avoid executing code in utils.localinterfaces at import time

moves away from global constants to utility functions.

The code is still only executed once,
but instead of at import time, it is executed at first request.

IPython/html/notebookapp.py

@@ -78,7 +78,7 @@ from IPython.kernel.zmq.kernelapp import (
     kernel_aliases,
 )
 from IPython.utils.importstring import import_item
-from IPython.utils.localinterfaces import LOCALHOST
+from IPython.utils.localinterfaces import localhost
 from IPython.utils import submodule
 from IPython.utils.traitlets import (
     Dict, Unicode, Integer, List, Bool, Bytes,
@@ -293,9 +293,11 @@ class NotebookApp(BaseIPythonApplication):
 
     # Network related information.
 
-    ip = Unicode(LOCALHOST, config=True,
+    ip = Unicode(config=True,
         help="The IP address the notebook server will listen on."
     )
+    def _ip_default(self):
+        return localhost()
 
     def _ip_changed(self, name, old, new):
         if new == u'*': self.ip = u''
@@ -694,7 +696,7 @@ class NotebookApp(BaseIPythonApplication):
         info("Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).")
 
         if self.open_browser or self.file_to_run:
-            ip = self.ip or LOCALHOST
+            ip = self.ip or localhost()
             try:
                 browser = webbrowser.get(self.browser or None)
             except webbrowser.Error as e: