fix clientside notebook manager and assume tree is dir by default

Assume tree url are dir by default instead of file, and if
not do a redirect. Usefull for Clientside manager where the server
does not know the content of the drive, and think that everythong exists.

IPython/html/services/contents/clientsidenbmanager.py

@@ -13,8 +13,8 class ClientSideContentsManager(ContentsManager):
     this class as the contents manager allows those pages to render without
     checking something that the server doesn't know about.
     """
-    def path_exists(self, path):
-       return True
+    def dir_exists(self, path):
+        return True
 
     def is_hidden(self, path):
         return False

IPython/html/tree/handlers.py

@@ -36,21 +36,8 class TreeHandler(IPythonHandler):
     def get(self, path=''):
         path = path.strip('/')
         cm = self.contents_manager
-        if cm.file_exists(path):
-            # it's not a directory, we have redirecting to do
-            model = cm.get(path, content=False)
-            # redirect to /api/notebooks if it's a notebook, otherwise /api/files
-            service = 'notebooks' if model['type'] == 'notebook' else 'files'
-            url = url_escape(url_path_join(
-                self.base_url, service, path,
-            ))
-            self.log.debug("Redirecting %s to %s", self.request.path, url)
-            self.redirect(url)
-        else:
-            if not cm.dir_exists(path=path):
-                # Directory is hidden or does not exist.
-                raise web.HTTPError(404)
-            elif cm.is_hidden(path):
+        if cm.dir_exists(path=path):
+            if cm.is_hidden(path):
                 self.log.info("Refusing to serve hidden directory, via 404 Error")
                 raise web.HTTPError(404)
             breadcrumbs = self.generate_breadcrumbs(path)
@@ -61,6 +48,18 class TreeHandler(IPythonHandler):
                 breadcrumbs=breadcrumbs,
                 terminals_available=self.settings['terminals_available'],
             ))
+        elif cm.file_exists(path):
+            # it's not a directory, we have redirecting to do
+            model = cm.get(path, content=False)
+            # redirect to /api/notebooks if it's a notebook, otherwise /api/files
+            service = 'notebooks' if model['type'] == 'notebook' else 'files'
+            url = url_escape(url_path_join(
+                self.base_url, service, path,
+            ))
+            self.log.debug("Redirecting %s to %s", self.request.path, url)
+            self.redirect(url)
+        else:
+            raise web.HTTPError(404)
 
 
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