# Buggy code
def render_GET(self, request):
    """
    .. http:get:: /torrentinfo

    A GET request to this endpoint will return information from a torrent found at a provided URI.
    This URI can either represent a file location, a magnet link or a HTTP(S) url.
    - torrent: the URI of the torrent file that should be downloaded. This parameter is required.

        **Example request**:

            .. sourcecode:: none

                curl -X PUT http://localhost:8085/torrentinfo?torrent=file:/home/me/test.torrent

        **Example response**:

            .. sourcecode:: javascript

                {"metainfo": <torrent metainfo dictionary>}
    """
    metainfo_deferred = Deferred()

    def on_got_metainfo(metainfo):
        if not isinstance(metainfo, dict) or "info" not in metainfo:
            self._logger.warning("Received metainfo is not a valid dictionary")
            request.setResponseCode(http.INTERNAL_SERVER_ERROR)
            request.write(json.dumps({"error": "invalid response"}))
            self.finish_request(request)
            return

        infohash = hashlib.sha1(bencode(metainfo["info"])).digest()
        # Save the torrent to our store
        try:
            self.session.save_collected_torrent(infohash, bencode(metainfo))
        except TypeError:
            # Note: in libtorrent 1.1.1, bencode throws a TypeError which is a known bug
            pass

        request.write(json.dumps({"metainfo": metainfo}, ensure_ascii=False))
        self.finish_request(request)

    def on_metainfo_timeout(_):
        if not request.finished:
            request.setResponseCode(http.REQUEST_TIMEOUT)
            request.write(json.dumps({"error": "timeout"}))
        # If the above request.write failed, the request will have already been finished
        if not request.finished:
            self.finish_request(request)

    def on_lookup_error(failure):
        failure.trap(ConnectError, DNSLookupError, HttpError)
        request.setResponseCode(http.INTERNAL_SERVER_ERROR)
        request.write(json.dumps({"error": failure.getErrorMessage()}))
        self.finish_request(request)

    if "uri" not in request.args or len(request.args["uri"]) == 0:
        request.setResponseCode(http.BAD_REQUEST)
        return json.dumps({"error": "uri parameter missing"})

    uri = unicode(request.args["uri"][0], "utf-8")
    if uri.startswith("file:"):
        try:
            filename = url2pathname(uri[5:].replace("+", " "))
            metainfo_deferred.callback(bdecode(fix_torrent(filename)))
        except TypeError:
            request.setResponseCode(http.INTERNAL_SERVER_ERROR)
            return json.dumps({"error": "error while decoding torrent file"})
    elif uri.startswith("http"):

        def _on_loaded(metadata):
            metainfo_deferred.callback(bdecode(metadata))

        http_get(uri.encode("utf-8")).addCallback(_on_loaded).addErrback(
            on_lookup_error
        )
    elif uri.startswith("magnet"):
        infohash = parse_magnetlink(uri)[1]
        if infohash is None:
            request.setResponseCode(http.BAD_REQUEST)
            return json.dumps({"error": "missing infohash"})

        if self.session.has_collected_torrent(infohash):
            try:
                tdef = TorrentDef.load_from_memory(
                    self.session.get_collected_torrent(infohash)
                )
            except ValueError as exc:
                request.setResponseCode(http.INTERNAL_SERVER_ERROR)
                return json.dumps({"error": "invalid torrent file: %s" % str(exc)})
            on_got_metainfo(tdef.get_metainfo())
            return NOT_DONE_YET

        self.session.lm.ltmgr.get_metainfo(
            uri,
            callback=metainfo_deferred.callback,
            timeout=20,
            timeout_callback=on_metainfo_timeout,
            notify=True,
        )
    else:
        request.setResponseCode(http.BAD_REQUEST)
        return json.dumps({"error": "invalid uri"})

    metainfo_deferred.addCallback(on_got_metainfo)

    return NOT_DONE_YET


# Traceback (TypeError)
Traceback (most recent call last):
File "/home/quinten/Documents/tribler/TriblerGUI/tribler_window.py", line 633, in on_confirm_add_directory_dialog
self.tribler_settings['download_defaults']['saveas'], [], 0)
File "/home/quinten/Documents/tribler/TriblerGUI/tribler_window.py", line 369, in perform_start_download_request
if not is_dir_writable(destination):
File "/home/quinten/Documents/tribler/TriblerGUI/utilities.py", line 216, in is_dir_writable
if not os.path.exists(path):
File "/usr/lib/python2.7/genericpath.py", line 26, in exists
os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found

# Fixed code
def render_GET(self, request):
    """
    .. http:get:: /torrentinfo

    A GET request to this endpoint will return information from a torrent found at a provided URI.
    This URI can either represent a file location, a magnet link or a HTTP(S) url.
    - torrent: the URI of the torrent file that should be downloaded. This parameter is required.

