Allow JupyterHub.default_url to be a callable based on user

jupyterhub/app.py

@@ -55,6 +55,7 @@ from traitlets import (
     Instance,
     Bytes,
     Float,
+    Union,
     observe,
     default,
     validate,
@@ -1314,12 +1315,23 @@ class JupyterHub(Application):
         """
     ).tag(config=True)
 
-    default_url = Unicode(
+    default_url = Union(
+        [Unicode(), Callable()],
         help="""
         The default URL for users when they arrive (e.g. when user directs to "/")
 
         By default, redirects users to their own server.
-        """
+
+        Can be a Unicode string (e.g. '/hub/home') or a callable based on the user object:
+
+            def default_url_fn(user):
+                if user:
+                    if user.admin:
+                        return '/hub/admin'
+                return '/hub/home'
+
+            c.JupyterHub.default_url = default_url_fn
+        """,
     ).tag(config=True)
 
     user_redirect_hook = Callable(

jupyterhub/handlers/base.py

@@ -638,8 +638,15 @@ class BaseHandler(RequestHandler):
             )
 
         if not next_url:
-            # custom default URL
-            next_url = default or self.default_url
+            # custom default URL, usually passed because user landed on that page but was not logged in
+            if default:
+                next_url = default
+            else:
+                # As set in jupyterhub_config.py
+                if callable(self.default_url):
+                    next_url = self.default_url(user)
+                else:
+                    next_url = self.default_url
 
         if not next_url:
             # default URL after login