changelog for DASH_PROXY and lint

CHANGELOG.md

@@ -4,6 +4,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [UNRELEASED]
 ### Added
+- [#1289](https://github.com/plotly/dash/pull/1289) Supports `DASH_PROXY` env var to tell `app.run_server` to report the correct URL to view your app, when it's being proxied. Throws an error if the proxy is incompatible with the host and port you've given the server.
 - [#1240](https://github.com/plotly/dash/pull/1240) Adds `callback_context` to clientside callbacks (e.g. `dash_clientside.callback_context.triggered`). Supports `triggered`, `inputs`, `inputs_list`, `states`, and `states_list`, all of which closely resemble their serverside cousins.
 
 ### Changed

dash/dash.py

@@ -1599,7 +1599,7 @@ def verify_url_part(served_part, url_part, part_name):
             else:
                 display_url = (protocol, host, ":{}".format(port), path)
 
-            self.logger.info("Running on {}://{}{}{}".format(*display_url))
+            self.logger.info("Running on %s://%s%s%s", *display_url)
 
         if not os.environ.get("FLASK_ENV"):
             os.environ["FLASK_ENV"] = "development"