Use `dict()` to convert the mapping to a dictionary

notebook/log.py

@@ -43,7 +43,6 @@ def log_request(handler):
         msg = msg + ' referer={referer}'
     if status >= 500 and status != 502:
         # log all headers if it caused an error
-        headers = { k: request.headers[k] for k in request.headers }
-        log_method(json.dumps(headers, indent=2))
+        log_method(json.dumps(dict(request.headers), indent=2))
     log_method(msg.format(**ns))