        **Example request**:

            .. sourcecode:: none

                curl -X PUT http://localhost:8085/torrentinfo?torrent=file:/home/me/test.torrent

        **Example response**:

            .. sourcecode:: javascript

                {"metainfo": <torrent metainfo dictionary>}
    """
    metainfo_deferred = Deferred()

    def on_got_metainfo(metainfo):
        if not isinstance(metainfo, dict) or "info" not in metainfo:
            self._logger.warning("Received metainfo is not a valid dictionary")
            request.setResponseCode(http.INTERNAL_SERVER_ERROR)
            request.write(json.dumps({"error": "invalid response"}))
            self.finish_request(request)
            return

        infohash = hashlib.sha1(bencode(metainfo["info"])).digest()
        # Save the torrent to our store
        try:
            self.session.save_collected_torrent(infohash, bencode(metainfo))
        except TypeError:
            # Note: in libtorrent 1.1.1, bencode throws a TypeError which is a known bug
            pass

        request.write(json.dumps({"metainfo": metainfo}, ensure_ascii=False))
        self.finish_request(request)

    def on_metainfo_timeout(_):
        if not request.finished:
            request.setResponseCode(http.REQUEST_TIMEOUT)
            request.write(json.dumps({"error": "timeout"}))
        # If the above request.write failed, the request will have already been finished
        if not request.finished:
            self.finish_request(request)

    def on_lookup_error(failure):
        failure.trap(ConnectError, DNSLookupError, HttpError)
        request.setResponseCode(http.INTERNAL_SERVER_ERROR)
        request.write(json.dumps({"error": failure.getErrorMessage()}))
        self.finish_request(request)

    if "uri" not in request.args or len(request.args["uri"]) == 0:
        request.setResponseCode(http.BAD_REQUEST)
        return json.dumps({"error": "uri parameter missing"})

    uri = unicode(request.args["uri"][0], "utf-8")
    if uri.startswith("file:"):
        try:
            filename = url2pathname(
                uri[5:].encode("utf-8") if isinstance(uri, unicode) else uri[5:]
            )
            metainfo_deferred.callback(bdecode(fix_torrent(filename)))
        except TypeError:
            request.setResponseCode(http.INTERNAL_SERVER_ERROR)
            return json.dumps({"error": "error while decoding torrent file"})
    elif uri.startswith("http"):

        def _on_loaded(metadata):
            metainfo_deferred.callback(bdecode(metadata))

        http_get(uri.encode("utf-8")).addCallback(_on_loaded).addErrback(
            on_lookup_error
        )
    elif uri.startswith("magnet"):
        infohash = parse_magnetlink(uri)[1]
        if infohash is None:
            request.setResponseCode(http.BAD_REQUEST)
            return json.dumps({"error": "missing infohash"})

        if self.session.has_collected_torrent(infohash):
            try:
                tdef = TorrentDef.load_from_memory(
                    self.session.get_collected_torrent(infohash)
                )
            except ValueError as exc:
                request.setResponseCode(http.INTERNAL_SERVER_ERROR)
                return json.dumps({"error": "invalid torrent file: %s" % str(exc)})
            on_got_metainfo(tdef.get_metainfo())
            return NOT_DONE_YET

        self.session.lm.ltmgr.get_metainfo(
            uri,
            callback=metainfo_deferred.callback,
            timeout=20,
            timeout_callback=on_metainfo_timeout,
            notify=True,
        )
    else:
        request.setResponseCode(http.BAD_REQUEST)
        return json.dumps({"error": "invalid uri"})

    metainfo_deferred.addCallback(on_got_metainfo)

    return NOT_DONE_YET