Docserver: Update Future.Then() to be more Promise-like

chrome/common/extensions/docs/server2/file_system.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import posixpath
 import traceback
 
 from future import Future
 from path_util import (
     AssertIsDirectory, AssertIsValid, IsDirectory, IsValid, SplitParent,
@@ skipping 88 matching lines @@
 
     There are several ways to implement this method via the interface but this
     method exists to do so in a canonical and most efficient way for caching.
     '''
     AssertIsValid(path)
     if path == '':
       # There is always a root directory.
       return Future(value=True)
 
     parent, base = SplitParent(path)
-    list_future = self.ReadSingle(ToDirectory(parent))
-    def resolve():
-      try:
-        return base in list_future.Get()
-      except FileNotFoundError:
+    def handle(error):
+      if isinstance(error, FileNotFoundError):
         return False
-    return Future(callback=resolve)
+      raise error
+    return self.ReadSingle(ToDirectory(parent)).Then(lambda l: base in l,
+                                                     handle)
 
   def Refresh(self):
     '''Asynchronously refreshes the content of the FileSystem, returning a
     future to its completion.
     '''
     raise NotImplementedError(self.__class__)
 
   # TODO(cduvall): Allow Stat to take a list of paths like Read.
   def Stat(self, path):
     '''Returns a |StatInfo| object containing the version of |path|. If |path|
@@ skipping 60 matching lines @@
             self.GetIdentity() == other.GetIdentity())
 
   def __ne__(self, other):
     return not (self == other)
 
   def __repr__(self):
     return '<%s>' % type(self).__name__
 
   def __str__(self):
     return repr(self)