def lookup(self, hostname):
        """
        Find a hostkey entry for a given hostname or IP.  If no entry is found,
        ``None`` is returned.  Otherwise a dictionary of keytype to key is
        returned.  The keytype will be either ``"ssh-rsa"`` or ``"ssh-dss"``.

        :param str hostname: the hostname (or IP) to lookup
        :return: dict of `str` -> `.PKey` keys associated with this host
            (or ``None``)
        """

        class SubDict(MutableMapping):
            def __init__(self, hostname, entries, hostkeys):
                self._hostname = hostname
                self._entries = entries
                self._hostkeys = hostkeys

            def __iter__(self):
                for k in self.keys():
                    yield k

            def __len__(self):
                return len(self.keys())

            def __delitem__(self, key):
                for e in list(self._entries):
                    if e.key.get_name() == key:
                        self._entries.remove(e)
                else:
                    raise KeyError(key)

            def __getitem__(self, key):
                for e in self._entries:
                    if e.key.get_name() == key:
                        return e.key
                raise KeyError(key)

            def __setitem__(self, key, val):
                for e in self._entries:
                    if e.key is None:
                        continue
                    if e.key.get_name() == key:
                        # replace
                        e.key = val
                        break
                else:
                    # add a new one
                    e = HostKeyEntry([hostname], val)
                    self._entries.append(e)
                    self._hostkeys._entries.append(e)

            def keys(self):
                return [
                    e.key.get_name()
                    for e in self._entries
                    if e.key is not None
                ]

        entries = []
        for e in self._entries:
            if self._hostname_matches(hostname, e):
                entries.append(e)
        if len(entries) == 0:
            return None
        return SubDict(hostname, entries, self)
Find a hostkey entry for a given hostname or IP.  If no entry is found,
        ``None`` is returned.  Otherwise a dictionary of keytype to key is
        returned.  The keytype will be either ``"ssh-rsa"`` or ``"ssh-dss"``.

        :param str hostname: the hostname (or IP) to lookup
        :return: dict of `str` -> `.PKey` keys associated with this host
            (or ``None``)