECUTABLE = '/usr/bin/gpgv'

            necessary in order to properly interpret the strings.)

        """

        if _have_apt_pkg and use_apt_pkg and isinstance(sequence, file):

        if _have_apt_pkg and use_apt_pkg and _is_real_file(sequence):

            parser = apt_pkg.TagFile(sequence)

            for section in parser:

                paragraph = cls(fields=fields,