Don't use format in log. Use log builtin capabilities.

Author: Carreau

nbviewer/app.py

@@ -497,7 +497,7 @@ def fetch_kwargs(self):
         if self.proxy_host:
             fetch_kwargs.update(proxy_host=self.proxy_host, proxy_port=self.proxy_port)
             self.log.info(
-                "Using web proxy {proxy_host}:{proxy_port}." "".format(**fetch_kwargs)
+                "Using web proxy %(proxy_host)s:%(proxy_port).", fetch_kwargs
             )
 
         if self.no_check_certificate:
@@ -579,7 +579,7 @@ def static_paths(self):
     def template_paths(self):
         default_template_path = pjoin(here, "templates")
         if self.template_path:
-            self.log.info("Using custom template path {}".format(self.template_path))
+            self.log.info("Using custom template path %()s", self.template_path)
             template_paths = [self.template_path, default_template_path]
         else:
             template_paths = [default_template_path]